Peavey XR 600E repairs

[spsimmons]

This mixer was blowing the main fuse immediately on power up when it was brought to me. One side of the board that the output transistors mount to was reading as shorted all over. I pulled the output transistors from that side and sure enough, one of the 84140s and one of the 74140s (Q201 & Q207) are shorted in every direction. I did a quick check of the other components in close proximity, without removing them from the board, and everything looks good.

What doesn't necessarily look good is the first two resistors (R1 & R2) and the first two zener diodes (CR5 & CR6) off of the filter capacitors appear to have gotten very hot (badly discolored board). I would guess that this happened while the transistors were burning up?

Without pulling them from the board, R1 & R2 seem to be fine (within 10% of 700 ohms) and CR5 & CR6 both measure at least 3 times as much resistance in one direction as the other.

What I'm wondering is, is it good practice to replace components that have more than likely been stressed? Obviously Q201 and Q207 have to be replaced, but should the other pair (Q202 & Q206) be replaced as well, even though they seem fine? The two resistors and zeners obviously got way hot, are they more prone to failure now? Should all these be replaced simply as a precautionary measure?

[Amp Kat]

Sometimes the drivers don't show bad until they are loaded down so I like to change them if there is a question. Try doing a diode check instead of resistance because sometimes it will give you a clue as to how stressed they are. The resistors should be ok as long as they are in tolerance and the zeners should be also tested with the diode checker. They are so cheap it may not hurt to swap them but it seems like something may have gone and took the output xistors with them so a good thing would be to use a lightbulb limiter when bringing it up and a variac to assure everything is coming up ok. I like to monitor the +/- rails while bringing it up to see that nothing else is shorted bringing them down. If you see asymetrical voltages you can almost be assured something is yanking on one side. There can also be a threshold to where they don't show full potential until they reach a certain point which is where the lightbulb limiter tester really comes in handy.

[Enzo]

Clearly R1,R2 are not related to the output circuits and so were not stressed by it. Those are dropping resistors for the zeners, they naturally run hot. They are probably fine, but if you feel more comfortable changing them, they are also cheap.

Think about two power transistors in parallel. Each channel has two such pairs. Of one of them shorts, then the other one gets no current - the bad one shunts all the current past it. SO as a rule, it is under less stress than normal operation. The remaining outputs are probably fine.

Make sure all the power sesistors are intact - a shorted transistor can burn out its ballast resistor. I would replace the drivers, Q200,Q213 - the 5331 and 5332 types. Make sure R203,204, AND R230 are intact.