Peavey Special blowing resistor

[Apexelectric]

Hey all,

A friend of mine gave me his solid state Peavey Special as practice starting to familiarize myself with repairing solid state amps. The amp worked fine until the last time he used it after being in storage for 6 months to a year.

I've traced the problem to the output section. The symptoms include burning up a 47ohm 1/4w resistor just after a transistor. It appears that a 1W resistor in the circuit is overheating due to some discoloration of the silicone that is covering it. The amp makes a loud 120hz (?) hum when it is turned on. This is unaffected by the volume control. There doesn't appear to be any issues in the power supply filter capacitors or diodes. No fuses have blown due to the issue. Not sure where to go from here.  The 47 ohm resistor is shorting out immediately after replacing it once. Hum was present before and after the shorting of the resistor.

Any help in where to go from here would be appreciated.

The picture attached is of the output section with the two resistors in question circled.

[PRR]

> Not sure where to go from here.

First, you don't leave a speaker on a sick transistor amp. You will find this one is putting BIG DC on the speaker. When trouble suspected, do a DC check at the speaker output before you connect speaker. Stage speakers may stand a few Volts, but anything over a few tenths Volt means a sick amplifier.

Voltage checks. Lots of numbers on schematic; where are you way out?

Ohms Law and Watts. We already see that R89 must be WAY more than 0.51V to burn-up. Transistor action: Vbe will normally be 0.5V-1.0V, so Q11 Q12 should be clamping R89 way under its burn-up voltage, so Q11 Q12 must be blown-open.

[pompeiisneaks]

If it's old enough, the hum is likely due to capacitors being dried out and failing, (electrolytics) they may need to be tested/replaced.  The burnt up resistor means that  likely many of the transistors around that area got cooked as well.  If it's not too expensive, you may want to replace the transistor you said was directly connected to the scorched resistor, and any very near it, because you can replace/repair one, and have another dead and burn up things again. 

You can use diode test mode to see if some fets/mosfets/etc are working, there are a few good video's out there on it, that may help. 

I'm still not yet outstanding with solid state but have fixed a thing or two with them.

~Phil

[Apexelectric]

Ill pull the two closer transistors after I check DC on the output.  I will unplug the speaker first. I suspect there is DC on the output due to the noise it makes when turned on.  I didn't leave it on for more than a second or two when I heard it.

All the filter caps were within 10% and rectifier diodes checked out ok.

I suspected there would be a breakdown in the nearby transistor allowing it to fry the resistor but wanted a second opinion before I pulled the transistors from the board to check them.

Anything in particular that would cause the transistors to go bad besides age in this situation?

[PRR]

> cause the transistors to go bad besides age

They last forever unless abused.

This is a very rudimentary power amp. It will survive some types of shorts, not others. When one transistor blows, the amp tries to force output, blowing several others. It is not uncommon to have to replace all but the input pair.

Aside from being very 1970, it has that really odd auto-transformer on the output.

Is this a Peavey Special 130? Is it supposed to be 130 Watts? That's on the bold side for a stage amp with just four power devices.

It is sadly just a bit too big for the available easy-install chips.

> to familiarize myself with repairing solid state amps

Go ahead. Been there. Done that. It can be frustrating; also unprofitable. You need the experience, both diagnostics and learning when to fold your hand and cut your losses.

[Apexelectric]

It's a peavey Special Solo Series from 1982. Found a short in Q11, one of the 6392 higher power transistors. Was reading a short on all but one of the 6392's but after pulling them only one had the short. The other smaller transistors showed no shorts while installed and tested fine when pulled. 

This is just a learning process for me. I'm just trying to hone some troubleshooting skills and get to the point where I know whether it's worth it or not to take on some of these repairs. I'm enjoying the education but it most definitely can be frustrating. I've got a small 60W Hartke bass combo to tackle next.

[shooter]

Found a short in Q11

pick up at least 2, they are basically paralleled an the "other" one might have been stressed before the other shorted.

it most definitely can be frustrating

yup, once you, learn to half-split, fix the obvious, shot-gun whole PA section, then test "spare" parts afterwards on a bread board, you'll be doing the happy dance n busting out your guitar!

[Apexelectric]

Thanks for all the great advice guys. Shooter, you'll have to go into some more detail sometime for me regarding the tech talk as I have no idea what you mean by half-splitting and shot gunning actually mean,lol. I can only guess.

From what I can gather, these transistors should be fairly robust under normal conditions and it makes me wonder whether there is another issue at hand causing stress on the transistor. Anything else to look for before firing it up? Would the shorted Q11 explain the resistor blowing when it was replaced? I always like to be fairly clear on the root cause and not just the obvious issues. I fixed a couple amps for the same guy and both of them had bad rectifier diodes. So I'm guessing he had a power surge/quality issue where ever he was living.