Peavey Renown Amp, blowin' fuses

[mfreqmaster]

After spending some time searching the threads, I could not find a basic tutorial for checking power transistors so I decided to start this thread. Here is the situation:

This amp was blowing the main 5 amp fuse. After disconnecting the power to the power amp rectifier, I replaced the fuse and the amp stayed turned on. So I suppose the +15V/-15V supply is good, the problem is with the +42V/-42V supply. The bridge rectifier appears to be ok, at least with my checking in curcuit. I tested Q10,Q11&Q13. These have little or no resistance between the two pins (approx 1-2 ohms). I then tested Q6,Q7,&Q12. These had around 42 ohms resistance between the pins. Am I going in the right direction or am I missing something? Thanks in advance for your help.

[Enzo]

Sure.

Those power transistors have the two pins underneath, which are the emitter and base connections. The metal case itself is the collector of the transistor. Check for shorted between the case and each pin. If you get low resistance, it is shorted. You were checking between base and emitter. They should not be shorted together either, as your Q10 side was, but in the circuit, there is a 47 ohm resistor across them, so the 42 ohms you got seems OK.

But the 42 ohm guys still could have a collector to emitter short.

In the amp, these parts are wired in parallel, so if one is shorted, it will look like they all are. Just pull them all and test each out of circuit. You may find only one is bad. But they could also all be bad, so you check.

[mfreqmaster]

I finally got around to this thing. I unsoldered and pulled the power transistors out. They all appear to check out ok. The circuit board is still reading approx 48 ohms or so where I previously checked the base & emitter. I did check Q5 and found a dead short between (looking from the top down), pins 2 and 3. I am still trying to find a pinout online but my gut feeling is that this is not OK. So far this is the only transistor I have definitely found wrong. Any other suspected areas to look for in the schematic? And could Q5 being blown have caused the problem? Thanks again.

[R.G.]

Light bulb limiter.
Light bulb limiter.
Light bulb limiter.

Did I mention the Light bulb limiter?

[Gtr_tech]

Anyone *serious* about repairing SS amps really needs a transistor checker. Your meter can check the junctions, but it can't check for leakage current. A leaky transistor can cause the same failure modes as a shorted one....and yer meter will tell you its ok.

The B&K 520b is a good machine for go/no go tests. I use one every day in my shop. It doesn't display gain, but just for repairs we're not concerned with it anyway. They are cheap and plentiful on Ebay....usually around $50 or less.

[carbro]

peavy renown blowing fuses

Q5 and Q9 drive the parallel transistors if you see a short across the emitter and collector this will short the 42volt supply and blow fuses.