Peavey Renown

[Hawk]

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? 

[Hawk]

Here's the schematic.

[Hawk]

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...

[Hawk]

Do these transistors need to be removed to get a  proper reading?

[J M Fahey]

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.

[Enzo]

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.

[Hawk]

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.

[Hawk]

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....

[Hawk]

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)...

[Hawk]

Can Q5 and 9 be tested in circuit? Are these the driver transistors?

[Hawk]

Crickets........I'd say from the C-E readings that these output transistors are shorted. I'm going to replace them.

[Enzo]

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.

[Hawk]

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. 

[J M Fahey]

I'm away for a few days but will get back to the amp on Thursday.

No, no and no   

Now WE got impatient  , so you send Family away and go sit back at the slave bench and measure whatever we tell you ... or else 
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[Hawk]

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!:)