Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: Hawk on June 26, 2015, 09:12:28 AM

Title: Peavey Renown
Post by: Hawk on June 26, 2015, 09:12:28 AM
Trying to repair an amp for a friend. Keeps blowing fuses. Drawing over 5 A current rating at 40v, not good. Check PT and found .3 ohms from Red/Centertap, both sides. Other fuses on lower voltage are fine. Need to pull board to check diodes--one half of their legs are under the board. Caps show good on ESR meter.

Did the lightbulb test on the PT, RG Keen's method, and no light at all...hmmm, internal short? Any other tests just to make sure?

I've included the schem and a pic of the output transistors. Always felt intimidated by these output transistors. Do I need to take out the pcb to check them?  :-[
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 09:15:05 AM
Here's the schematic.
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 09:55:10 AM
Here's an image of the underside of  one of the Output Transistors. Two screws  plus what looks like the base and emitter. Is it a matter of measuring from between these two pins and the metal case to figure out if there is a short? Diode checker...
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 09:57:14 AM
Do these transistors need to be removed to get a  proper reading?
Title: Re: Peavey Renown
Post by: J M Fahey on June 26, 2015, 11:28:10 AM
If it's a plain short, it will measure the same in or out.
A short, by definition, swamps anythin in parallel with it ...even another short.

Now if you are getting other readings, such as a reverse bisade junction which should read open but reads some other value, you are probably measuring "something else" in the circuit, so for a proper reading you must pull the btransistor.

No big deal, just suck solder , unscrew and pry it out.

When remounting, remember to clean old thermal paste first and apply fresh one and it's better to use a new mica.
Title: Re: Peavey Renown
Post by: Enzo on June 26, 2015, 11:46:42 AM
First, unplug that output transformer, if ther is any DC on the output of the amp, that transformer will act as a short.  Also, until you know the amp is stable and not producing DC, do not connect a speaker load.

Look up light bulb limiter and make one.  The variac  is a great tool with the ammeter, but there is no reason to go up to 5A if it is starting up that fast, just back right off.

Either way, You can check the outputs where they sit.  Every output transistor has the case exposed, the collector, and each has a ballast resistor connected to its emitter.  Those are the 0.33 ohm 5w resistors.  So you can measure resistance from the transistor case to the emitter resistor from up top easily.  Another trick is to measure resistance from the case of the positive side transistors to the output bus, shorted or not?  And on the negative side, from the output bus to those emitter resistors.

Want to check the rectifiers?  OK, look at the layout drawing.  Right next to the main rectifier diodes are the two main filter caps.  The ends nearest the output transistors are the hot ends, one positive and one negative.  You can see on the drawing the copper traces on the underside of the board.  The hot ends of those caps go directly to the diodes, two per cap.  You can simply unplug the red transformer wires, then measure resistance from the end pins of the three pin connector on the board to the two hot ends of the cap.  That will reveal any shorted rectifiers.

Shorted outputs are a lot more common than shorted rectifiers, but they are worth checking.

Power transformer failures are rare, but it takes only seconds to check it.  Just unplug both the red wires and the orange wires from the board, leaving only the mains wires, blue and black I think, connected.  Now see if it still draws current. If it does, the transformer is shot, but if it powers up with almost no current draw, it is OK.
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 04:55:22 PM
Thanks Enzo and JM. So tried the PT the way suggested and no current draw so transformer good. With Output Transistors still intact, I took measurements the way that was suggested by Enzo. Hope I got it right! ( A little hard to tell from the PCB layout diagram if I'm connecting Q7, Q10 to the correct resistor, but I put my probe from one to another of the three closest just to make sure and they measurements came out high. The resistor closest to Q6 was a half watt .33 ohm and I used that for my measurement which gave me .2 ohms).

Collector to Emitter,
Q6 .2 ohms, Q7 4.3M, Q12 4.3M,

Q10, 2.2, Q11 2.1 Q13 1.3.

Measure O ohms from collectors on Q6,7,12 to the positive ouput bus. And close to 0 ohms--.1 or sometimes .2 ohms from the collectors on Q10,11,13 to the negative bus.

I find transistors daunting!! What do you make of my measurements so far?

Thanks for the rectifier measuring tip with board in. Awesome! They all check out.

Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 05:36:58 PM
When I used the Diode Function, from C to E, I found .410 Volts one way, then .OL V  the other way on Q6,7,12.

On Q10,11 and 13 I got .0V both ways, C to E....
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 05:58:09 PM
Also, when the amp came to me one of the speaker wires was disconnected and my friend said he'd been using the 2 ohm jack with the speakers at 4 ohms (two 8's in parallel)...
Title: Re: Peavey Renown
Post by: Hawk on June 26, 2015, 06:15:58 PM
Can Q5 and 9 be tested in circuit? Are these the driver transistors?
Title: Re: Peavey Renown
Post by: Hawk on June 27, 2015, 10:58:29 PM
Crickets........I'd say from the C-E readings that these output transistors are shorted. I'm going to replace them.
Title: Re: Peavey Renown
Post by: Enzo on June 28, 2015, 12:47:24 AM
Some of us are only on here a couple times in a day, especially on a Friday, please don't be impatient.

yes, close to zero ohms C-E is a shorted part.  So it has to come out.  At that point retest them out of circuit.  They are in parallel in circuit, so one shorted can make all three look so.  That is the bottom three.

For the same reason, if Q6 is shorted, as it appears to be, the other two ought to measure short as well.  So look at the three emitter resistors there, R113,116,121.  Are any open?  If any are open, then recheck the associated transistor for shorts.  Do it right at the transistor legs.  In fact with at least four blown, I'd probably replace all six myself.

Might as well check the lower ones too, R115,117,122.

Yes Q5,Q9 are drivers.  With mega blown outputs, those are instantly suspect, in fact I'd probably just replace them, even if they test OK with a meter.  There is no way to test if they have been weakened.

But sure you can test them in circuit, like any other part, you just have to be aware of the circuit.  If it tests shorted, it might be something else in parallel, so we THEn remove it for further tests.  But if it does not test shorted, well there is nothing in a circuit that can make a shorted part look not shorted.

Always check the resistors associated with any bad transistor, in this case, R111,109,110,112.  They can burn open.

testing in circuit:  look at the uppers, Q6,7,12.  See that 47 ohm resistor from their bases to the output bus?  That is across their B-E junction.  Your meter can't tell whether there is 47 external ohms or the part has 47 ohms of leakage.  So we keep that in mind before we decide it must be shorted from the low reading.  Flip the meter to ohms and find 47, and whatever the junction voltage, there is a good chance the part is actuaqlly OK.
Title: Re: Peavey Renown
Post by: Hawk on June 28, 2015, 03:44:12 PM
Enzo solid info! I'm away for a few days but will get back to the amp on Thursday.  I get impatient because I'm afraid I won't be able to fix this amp and I'll come across as incompetent!  I can't say how much I appreciate you and others taking the time! It means a lot.  :dbtu:
Title: Re: Peavey Renown
Post by: J M Fahey on June 28, 2015, 07:11:18 PM
Quote
I'm away for a few days but will get back to the amp on Thursday.
No, no and no   >:(

Now WE got impatient  :grr , so you send Family away and go sit back at the slave bench and measure whatever we tell you ... or else  :trouble
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Title: Re: Peavey Renown
Post by: Hawk on June 28, 2015, 09:12:59 PM
I'm not worthy! Four hours away from the slave bench...I should ask my wife what she thinks of a transportable bench ...on second thought not a good idea!:)
Title: Re: Peavey Renown
Post by: LateDev on June 28, 2015, 10:23:13 PM
Standard practice for most people, is to rip out all power transistors and check them, then forget about the protection circuit, then wonder why nothing works.
Many times you will find a protection circuit fails, with the result that there is no output, despite everything else looking OK.

Power amps are a pain, take nothing for granted as it is all DC coupled and a blown driver can cause you to loose a new set of output transistors. Check everything, all it cost is a little time.
Title: Re: Peavey Renown
Post by: Hawk on July 02, 2015, 09:57:06 AM
Interesting! So I checked all resistors mentioned in post and all tested good. I removed all output transistors and Q13 was the only one shorted. Could that alone explain the fuse blowing and the amp not working? Do others find that one output transistor alone can and will effect the current draw but not effect the resistors enough to make them blow. Seems to good to be true.....

I haven't checked the other transistors mentioned but will soon. Thanks.
Title: Re: Peavey Renown
Post by: J M Fahey on July 02, 2015, 10:09:38 AM
Well, if the mains fuse is properly rated it may blow fast enough to protect other parts, sadly not semiconductors which are even faster.

Of course, sometimes (quite often in fact) , some fearless users replace the fuse with a piece of rolled up aluminum paper, a piece of wire or simply "a fuse" ..... pulled from a car or motorcycle nearby ... typically rated from 10 to 30 Amperes.

Then you open the amp and find a small version of Hiroshima inside.
Title: Re: Peavey Renown
Post by: Hawk on July 02, 2015, 10:26:05 AM
Thanks JM. Fuse was proper fuse. Have you come across situations like mine with only one shorted output transistor? Is that Common? Uncommon? Thanks
Title: Re: Peavey Renown
Post by: Enzo on July 02, 2015, 02:25:54 PM
yes indeed, all the time.  One shorted output will blow fuses.  And once the fuse blows, the amp won;t work...
Title: Re: Peavey Renown
Post by: Hawk on July 02, 2015, 03:54:46 PM
Okay great! I'll check the other transistors and build that light limiter and give it a whirl. So can anyone suggest a good supplier for one off transistors? Seems most charge a basic shipping fee regardless of only needing small and few components.
Title: Re: Peavey Renown
Post by: Enzo on July 02, 2015, 08:40:10 PM
Peavey has a minimum charge of $5 I think.   Their prices may not be the lowest you can find, but they are generally reasonable, not outrageous like some places.  if you need nothing else and wind up paying $5 for a $3 part, oh well.

There are three parallel transistors on the positive side, and three on the negative side.  You found one was bad.  My rule is as soon as you identify a part as bad, either throw it away NOW, or at least bend its leads over so it will not get confused for a good part.  My work area gets cluttered so this matters to me.  So remove the bad part and install the others.  The amp will function with a transistor missing on each side.  I wouldn't expect it to go to full power, but you can play it at reasonable volume.  I also wouldn't play it for extended periods that way.

But the value of doing that is you have the opportunity to find out if other parts are bad that were not discovered while the amp was blowing fuses.  We don't want to order our one part only to find one more part reveals itself after we get the first new one.  You may find you need a few parts, and that helps put together a good order.

Common or uncommon doesn't really matter.  Whether it happens to most everyone or if it only happened to you, the fact remains it happened, and you need to solve the problem regardless of how many other people had to do so.
Title: Re: Peavey Renown
Post by: LateDev on July 03, 2015, 06:51:58 AM
I removed all output transistors and Q13 was the only one shorted. Could that alone explain the fuse blowing and the amp not working?
You are quite lucky it was that transistor that blew, as it is the output driver on one half of the power amp section.
I did mention the protection system on these amps, which, on this occasion, did its job.
With a shorted Q13 the bias to the output transistors would have been pulled up to rail, turning the o/p transistors hard on. However one of the output transistors is connected to the protection circuit (this is the lucky bit), which would have clamped the bias down. It would have done this via R134 a shorted Q13 to diode CR34 and Q12 turned hard on.
This is where protection circuits can go bang.

So the transistor goes short, a big current spike and the protection circuit kicks in to clamp the bias, in the mean time the fuse is still being heated to the point of breakage from the original short. or, hopefully not in this case, the protection circuit fries and is the cause of the blown fuse.

Make sure you check every component as power amps are DC connected throughout. For all you know at this stage, Q12 could have gone into meltdown.

Bending twisting or snipping legs of defunct transistors are always advisable, just don't forget to throw then away after  ;)
Title: Re: Peavey Renown
Post by: Hawk on July 03, 2015, 09:27:42 AM
Thanks  LateDev.
Quote
It would have done this via R134 a shorted Q13 to diode CR34 and Q12 turned hard on.

I can't locate R134...hmm. Also, aren't Q4 and Q8 part of the protection circuit?

So if everything else is working but I take out Q12 and Q13, the amp should work, although not as powerful as I've removed a gain stage?

Thanks again!
Title: Re: Peavey Renown
Post by: Hawk on July 03, 2015, 10:20:51 AM
Re: CR9 and CR10. Other diodes that I tested in circuit test normal using the diode checker function on my meter. But with these two, in circuit, I get a read of .553v in one direction and .555v in the other direction. So that would be incorrect. But just to make sure should I  take one leg out of circuit and re-test just to make sure these are indeed fried?
Title: Re: Peavey Renown
Post by: Hawk on July 03, 2015, 10:22:22 AM
In fact, now that I'm testing other diodes I'm getting the same mixed messages. Can't imagine all these diodes are fried. :-\
Title: Re: Peavey Renown
Post by: Hawk on July 03, 2015, 11:18:06 AM
Also, fed a 1khz sinewave to do some signal tracing from the input on the pre-amp. Starts off as a nice clean sinewave, tests as showing in multiple locations throughout pre-amp and beyond, but the sign wave is fuzzy, although its retains its original shape and amplitude. :-\
Title: Re: Peavey Renown
Post by: g1 on July 03, 2015, 12:07:30 PM
  Just reading above, it sounds like Latedev is speaking of Q13 as if it is a driver, Enzo speaking of it as if it is an output transistor.
After looking at the schematic you supplied, Q13 is an output device.
Latedev, are you looking at a different schematic?  A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Operating with one pair of the output transistors missing is not one less gain stage, it is less current gain.  This translates to less power available to the load, and more strain on the other output transistors at higher power levels.
Title: Re: Peavey Renown
Post by: g1 on July 03, 2015, 12:10:19 PM
Re: CR9 and CR10. Other diodes that I tested in circuit test normal using the diode checker function on my meter. But with these two, in circuit, I get a read of .553v in one direction and .555v in the other direction.
Look at them closely, they are parallel but opposite polarity.  So either way you connect your meter, one of them is forward biased and gives you a reading.  This is normal.
Title: Re: Peavey Renown
Post by: LateDev on July 03, 2015, 12:45:24 PM
  Just reading above, it sounds like Latedev is speaking of Q13 as if it is a driver, Enzo speaking of it as if it is an output transistor.
After looking at the schematic you supplied, Q13 is an output device.
Latedev, are you looking at a different schematic?  A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Operating with one pair of the output transistors missing is not one less gain stage, it is less current gain.  This translates to less power available to the load, and more strain on the other output transistors at higher power levels.
I didn't see the cct diag Hawk supplied and yes I was referring to my own circuit which is exactly the same just different component numbering for some odd reason. Oh well thanks for pointing that out :)
Quote
A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Actually it would be a miracle if it did not effect the base as the emitter and collector is physically separated by the base on a bipolar transistor, unlike an FET which you may be getting confused with.

Even more amazed nothing else went pop.
Protection circuit is CR29, Q4, Q8, CR30, R105, R106. These act as clamps on the bases of Q5 and Q9, which are the drivers, via R107 and R108 . All transistors and diodes should be checked out of circuit.
Never assume you can run the amp with one O/P transistor removed, as this could blow the rest. If you need to know why, just ask.
Title: Re: Peavey Renown
Post by: Enzo on July 03, 2015, 01:06:47 PM
I need to know.  How does the removal of one parallel transistor cause the rest to fail?  Remember we already stipulated the amp would not be producing full power this way, but will work sufficiently to service.
Title: Re: Peavey Renown
Post by: g1 on July 03, 2015, 01:17:43 PM
Quote
A shorted Q13 could be C-E, so it would not necessarily affect the bias.
Actually it would be a miracle if it did not effect the base as the emitter and collector is physically separated by the base on a bipolar transistor, unlike an FET which you may be getting confused with.
No I'm not getting it confused with anything  ;).  We often see C-E shorts in power transistors yet the base is shorted to neither C or E.
But this is aside from the point, you were originally speaking of it as a driver.  In which case a C-E short would have affected the bias.  So no need to argue a point you never originally intended, right?


Never assume you can run the amp with one O/P transistor removed, as this could blow the rest. If you need to know why, just ask.
  Ok, I'm asking.  No one said to run an amp with one O/P transistor removed.  What was said was that when there are multiple pairs, extra pairs can be removed, for test purposes at lower power levels.  We do this all the time in the repair world.  You saw the caution about power levels when doing this?
Title: Re: Peavey Renown
Post by: Hawk on July 03, 2015, 02:21:38 PM
CR28 reads .014v one way and the other, in circuit?? As LateDev suggest testing diodes out of circuit so will try that.

Quote
Protection circuit is CR29, Q4, Q8, CR30, R105, R106. These act as clamps on the bases of Q5 and Q9, which are the drivers, via R107 and R108 . All transistors and diodes should be checked out of circuit
. Okay, good.

 Thanks for the post on parallel diode readings. :cheesy:

 
Quote
No one said to run an amp with one O/P transistor removed.  What was said was that when there are multiple pairs, extra pairs can be removed, for test purposes at lower power levels.  We do this all the time in the repair world.  You saw the caution about power levels when doing this?
  If, for example, you mean Q3, Q12 and Q7, Q11, and Q6, Q10 then I get it. Correct?

Quote
Operating with one pair of the output transistors missing is not one less gain stage, it is less current gain.  This translates to less power available to the load, and more strain on the other output transistors at higher power levels.
Understood!
Title: Re: Peavey Renown
Post by: LateDev on July 03, 2015, 05:17:28 PM
But this is aside from the point, you were originally speaking of it as a driver.  In which case a C-E short would have affected the bias.  So no need to argue a point you never originally intended, right?
Did you not read my post ? I did say the numbering was different duh, in other words on the diagram I have, Q13 is shown at the position of Q5 on Hawk's cct, which is a driver.

 I can only go by how a bipolar transistor is physically made. For there to be a short between collector and emitter the base region of a transistor must also be shorted, this does not mean that you have to see a short between the base leg to either the collector or the emitter as the leg is connected to a small portion of the base. You should really read up on how a bipolar transistor works to understand why the base current is effected when there is a CE short. Pay particular attention to doping of the base emitter region, and the now defunct depletion layer. which is blown away when there is a short of this nature.

I need to know.  How does the removal of one parallel transistor cause the rest to fail?  Remember we already stipulated the amp would not be producing full power this way, but will work sufficiently to service.
In actual fact Hawk was more sensible than this, and mistakenly suggested he remove a pair of transistors.
What we have here is that no explanation was given when it was said it would be fine to do so, and as you should know that without checking other components within the output section of the power amp, you cannot know that powering up will be safe.
 Transistors are current operating devices, which means that the bias current for the output transistors is calculated to be shared between each of the 3 transistors. See Kirchoffs current law(KCL) What you now have is a circuit designed for 3 transistors each side, being biased on harder because there is only 2 transistors each side and you still have not checked other components in a DC connected circuit.

This is in answer to Hawk's question
Quote
So if everything else is working but I take out Q12 and Q13, the amp should work, although not as powerful as I've removed a gain stage?
No, but it could cause damage, so take your time. don't rush, and check everything. I do not mean the whole circuit as it is an output transistor that blew.
If Q3 is OK and everything after that is fine, then there should be no problems. The furthest back you really need to check is to Q1 and Q2 but these never really blow, or it would be farer to say I have never seen them blown.
Once you are happy that you cannot see any other problems, replace any blown components, connect it up with the light bulb on the mains, then take voltage readings at different points. Never inject a signal until you know all other conditions, in a DC sense are correct.
Title: Re: Peavey Renown
Post by: Enzo on July 03, 2015, 08:46:24 PM
I don't know what to make of this.  If the C-E short exists, you say the base must also be "shorted".  Well, I will believe the base region may well be damaged.  But then you add that it doesn't mean there has to be a short from the base leg to the other terminals.  We do not tear transistors apart, so all we CAN do is test the relationship from one leg to another.  You might make some philosophical point about what goes on inside, but from a reasonable practical view, such as during troubleshooting, if we measure no short from the base leg, we will simply say ther is no short.  The transistor is already failed C-E, so anything else is moot.

Quote
In actual fact Hawk was more sensible than this, and mistakenly suggested he remove a pair of transistors.
What we have here is that no explanation was given when it was said it would be fine to do so, and as you should know that without checking other components within the output section of the power amp, you cannot know that powering up will be safe.

Removing a pair is fine to do, more below.

What I suggested was that the amp does not need that one transistor to operate during test, I did not imply nothing else could be wrong.  I suggested that service could continue with that one part removed - or with two removed as a pair - and in fact I did explain that he should do so in order to find pout whatever else might still be wrong with the amp, so as to get all the parts into one order.  That specifically implies something additional may well be wrong.  So powering up would be done in the same service context as before.  I am sorry you did not understand that. And that is where current limiting bulbs or variacs with current meters would be exactly the tools to use.


As to removing Q12 and Q13, there will be no detriment to the amp.  In fact, MANY solid state amplifies are built on boards intended for more than one model, and the difference between a higher and lower power version will be the addition or deletion of a pair of output transistors on that board.  The bias of the solid state output is determined by the voltage difference at opposing bases.  removing a pair doesn't change that.

Yes, if we remove 1/3 of the output devices AND TRIED TO GET FULL OUTPUT from the amp, then each transistor would have to try to conduct the extra current.  But we already specified th amp would not produce full power under those conditions and should not be asked to.  SO again, your point is moot because it does not apply here.

It will not damage the amp in any way to operate it with Q12 and Q13 removed during test.
Title: Re: Peavey Renown
Post by: LateDev on July 03, 2015, 09:21:28 PM
I don't know what to make of this.  If the C-E short exists, you say the base must also be "shorted".

and where did I state that ?
Well, I will believe the base region may well be damaged.  But then you add that it doesn't mean there has to be a short from the base leg to the other terminals.  We do not tear transistors apart, so all we CAN do is test the relationship from one leg to another.  You might make some philosophical point about what goes on inside, but from a reasonable practical view, such as during troubleshooting, if we measure no short from the base leg, we will simply say ther is no short.  The transistor is already failed C-E, so anything else is moot.
Of course there does not have to be a short, if you care to reread the post I made, it was g1 that made that comment, and that was in respect to the different numbering of the schematic I was looking at, having different numbers to Hawks, again I explained all this.
Quote
g1 stated : A shorted Q13 could be C-E, so it would not necessarily affect the bias.

I merely pointed out that it would effect the bias. If you cannot understand this, then fine.

Same goes for the biasing of the power amp with a pair of transistors removed, if you note Hawk had decided to inject a signal, which should never be done at this stage. the four transistors are actually biased on more, than they normally would be, so the test is useless.
Title: Re: Peavey Renown
Post by: J M Fahey on July 04, 2015, 12:43:20 AM
Quote from: LateDev

Quote from: Enzo
I don't know what to make of this.  If the C-E short exists, you say the base must also be "shorted".

Quote from: LateDev
and where did I state that ?

Quote from: LateDev
For there to be a short between collector and emitter the base region of a transistor must also be shorted
Title: Re: Peavey Renown
Post by: Enzo on July 04, 2015, 01:33:50 AM
Quote
Same goes for the biasing of the power amp with a pair of transistors removed, if you note Hawk had decided to inject a signal, which should never be done at this stage. the four transistors are actually biased on more, than they normally would be, so the test is useless.

Sorry, flat out wrong.  Removing one of the three pairs of outputs does not change the bias relationship between the bases.
Title: Re: Peavey Renown
Post by: Hawk on July 04, 2015, 08:58:19 AM
Thanks for everyone's input. I've learned a lot as usual. Will print off this thread and attach to schematic for future reading for future solid state repair work.

Will order parts and power up with my current meter, variac, and limiter and let everyone know my results.  Thanks :dbtu:
Title: Re: Peavey Renown
Post by: Hawk on July 10, 2015, 10:57:52 AM
Best techniques to save transistor legs from breaking!!!

In the images I attached (sorry one is blurry but you'll see that the middle leg broke off), I removed the transistors to test out of circuit and they both tested good. So I was feeling good about that. But when I tried inserting the 5331 the middle leg broke off. I had kept the transistor screwed down and had pried the legs out as I applied the iron from the other side of the board. After testing, I tried to slide the legs back into the holes using a small screwdriver--the middle leg wore out very quickly and broke.
Is it best to have the transistor standing at a 90 degree angle to the board when removing then simply pull upwards? When re-inserting should I insert legs through holes, solder, then bend the entire trans. down to the heatsink and screw it down??

On the smaller transistor, when I removed it, the legs soon became bent at odd angles that were very hard to straighten out and after trying several times to get all three legs in the hole the transistor looked like an arthritic octopus and the middle leg broke off. What is the best method for taking out and re-insertion with these small sensitive resistors??

I've ordered duplicates from Peavey and I don't want to screw up putting the new ones back in the board.

What have the experts been doing for years that works?  Thanks for  your input in advance!


Also, when I ordered a replacement transistor (Q13) from Peavey they told me it had to be a matched pair so I purchased two.  So, like output tubes, does this mean I have to replace Q12 with the other new transistor?
Title: Re: Peavey Renown
Post by: Enzo on July 10, 2015, 06:28:10 PM
First thing I do is remove the nut from the screw, so the transistor can be moved.  I then suck the solder from around the leads, then a worry the part free and lift it up.  I do not unkink the leads.  I rarely have to remove thse to determine if they are faulty.
Title: Re: Peavey Renown
Post by: J M Fahey on July 11, 2015, 12:15:07 AM
Agree and add: the main point is to remove all solder from the joint, any left bridging pad to leg will make you twist and pull, and we want as little mechanical effort as possible.
Sometimes there'sonly a few atoms left, so lightly pulling the leg from above or lightly pushing it from below with a screwdriver blade is all that's needed.

Sometimes it pays to have some solder wick .

Obsolete for many, but helps remove that last tiny bit of solder.
Title: Re: Peavey Renown
Post by: Hawk on July 11, 2015, 08:02:08 AM
Thanks guys.

Quote
I rarely have to remove these to determine if they are faulty.

Well, from what I've read most people say to get a proper reading the transistor must be removed from circuit. Enzo, I'd rather not for sure. So, how would you have tested this one in-circuit to save you the hassle of removing it? Would none of the associated circuit effect the reading?  Thanks!

I will certainly try your method to save future removals...
Title: Re: Peavey Renown
Post by: Enzo on July 13, 2015, 01:21:27 AM
Of course external things can affect the readings.  I admit to a great deal of experience, which may color my approach, but I keep in mind what I am looking for.  For example, I do not just take a zillion readings and hope they suggest something. I am looking for specific information.  If I am blowing fuses or the amp is going to DC, I start to look for shorted or open parts or circuit paths.  If the transistor is shorted, there is nothing in the world external to it that can make it look not shorted.  So if I am looking for shorts, then no need to remove anything.  On the other hand, when I do see a short, it may be the part itself or the things parallel to it.   So since the part had to come out to replace if shorted anyway, THE I remove it and retest.  If the part still is shorted out of circuit, then I know and replace it.  If it now seems OK, I know the short was, and still is, elsewhere in parallel.

The part may not be good, but at least my meter will tell me it is not shorted, or may be.

Sometimes two transistors are connected together, this is common, and so a couple of their junctions may be in parallel.  If they are such that one transistor conducts on direction and the other the other, I will see a diode indication it conducts both ways.   In one transistor that would mean a bad junction, but I am looking at the schematic, and will see the parallel reverse junctions and know it is an expected reading.

To get a proper reading it must be removed?  Well, it depends what you are doing.  If you want to measure gain and leakage and stuff, sure.  But if we are looking for evidence of failure, we can usually find that while in circuit.   None of this is hard and fast rules, just general tendencies.  We roll with the punches.

A lot of output stages feature low resistance resistors.  SO some PV solid state may have a 22 ohm resistance from base to emitter.  Your meter will see that as a short on diode test, but n resistance, voila, the 22 ohms the schematic calls for.  The part could still be bad somehow, but not shorted.

There is of course an endless tale of ways to do this stuff.
Title: Re: Peavey Renown
Post by: Hawk on July 30, 2015, 10:39:44 AM
Quote
yes in tube radio world it is known as a Dim Bulb Tester.  high current is dropped across the bulb protecting your equipment instead of high current across expensive delicate components such as transformers.  you can gage the bulb size by device power consumption.  100W power consumption = 100W bulb.  not to be confused with power output which is not close to power consumption

Replaced power output transistor. So, according to this statement above, I need to have a 450 watt bulb as the Renown Amp is rated at 120V/450 Watts. Not sure this statement agrees with some of the other info on the current limiter info discussed in this forum.  I read the info but not sure exactly how to proceed. I have a current limiter and a variac. I know that SS amps can operate at lower voltages so would it be okay to start with a 60W/100 W bulb? The amp  has a 5 Amp fuse.
I also have a current monitor.

Thanks.
Title: Re: Peavey Renown
Post by: Hawk on July 30, 2015, 11:17:21 AM
So, thought about it some more and fired up the amp with the limiter and slowly brought up the voltage to about 94 volts using a 100 W bulb. Bulb is very bright, current draw reads approx. .6 volts, 1mv DC on output which seems reasonable. But the rails are reading -/+ 1.47 volts. Seems way too low. Any thoughts? Ground to  Collector Voltage on Transistors 6,7,12 shows 1.47V, and the emitter voltage on 10,11,and 13 show -1.47.  Rail voltage on Schematic, when operating normally, is  +42 volts.  How much voltage is being dropped across the lightbulb? I checked the line volts on the amp and it reads 11 volts. hmmm,not a lot of voltage, the amp has far to go, so we should see an increase in rail voltage, but is this voltage too low at this stage and should I be concerned? ???
Title: Re: Peavey Renown
Post by: g1 on July 30, 2015, 01:47:01 PM
  The quote about the bulb wattage said to use the wattage of the output power, not the input from the AC line (120V).  The 450W is input wattage, the actual amp power output is much less, I think around 150W ?
If the bulb lights up (more than dim), then there is a problem.  That is why the voltages are so low.
  Check for low resistances across the supply rails, which are usually caused by shorted power transistors or rectifier diodes.
Title: Re: Peavey Renown
Post by: Hawk on July 30, 2015, 06:20:47 PM
.7V AC across 42 volt supply, line side. 3.5v AC across 15v supply. As an aside I orginally checked all the Output Transistors and all the others on the power amp side and they checked out okay but one which was shorted and I replaced it and the other in last stage with a matched set from Peavey.
42 v supply diodes check out okay. What kind of resistance should I find across the 42 volt transformer winding?   Info so far...
Title: Re: Peavey Renown
Post by: Enzo on July 30, 2015, 06:57:30 PM
I have no idea what the transformer winding measures, I have never had to measure one.  The transformer is likely just fine, it is the last thing on the list of parts failing.  If you suspect the transformer, then pull the wires off the board, power up the transformer and see what AC you get on its wires.

450 watts is the maximum the amp can ever pull from the mains, not a steady need.  A 100 watt bulb is just fine.  When the bulb glows brightly, that means the amp is acting like a big short.  And when the bulb is glowing bright, that means very little voltage is getting into the amp itself, so of course any voltages in the amp will be very low.  That is the point of the bulb.  You are testing the amp at idle anyway, and it doesn't remotely draw 100 watts, let alone 450 of them.  At least when it is working.

The bulb is there to save the amp from blowing fuses.  it saves fuses, but also prevents full current from flowing in a damaged amp. It is not ever meant to be used while operating an amp.   Once the amp is stable and not making the bulb bright, we get rid of the bulb.
Title: Re: Peavey Renown
Post by: Hawk on July 30, 2015, 10:46:55 PM
Thanks gents!  So the AC voltage off the transformer is good. Checked the diodes using  a resistance check and they don't look so good. D36--8.45Mohm, 5.9Mohm  D34 .1ohm, 0 ohm,
D37 3.67 Mohm, 7.95 Mohm D35 0ohm, 0ohm. The voltage that is fed from the diodes is only 1.4 volts which is what I'm getting on the rails. The diodes look good visually. My readings were with the diodes in circuit.

I checked all the resistors in the power amp circuit and they checked out good except for R97 which should be 4.7K but reads as .8k, again, in circuit. How important is this to getting the amp up and running?

I'm including a photo of something wierd that I found. C53 and 54 don't exist in the amp. C53 only has the leads left, and C54 looks like a resistor and measures like one, 107kohm?. My friend didn't give this amp to anyone else to fix and it's been sitting in his basement for 20 years. I suppose C53 could  have broken off after I opened up the amp but I wasn't tinkering with that side of the amp. Could C53 have anything to do with the amp not working? :-\
Title: Re: Peavey Renown
Post by: Enzo on July 31, 2015, 03:20:23 AM
Visual inspection is important, because it something looks burnt or cracked, it needs replacing.  However, MOST defective parts do not look any different.  Does your car look different if a motor bearing is worn?

Those rectifiers are shorted, I'd just replace all four.  The shorted ones may have stressed the other two.  They are just 6 amp rectifiers, 100v or higher.  Whatever voltages are in the circuit now are irrelevant, because the diodes are shorted and because the bulb affects all voltages.

R97.  Look at your schematic.  One end is grounded, the other connects to a cap and R99.  The cap offers no DC path, but follow the line from R99 down and over and you will see it has a parallel pair of 0.1 ohm resistors to ground, R119,R120.  That makes 0.05 ohms, which is essentially zero, so the lower end of R99 is really grounded..  That means 1k R99 is now in parallel with 4.7k R97.  What do they total then?   I get 824 ohms, or 0.8k

Don't worry about resistor readings until the big stuff is fixed, unless the readings show open or shorted.

C53,C54.  C54 is there, it is the green one that looks like a resistor.  C53 has been snipped out.  The reason C54 reads like a resistor is that there are resistive paths to ground from either end, and your meter is measuring them.  Don't worry about it.
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 12:10:22 PM
Thanks Enzo, greatly appreciated. So I replaced the rectifiers and am now getting a mere .31 vDC on the rectifiers and on the rails. The 100 W bulb  is shining very brightly, at 90 V on the variac :'(

 I was getting 1.4 volts with the shorted rectifiers...
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 12:29:45 PM
Okay, back to getting 1.4VDc on supply rail and other side of rectifiers....light bright as ever.
Title: Re: Peavey Renown
Post by: DrGonz78 on July 31, 2015, 04:52:12 PM
Try disconnecting the secondary leads of the transformer at this point. With all the secondary taps disconnected does the light bulb shine bright? I think at this point I would want to know the health of that transformer.

Edit: Perhaps I missed something here though...
Thanks gents!  So the AC voltage off the transformer is good.

So you have checked the AC voltage coming off the transformer that leads to the rectifier and it is good? Then are there burnt or open traces any where on that part of the board around the rectifiers etc. Should be even looking for shorts caused by solder bridges even.
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 05:07:27 PM
Yes, all the secondary taps  disconnected and the lightbulb shines very dimly at 120V so all appears well. Will check for solder bridges, open traces...
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 05:28:24 PM
Flipped board over--no solder bridges or burnt marks on rectifiers. I put in new rectifiers and the problem still exists.
Title: Re: Peavey Renown
Post by: DrGonz78 on July 31, 2015, 06:16:49 PM
So what resistance readings are you getting across the +/- ends of C66 and C67? Are either of those caps shorted?
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 06:30:22 PM
Quote
So what resistance readings are you getting across the +/- ends of C66 and C67? Are either of those caps shorted?
Thanks for this Dr. Gonz. I used my ESR meter and both caps checked out good.

So is  this a case of a shorted supply that could have something to do with the Output Transistors shorted out and pulling tons of current? I tested them with my Peak Atlas DC55, out of circuit, and they all showed good but one. I ordered a matched pair from Peavey and installed them. I haven't fired up the amp without a bulb limiter.

I installed them the same direction as the original transistors--serial number/ brand--unless for some wierd reason these new ouput transistors have different pinouts than the originals.

With the variac at 100V, .69Amps and the bulb limiter very bright, I have 4.9 VAC coming in for the rails and 1.4V DC leaving to feed the rails. So with the voltage drop of the rectifiers this DC voltage doesn't seem too far off.

Again, I've measured the resistors in the power amps  and they appear good. Is there anything else in the power amp circuit that could be hauling current through the amp? "Shorts between transistor case and heatsink, due to a punctured mica washer"? This is my toughest amp yet :grr
Title: Re: Peavey Renown
Post by: DrGonz78 on July 31, 2015, 08:27:02 PM
With the variac at 100V, .69Amps and the bulb limiter very bright, I have 4.9 VAC coming in for the rails and 1.4V DC leaving to feed the rails. So with the voltage drop of the rectifiers this DC voltage doesn't seem too far off.

I am having a difficult time understanding exactly what this means. You say you only have 4.9VAC going in and 1.4VDC going out, but that seems very strange indeed. So we should be seeing a way higher AC voltage (of course) and I would expect the rectified DC voltage to be higher than the AC coming in. Then again these voltages are way off (of course).

One more thought is that there is a C69 0.1uf 1kv cap right between the two AC voltage inputs on the bridged rectifier circuit. How is that part doing? A quick test of shorted will be more than enough, but you'll have to remove at least one leg from circuit to get an accurate test.
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 08:53:40 PM
Sorry about that will try to be  clearer. 4.9VAC, 2.45VAC per side, on the secondary side of the transformer, measured from one side of the red wire connector to the other with the connector plugged in. 1.4VDC  rectified. And this same 1.4VDC shows up on the rails.

What's also strange is how, when I remove the red transformer wires from the board and test them, it has 53.6 AC volts from red to red, the light barely lit, but, again, when I connect it and measure, on the pins, I get 4.9VAC and the light goes very bright!

Silly question but if the cap is short should I get an infinite reading? The .1uf is too low a value for the ESR meter to read it.  I have a multi-meter. Also if this cap is shorted then will it be pulling the voltage to ground?Thanks.
Title: Re: Peavey Renown
Post by: DrGonz78 on July 31, 2015, 09:08:10 PM
Try to just measure each side of the bridge rectifier red probe on (~) AC in to bridge and black probe to ground. Give us the voltages that way to be clearer.

Also, an infinite reading would indicate an open condition, as in infinite resistance. A shorted reading would simply be low resistance, such as the same reading as measuring a piece of wire. Use your ohms setting on your meter and if it is a low resistance (hence 1ohm or less) then it is shorted. Although measuring a capacitor we should see a very high resistance.

Edit: Note that we still need to know if that C69 is shorted. Measuring the voltage at the bridge, the way you did, is also a correct way to go about this too. I am concerned about that C69 still. If the secondary leads are connected you will not get an accurate reading and you will have to lift one leg of C69 to do an ohms test.
Title: Re: Peavey Renown
Post by: Hawk on July 31, 2015, 09:33:12 PM
Thanks! Up early for work. Will post tomorrow :tu:
Title: Re: Peavey Renown
Post by: Enzo on August 01, 2015, 01:45:05 AM
Don't focus on that 1.4v, it tells us nothing useful.

You have SOMETHING loading down the supply, that is why the bulb is bright.  Have you removed the shorted output or outputs? Oh, you said you did. If they are still in there, then that explains it, at least in part.  The schematic shows an output matching transformer.  If you have this in your amp, unplug it from the board.  And remember, we are using NO speaker or load.  If the output transformer is unhooked, NOW does the bulb act the same?

About the power transformer, it is supposed to provide 60v red to red, but that is center tapped, so it is important that the center tap works.  So with the red wire connector pulled off, measure AC volts from the center pin to either end, and you ought to get roughly 30v.  Yes, a little less with the bulb in place.   I suspect it is OK.


The rectifiers are now new?  so that ought to be good.  Assuming they are in the right way.  If neither rail is shorted to ground by C66, C67, then the power amp has the problem.  A shorted filter cap there is pretty rare. 

You checked the outputs, found and replaced a shorted one.  But did you also check all the 0.33 ohm 5 watt resistors associated with them?  They will either be open or they will be OK.  Also the three 47 ohm resistors asociated with the 5331 and 5332, are they open?  Those two transistors, Q5, Q9 are your drivers.  They are often damaged when outputs blow.  Check them, I usually replace them when I have to replace outputs.

A little left of those are CR26, CR27 diodes.  Make sure neither is open.  CR26 is a dual one - two diodes in one part.  So on your meter, it will look more like 1v on diode test instead of 0.5v.  CR27 is a regular diode.

My suspicion is that if you have the output matching transformer, it may be the cause of the bulb lighting, so do unplug it.  I don't think it is what is wrong, but it will cause the bulb to light if the amp is making DC.

For the power amp to draw current like that, we either have both plus and minus sides turning on at once, or we have shorted something in the output stage.
Title: Re: Peavey Renown
Post by: Hawk on August 01, 2015, 08:59:48 PM
Quote
And remember, we are using NO speaker or load.  If the output transformer is unhooked, NOW does the bulb act the same?
Yes, acts the same.

Quote
So with the red wire connector pulled off, measure AC volts from the center pin to either end, and you ought to get roughly 30v.  Yes, a little less with the bulb in place.   I suspect it is OK.
27VAC

Quote
The rectifiers are now new?  so that ought to be good.  Assuming they are in the right way.  If neither rail is shorted to ground by C66, C67, then the power amp has the problem.  A shorted filter cap there is pretty rare.
  Yes, rectifiers new


Dr. Gonz:Edit:
Quote
Note that we still need to know if that C69 is shorted. Measuring the voltage at the bridge, the way you did, is also a correct way to go about this too. I am concerned about that C69 still. If the secondary leads are connected you will not get an accurate reading and you will have to lift one leg of C69 to do an ohms test.
Removed one leg and got an infinite reading. Checked other good caps out of circuit and got an infinite reading so I guess is infinite is normal for a good cap.

Quote
But did you also check all the 0.33 ohm 5 watt resistors associated with them?  They will either be open or they will be OK
They test OK.

Quote
Those two transistors, Q5, Q9 are your drivers.  They are often damaged when outputs blow.  Check them, I usually replace them when I have to replace outputs
I replaced them as I buggered them up when I removed for testing. Brand new. Only used with the light bulb limiter.

Quote
Also the three 47 ohm resistors asociated with the 5331 and 5332, are they open?
They all show 47 ohms.

Quote
A little left of those are CR26, CR27 diodes.  Make sure neither is open.  CR26 is a dual one - two diodes in one part.  So on your meter, it will look more like 1v on diode test instead of 0.5v.  CR27 is a regular diode.
CR26 .015 V on diode test, both directions. CR27 .515 one direction, 0L the other.


Thanks again.



Title: Re: Peavey Renown
Post by: DrGonz78 on August 02, 2015, 03:04:06 AM
Looks like CR26 is shorted out. Got to replace that and then just keep checking as many components as possible. I am glad Enzo knew more about that output transformer. I guess I was surprised to see an output transformer on this amp. If nothing else so far it seems you are getting better at testing for bad parts.
Title: Re: Peavey Renown
Post by: Enzo on August 02, 2015, 03:52:07 AM
The power amp works without the transformer, but it allows full power at both impedances.  However, when the amp makes DC, that trasnsformer then acts like a dead short.  So we disconnect it.

Actually I was more worried that CR26 was open.  All shorting it does is make the amp run cooler and add some crossover distortion.
Title: Re: Peavey Renown
Post by: Hawk on August 02, 2015, 07:29:31 AM
Thanks again guys.
Quote
The power amp works without the transformer, but it allows full power at both impedances.
So, by two do you mean with the transformer connected (one impedance) and not connected (another impedance, or lacking impedance because it's not connected)? You say the power transformer works without the transformer, but you mean not properly, correct, otherwise we wouldn't have a problem?

Tonight, I'll comb through every component on that power amp. Easy to want to give up at this point but I'll keep at it...................

Title: Re: Peavey Renown
Post by: phatt on August 02, 2015, 07:38:04 AM
Thanks again guys.
Quote
The power amp works without the transformer, but it allows full power at both impedances.
So, by two do you mean with the transformer connected (one impedance) and not connected (another impedance, or lacking impedance because it's not connected)? You say the power transformer works without the transformer, but you mean not properly, correct, otherwise we wouldn't have a problem?

Tonight, I'll comb through every component on that power amp. Easy to want to give up at this point but I'll keep at it...................

It's an impedance matching Auto transformer, yes the Amp would likely work fine without it but those older peavey's had many little extras.
Phil.
Title: Re: Peavey Renown
Post by: phatt on August 02, 2015, 07:40:59 AM
Hopefully without adding confusion,,,

OK if the amp runs colder with CR26 shorted then rail voltage *Should* go Up not down to 2 or 3 volts.???

With such low rails there has to be a major issue somewhere else. I may have missed it but, Have the main filter caps been checked? My old Laney combo came to me dead, instantly blowing fuses.
The main filter caps where the cause. Laney was likely similar age.

Hey Enzo What if?

As the open circuit AC voltage reads correct when not connected to board but returns very low DCV when reconnected to Board then;
1/ Main Filter Caps are dead or dying.
2/ A short on both rails.
(would I be right in thinking for voltages to drop that low surely the transformer would be humming/buzzing very loudly even with the limiting lamp)

Or 3/
Many years back (before I had much of a clue) while testing many different poweramp designs I did run into a rather odd problem which to this day I still don't know the cause but my supply transformer gave similar fault conditions as presented here.

Replaced all 4 diodes (Bridge) and both Caps,, same result. :grr :grr

With no circuit connected the DCV read 37 Volt rails but the moment you connected a load the DCV dropped like brick, close to zero DCV.
This was over 20 years back so I can't recall exact details but I don't think the transformer even hummed.

Just Maybe a bad winding would cause that kind of condition. xP

Replaced the transformer and all was normal.
It really ticked me off because that transformer was a big iron with large cross section and bell ends. (from a big Sansui Amp)

Lets hope it's not the transformer. :tu:
Phil.
Title: Re: Peavey Renown
Post by: Hawk on August 02, 2015, 08:36:59 AM
Quote
With no circuit connected the DCV read 37 Volt rails but the moment you connected a load the DCV dropped like brick, close to zero DCV.
This was over 20 years back so I can't recall exact details but I don't think the transformer even hummed.
Thanks for that Phatt. I'll do a transformer test when I get home from work.I believe I used this method before on this transformer but that was a while back. I'll re-test for sure. http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

Quote
With such low rails there has to be a major issue somewhere else. I may have missed it but, Have the main filter caps been checked? My old Laney combo came to me dead, instantly blowing fuses.
The main filter caps where the cause. Laney was likely similar age.
Tested with an ESR meter and they are good.
Title: Re: Peavey Renown
Post by: Hawk on August 02, 2015, 12:49:35 PM
Replacement of CR26, dual-diode.... What effect would this have?

Is the shorted CR26 pulling too much current through the amp before a signal is applied and turning on the drivers in advance, therefore drawing too much current through the Output Transistors and therefore popping the fuse?

Basically, what is it's function and could replacing this give good odds of bringing the amp back to life?  :-\
Title: Re: Peavey Renown
Post by: Enzo on August 02, 2015, 06:46:50 PM
As I said, a shorted CR26 dual diode will only make the amp run cooler and will increase crossover distortion. It won't cause any increases in current.  In fact, shorting those bias diodes is used as a troubleshooting technique.  SHorting that diode turns OFF the drivers.  It is an OPEN CR26 that would cause excess output stage current.

This exact power amp is used in multiple amp models, it doesn't always have the output matching transformer.  The transformer does a useful job, but is not necessary to the amp function, and in times of malfunction, that transformer can obscure problems, so we disconnect it during service.

If your amp is making DC voltage, with NO load, there is nothing to draw current from the output, so it sits there with DC on the outs. When you connect a load, NOW that load draws tons of current, which loads down your power supplies.

I mentioned before not to focus on the 1.4v rails.  With the bulb limiter, some serious short in the amp lights the bulb brightly meaning the amp itself sees very low voltage.
Title: Re: Peavey Renown
Post by: Hawk on August 03, 2015, 07:16:57 PM
Okay, have gone through all the resistors and diodes in power amp. Here are the readings that are suspect:


CR25 Diode Test .656/.515 . Shorted, I'd say
R92, 27K instead of 33K, as should be
R99 .824K not 1 K
R105 31.9K not 100 ohm
R107 54 Ohm not 100 Ohm
R108 54 Ohm not 100 Ohm
CR32 Diode Test .032/.032 shorted I'd say 1.
CR33 .457/.457 shorted
R118 26.5 ohms instead of 22
CR26 We know that this is already shorted.
CR23 Resistance Test 4.78M  one way, 365.5 Kohms the other (but the meter reading is still creeping up as we speak and has been doing for a while, why does that happen? Is it charging C56?) I switched to Diode Test and it reads  .709/0L so it seems good.

CR28 is also shorted .

Haven't taken out each leg of the small capacitors to measure but wonder if I should wait after replacing the bad resistor/diodes. Any thoughts? Thanks!!!!
Title: Re: Peavey Renown
Post by: Enzo on August 04, 2015, 03:14:44 AM
Your amp is blowing fuses, not just sounding distorted.  A 27k resistor in place of a 33k one will not blow fuses.

But I'll look through all this anyway.

CR25 has a resistor across it so it won't measure like a diode - but you can still see it is not shorted, shorted diodes measure close to zero.

R99 - didn't we already discuss that R97 and R99 were in parallel due to R119,R120? And didn't we calculate that exact number of ohms?

R105,R107 - they are in parallel through R113.  If R113 is not open, then around 50 ohms is what I'd expect.  I see no way to get 50 ohms at R107 f R105 is open, and 33k there is an indicator of open.  I suggest recheck R105.

R108, same as R107, parallel R106 through R114, expect 50 ohms.

CR32 has R110 and R114 across it in parallel, they make 47.33 ohms.  A diode with 47 ohms in parallel will test like shorted.

CR31,33 - VERY rare to find these bad.  Possible of course, but frankly more likely Q10,11,or 13 shorted E-C.

R118 - OK that is off a little.  If it looks like it got hot, replace it, otherwise I wouldn't sweat the difference.

CR28 - has a 22 ohm resistor across it in parallel, so it measures like a short.

CR23 -  it tests like the diode it is, trust that.  When you put it on resistance, then your meter is trying to charge up the capacitors in the circuits.  Your meter shows that as a slowly changing reading.   Get an electrolytic cap from your parts drawer and measure its resistance, and see the meter act similar.

The lesson her to learn is these things are not about parts, they are about circuits.  Every part in there is a piece of a larger system.  You cannot ignore all those parallel circuit paths.


When an amp blows fuses (or lights the bulb bright) then the first thing I look for is Emitter to Collector shorts on all the power transistors.
Title: Re: Peavey Renown
Post by: Hawk on August 04, 2015, 10:35:34 AM
Thank alot Enzo, after I fix this amp I'll be going over your notes and check out the  math and relationship between components in the circuit.

I feel at this point I've come full circle. I already pulled out the driver and output transistors and found them to be good but one--I used my Atlas DCA55 tester and it showed a short on T13.

Once I replaced the output transistors, plus the driver transistors which I had to replace due to destroying them when removing, I have only tested the amp using the bulb limiter. Is it possible to blow them when using the limiter? Can't see how as the current is so low.


I've retested the  .33 emitter transistors and my meter vacillates between .3 and .4 so hopefully that will not have an adverse effect. Should I replace them just to be sure?

I've included a pic of the output transistors. I was told by Peavey that I needed to use a matched set so I replaced 12 and 13. I used the same grey rubber insulators that came with the old transistors.

I'm going to re-test the output transistors.
Title: Re: Peavey Renown
Post by: Hawk on August 04, 2015, 11:33:39 AM
Readings:
I didn't tests the new transistors but might just in case. Here are test results from 4 of the six original output transistors in amp. All transistors were removed for all readings.

TR 7 Showed Good on DCA55 Tester. Resistance Readings trans. out of circuit.  E/C-C/E-0L/0L,  B/E-E/B 75M/0L, C/B-B/C- .71M/0L

TR10 Showed Good on Tester.  Res. readings trans. out of circuit. E/C-C/E-0L/0L, B/E-E/B 5.7M/0L, CB-B/C .6M//0L

TR11 Showed Good on Tester. Res. readings trans. out of circuit E/C-C/E-0L/0L, B/E-E/B 6.8M/0L, C/B-B/C .703M/0L

TR6 Showed Good  on Tester. Res. readings trans. out of circuit E/C-C/E-14.8M/0L, B/E-E/B 7.17M/0L, C/B-B/C .7M/0L.
Title: Re: Peavey Renown
Post by: phatt on August 05, 2015, 12:14:58 AM
Just remember;;When reading low Ohms on a DMM it's wise to self test first.
Bridge the tips together and note the reading of the leads,, it won't be zero.
Now measure the resistor value and subtract the lead resistance that will give a more accurate number. :tu:
Phil.
Title: Re: Peavey Renown
Post by: Hawk on August 05, 2015, 10:15:46 AM
Thanks Phil. My DMM zeros out completely when touching the leads. I'm still wondering about the power transformer as you suggested. I'll do some resistance readings on the 42V windings. From red-to-red I get .5ohms. Red to center tap-.3ohms, other side .2 ohms.....seems low!!
Title: Re: Peavey Renown
Post by: Hawk on August 05, 2015, 10:44:57 AM
I used R.G. Keen's transformer test. Since I used a 9V battery I reduced the resistance to 10K. The bulb didn't flash. But when I removed the resistance the bulb did flash. Not sure if I've used the correct ratio of resistance to voltage (my battery reads 7V so I reduced the resistance down to 10K instead of 100K). Hmmm....I then tried using a 100K and again the bulb didn't flash but did without the resistance.

Could a small internal short cause the voltage to drop when connected to the power amp as is happening to my amp?
Can putting a load on a transformer with an internal short create an Inductive Kickback in the transformer so that current tries to flow in the transformer to compensate for the short and therefore not send current out into the power amp?
 ??? ??? ???
Title: Re: Peavey Renown
Post by: phatt on August 06, 2015, 03:25:20 AM
Hello Hawk,

I think you will find the neon will only light up (flash) when you break the connection, if no flash then you have a problem. Unlikely the transformer has a fault but I thought it might be worth a mention.

I'm with Enzo, there is a short somewhere loading down the rails and I quote from his previous clarification;

Enzo said; "I mentioned before not to focus on the 1.4v rails.  With the bulb limiter, some serious short in the amp lights the bulb brightly meaning the amp itself sees very low voltage."

That short could be anywhere.  :grr :grr :grr

Even a breakdown in the isolation shims of the power transistors,, only takes one to fail.
A hairline crack in a trace,,, the list goes on.
Phil.

Title: Re: Peavey Renown
Post by: Hawk on August 06, 2015, 03:36:59 PM
So when the screws go through the output transistor hole they insert through the insulator but the thread makes contact with the case collector and that is cool. Right? I should test to make sure the collector isn't grounded out to the heat sink......or that the base and emitters aren't grounded out.....thanks!
Title: Re: Peavey Renown
Post by: g1 on August 06, 2015, 03:48:48 PM
I should test to make sure the collector isn't grounded out to the heat sink
Yes
Title: Re: Peavey Renown
Post by: phatt on August 07, 2015, 05:13:49 AM
See pic;;
If Any of the parts marked *C* come in contact with the heatsink then it's a dead short for sure.

The little washer down the bottom has a locating detent and that keeps the bolt from coming into contact with the sides of the hole as it passes through the heat sink.

Even If the parts and holes are well made and aligned you still need to take care when bolting it all together.

You only need a bit of conductive crap to lodge in the hole and it's all for nothing. :'( :'(

Yes ALWAYS :trouble :trouble Use your continuity buzzer to check for shorts to the collector when you mount TO3 transistors. :tu:

And as you have already noted,,, the B and E pins need to be checked for the same reason,, Some gear may even have protective sleeves on those pins to stop any chance of contact of pins to the heatsink.
Hope it helps,, Phil.
Title: Re: Peavey Renown
Post by: phatt on August 07, 2015, 08:56:35 AM
Just had closer look and cross referencing some of this;;

Now I just goo'd the number on those new looking power transistors.
Best I can make out from the photo is MJ150250.

Which seems non existent but MJ15025 is legit.

BUT  Ouch!! it's a PNP power transistor  :o :o and if that be the case then no wonder the amp does not work as the circuit uses all NPN power devices.
Phil.
Title: Re: Peavey Renown
Post by: Hawk on August 07, 2015, 09:35:57 AM
Quote
BUT  Ouch!! it's a PNP power transistor  :o :o and if that be the case then no wonder the amp does not work as the circuit uses all NPN power devices
Ahhhh!!!!!!!!!  I ordered these from Peavey and they sent me the wrong ones. I know I quoted them the original part and I even told them the cross-reference part as well. I didn't think to check before inserting. Okay, there's hope!! Thanks Phil for noticing! Awesome!! I'm going to order from a local supplier and get the right ones.

Also, Phil, thanks for the post on continuity buzzer. I never use it and now will always use it when looking for shorts. So, yeah, I'm finding shorts on the output transistors  from collector to heat sink, and it comes down to the screw making contact. However, Peavey did not supply any kind of isolating washer. The screws go right down through the holes in the heatsink and the holes are tight to begin with. Is there something I can use/purchase to stop the screw from making contact with the hole?
Title: Re: Peavey Renown
Post by: phatt on August 07, 2015, 10:24:48 AM
Re the buzzer which is likely the diode tester,  be aware they are not fool proof and can give false reading. I'm not sure of the threshold resistance but others may know more.

I like the buzzer because it allows you to focus on the circuit without having the distraction of keep looking at the meter.

I recall *Enzo* mentioning parallel components causing misreadings and this is the case here as power transistors are often paralleled by very low resistors.
I guess ideally you mount all the power transistors on the heat sink and test for shorts *Before* connecting to the rest of the circuit but (by what I can make out) that is not easy to do in this case.
Hopefully Enzo might be able to shed more light on this. 8|
Hang in there all is not lost. :tu:
Phil.
Title: Re: Peavey Renown
Post by: Hawk on August 07, 2015, 11:30:51 AM
Thanks Phil!  So some good news. The light bulb is not lighting up!!!!!!!!!!!!!!!!!

70 volts variac, 20 Volts collectors of Transistors 6 and 7 (12 removed, -20 volts on emitter resistors of 10 and 11, 13 removed. -.57 on collectors of 10 and 11.

So, here's my question: Yes, I understand that the collectors can't short out to the heat sink but the heat sink is part of ground, and 6 and 7 aren't shorting and 11 and 10 collectors even if I remove the screws they short out (beep test, so I get a beeping sound). I removed the screws so that I wouldn't get the beeping sound and I didn't so I figured no shorts and decided to power up the amp for a short while to see what I could find. Are 11 and 10 collectors supposed to be shorted  ground??? Do my collector voltage readings on 10 and 11 make sense? they are connected to ground so it makes sense to me:tu:



Title: Re: Peavey Renown
Post by: g1 on August 07, 2015, 01:27:32 PM
Can you post a pic of the underside of the board in the power transistor area?
Title: Re: Peavey Renown
Post by: g1 on August 07, 2015, 01:33:19 PM
  Also, there may be a short to ground from where ever those screws connect to, that's why the picture will help.
Title: Re: Peavey Renown
Post by: Enzo on August 07, 2015, 01:58:12 PM
Peavey will provide insulating stuff if you ask for it, but otherwise the transistors do not "come with" them.  And really, all insulating materials were, or should have been, already in the amp with the bad parts.  The screws need not to touch the heat sink, but also, under each transistor is an insulating piece, and those can get damaged.  A chip out of it can let the transistor body touch the heat sink.  SO make sure all those insulators are in place and intact.

If peavey sent you the wrong parts, first look on your packing slip or invoice to see exactly what part number they SAY they sent.  is it the same number as on your new parts?  We want to figure out if they placed an order for one thing but shipped a different thing, OR did they enter the wrong part in the first place and then shipped it.  Then CALL them and tell them you received the wrong parts.  Chances are they will just send the right ones to you and won't want the others back.  Give them a chance to correct any mistakes.  WHy pay twice?

Collectors of Q10,Q11 grounded?   Look at the schematic, any ground connections there?  I see none.  However, if you have that output transformer connected, it will look like a short to ground.  We were supposed to have that disconnected, is it?  Also, if you have a load on the output, that also will make those collectors look grounded.  We were supposed to be working unloaded, are we?

Beep tests are fine within their limits, but use the ohm meter.  There is a HUGE difference between zero ohms, as in a dead short, and something like 22 ohms or 50 ohms.  Those resistances will still beep the continuity test, but they indicate resistance instead of a short.  For that matter, most meter beep functions are diode tests which should present a voltage on the scale, so it may beep and show half a volt, whch sounds like a diode junction or transistor junction rather than a short.  Don't assume your test gear is telling you more than it is.

If the amp has MJ15025 installed instead of MJ15024, good catch.
Title: Re: Peavey Renown
Post by: Hawk on August 07, 2015, 04:18:11 PM
Thanks Enzo. I'm now getting 23.95 volts on all collectors at 77 volts from the variac.

Peavey sent me the wrong parts. They are going to send me the correct ones with no extra charge.

The original transistors in the amp only came with a shim per transistor and nothing else. Somehow,  the screws through the heat sink were not touching the heat sink when this amp was  built but when I replaced the transistors I was having a hell of a time getting them not to short against the chassis In fact, just as a quick fix, I've used electrical tape on the screw to see if that would work and it has. Crude yeah, but I was determined to see if that was the problem--it's been a rough ride!!

The shims appear to be in good shape.

The amp is unloaded.

So, wouldn't plastic screws be a good solution for these output transistors??

Thanks for all your help everyone. Is it too early to say I might be getting somewhere? :)
Title: Re: Peavey Renown
Post by: Enzo on August 07, 2015, 10:10:00 PM
There should be ample clearance for the screws through the holes.  No, you cannot use plastic screws.  The screws are what connects the collector - which is the transistor case - to the circuit board.  Also, as the parts get hot, the screws will soften and get weak.

Unless the heat sink is somehow detached from the circuit board, the holes should align.

really, what voltages you get at 77v on the variac are not meaningful, other than to tell us they are not shorted out.  I am hoping you get that 23v only on the upper side output transistors, remember the lower side ones have their EMITTERS connected to V-.  The collectors of the lower ones are connected to the output bus - the speakers.

See, I knew Peavey would want to make it right.  I have been running a pro shop the last 29 years, and PV has always been at the top of my list for customer friendly companies.

Title: Re: Peavey Renown
Post by: Hawk on August 19, 2015, 09:37:36 AM
Quote
The amp will function with a transistor missing on each side.  I wouldn't expect it to go to full power, but you can play it at reasonable volume.  I also wouldn't play it for extended periods that way.

But the value of doing that is you have the opportunity to find out if other parts are bad that were not discovered while the amp was blowing fuses.  We don't want to order our one part only to find one more part reveals itself after we get the first new one.  You may find you need a few parts, and that helps put together a good order.

This was Enzo's response earlier on in this thread.  So remembering this and, after making sure the collectors weren't shorting to the heat sink: I ran the amp with variac at 49 volts, at low volume, light bulb not lighting up, and two less output transistors (still waiting for the correct transistors to arrive), plugged my guitar into the amp and: Sound!!! Yes, sound!!! So this looks very good. I remembered Enzo saying that these transistors turn on at lower voltages so that we can troubleshoot the amp to find out if anything else needs replacing.
Will follow up once I insert the new transistors! Fingers crossed!  :tu:
Title: Re: Peavey Renown
Post by: Hawk on August 20, 2015, 07:20:34 PM
Okay so Peavey sends me the new transistors and they are NPN but the woman on the phone had to do a lot of cross-referencing and I'm concerned that what she gave me may be incorrect...again!

Tested on my Peak Atlas DCA55:

Old Transistors (Ic 2.5ma test current), Hfe-27 VBe-.58 (Test current Ib-4.83mA, Leakage Current Ic=0
New Transistor (same test currents) Hfe=59!!! VBe-.62 everything else the same.

What am I doing with transistors with current gain of 59? Can these be used, or will the extra current cause problems? ???
Title: Re: Peavey Renown
Post by: Enzo on August 21, 2015, 12:03:25 AM
I've never bothered to measure them, they are current amps.  Try measuring them with 3 or 4 amps running through.

WHAT transistors did they send you?  The originals were sj6392.  Those were generically replaced by either 2N3055A or MJ15015.  The order number was 70483100.  They have since changed some numbers to I think 304xxxx format.  Frankly I just use MJ15003 and am done with it.  If you don't tell us what you got, we cvan't tell you if they are right.
Title: Re: Peavey Renown
Post by: Hawk on August 21, 2015, 08:06:28 AM
They sent me 70484200/MJ15024G.  I have the Peavey cross reference chart which shows the replacement info you've given me.

On the phone, when I re-ordered 2x, I told them it's SJ6392. I also told them the replacement I see on my chart which is all over the internet and matches your info, and they say that this has been changed and they read a bunch of engineer's notes on the computer telling them to send me something else. I think I need to speak to a supervisor when I re-order for the 3rd time. Friendly, but confused.
If I can't get satisfaction from Peavey maybe I should follow your advice and pick up two MJ15003's--do they have to be a matched pair if only one transistor is shorted? Will one do the job?
Title: Re: Peavey Renown
Post by: Hawk on August 21, 2015, 09:56:27 AM
2N3055's. My local electronics supply shop has these in stock....hmmm
Title: Re: Peavey Renown
Post by: g1 on August 21, 2015, 12:07:30 PM
  Those will be fine.  They are equivalent to MJ15015 but with a higher voltage spec.
For Peavey, it means one less part to stock, as they can use them for both MJ15015 and MJ15024 applications.
  You can consider it an upgrade, or "beefier" part.  :)
Title: Re: Peavey Renown
Post by: Hawk on August 21, 2015, 12:37:49 PM
By "those" you mean MJ15024G? Just need to make sure! Thanks for your help!! Appreciated!
Title: Re: Peavey Renown
Post by: Enzo on August 21, 2015, 05:01:34 PM
MJ15024 is a perfectly fine sub for the MJ15015.  I often refer to the 15024 as universal replacements.  They have heftier specs than the smaller ones.  I stock both those and the MJ15003, mainly because there are a lot of other amps that use MJ15003.  Were that not the case, I would stock only the MJ15024 as it will cover the lesser ones.

I can tell you all the same engineering stuff too, but I will just say from experience, the MJ15024 will work fine.

There are variations on 2N3055, and the 2N3055A they used would have been tested for 100v, are the local ones thus tested?  Also, the 2N3055 as a commodity part of old is often counterfeited.   I only buy my parts from trusted suppliers.
Title: Re: Peavey Renown
Post by: J M Fahey on August 22, 2015, 05:13:05 AM
2N3055's. My local electronics supply shop has these in stock....hmmm
Sadly, any shop bought 2N3055 must be considered fake or at least unable to service old guitar amps.

There's 2 kinds:

1) FAKE FAKES , crap of the worst kind, with doubtful dies, no internal copper heat spreader, the works.
I have tested some which stood 48Vce or less (original spec is 60/70V), even have a couple which measures 24V  :trouble

2) real ones but barely meeting old manual specs, forget surpassing them.

Prices have plummeted: an old RCA would cost $5 to $7 today while I can buy all the 2N3055E I want, very good ST brand , for 67 cents each in closed Factory cartons, with all papers ... but they explode in my old amps.
And I can't complain, they (just) meet all datasheet specs.

Very good for 40W/ch home Hi Fi amps with, say, +/- 28V rails. :(
Title: Re: Peavey Renown
Post by: Hawk on August 22, 2015, 07:29:00 PM
Great info! Since only one is shorted can I replace just that one or should I replace both--i ask this because Peavey sells this particular transistor as a "matched pair" so I bought two....
Title: Re: Peavey Renown
Post by: Enzo on August 22, 2015, 09:18:51 PM
Often as not, these old peaveys kill a single transistor, and that is all it takes to fix it, but you now have a spare at least.
Title: Re: Peavey Renown
Post by: Hawk on September 01, 2015, 06:09:13 PM
Okay, this amp works!!! Thanks Fahey, Enzo, and G1 for sticking with me on this. Learned a ton! Couldn't have done it without everyone's help. :tu: :tu:
Title: Re: Peavey Renown
Post by: Hawk on September 14, 2015, 10:59:23 AM
Anyone know the pin out configurations for the foot pedal that comes with the amp? It has an xlr end. Has a red, green, black and ground wire. Wires were ripped off the terminals and I'm not sure how to reconnect.

Thanks.
Title: Re: Peavey Renown
Post by: Enzo on September 14, 2015, 05:57:51 PM
In your #2  post, you put up the schematic of the amp, it includes the footswitch wiring, and also indicates a DIN plug rather than an XLR.
Title: Re: Peavey Renown
Post by: Hawk on September 16, 2015, 11:22:08 AM
I see it. Thanks Enzo. :cheesy:
Title: Re: Peavey Renown
Post by: Hawk on September 16, 2015, 02:10:05 PM
Always more questions: so on the schematic it shows GN. Ground or Green?

I have two pedals that were supplied with the amp. Both Peaveys, one says Peavey Automixer and has three pushbuttons (Selector, Combiner, and Reverb) and has a green, black, red and ground wire. In this scenario there is no white wire which the Schematic calls for.

The other pedal says Peavey Automixer and has two pushbuttons (Selector and Combiner) and has a red,white, green and ground wire. In this scenario there is no black wire which the Schematic calls for.

According to the schematic we need a red, white, black and GN wire(ground or green?).

Each pedal appears to be missing the correct wire colour that matches the schematic. Wiring suggestions?? ???
Title: Re: Peavey Renown
Post by: Enzo on September 17, 2015, 04:09:18 AM
Wire it so it works.  I never worry about wire color nor do I assume wire colors are constant.

Open your pedal and note wire wire colors inside it.  According to the drawing the selects are pins 2 and 5, which are shown as green and black.   Inside the pedal, the select switch should have three wires.  One is a short one over to the combiner switch, and that could be any color.  Then the remaining two wires go up the cable.  Are they black and green?  If they are something else, just note that and adapt.  Even if they wind up purple and pink, they still get wired to pins 2 and 5.

Look between the two post controls and U2a.  See the balls marked norm select, lead select and reverb?   Those are the footswitch connections.  Channels are "selected" by grounding the other signal path.  Want lead?  Ground normal.  Those are pins 2 and 5.  You can ground either pin with a wire to see if it functions for you.  That is how I do them, I verify the pin is wired to the appropriate point in the amp, then verify the pedal is also wired the same.

The combiner simply opens the ground path to the selector switch.  Then neither channel can be grounded off, and so both are "combined".   The remaining switch is a reverb grounder.  Kills the reverb.