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Author Topic: Peavey Renown  (Read 34306 times)

LateDev

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Re: Peavey Renown
« Reply #15 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.

Hawk

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Re: Peavey Renown
« Reply #16 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.

J M Fahey

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Re: Peavey Renown
« Reply #17 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.

Hawk

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Re: Peavey Renown
« Reply #18 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

Enzo

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Re: Peavey Renown
« Reply #19 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...

Hawk

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Re: Peavey Renown
« Reply #20 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.

Enzo

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Re: Peavey Renown
« Reply #21 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.

LateDev

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Re: Peavey Renown
« Reply #22 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  ;)
« Last Edit: July 03, 2015, 06:53:06 AM by LateDev »

Hawk

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Re: Peavey Renown
« Reply #23 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!

Hawk

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Re: Peavey Renown
« Reply #24 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?

Hawk

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Re: Peavey Renown
« Reply #25 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. :-\

Hawk

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Re: Peavey Renown
« Reply #26 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. :-\

g1

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Re: Peavey Renown
« Reply #27 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.

g1

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Re: Peavey Renown
« Reply #28 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.

LateDev

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Re: Peavey Renown
« Reply #29 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.
« Last Edit: July 03, 2015, 12:54:08 PM by LateDev »