Just found a weird one, might be useful to you

[R.G.]

I just had a first. I was finishing a SS amp repair and it worked, but just looked a little odd on the scope. The positive-going output seemed a little compressed compared to the other polarity. The output had paralleled bipolars, and one of the top-side pair was hotter than the other. In fact, it got distinctly HOT. But nothing was really burning up, and the signal was coming out, even at 100W into 2 ohms.

Pulled the hot one out. It was fine.
Tested wiring to it. OK.
Tested base drive - just fine.
I cleaned up the socket contacts, and spend a half-hour searching for my heat sink goo, then put it back in. Still fine.

It finally got through to me to wonder not why this one was hot, but why the other parallel device was cold. I probed the cold one and found that its emitter was always about half a volt below the collector, up at about 40Vdc. Shorted?

Nope, can't be. The output is rising just fine, and the fuses are not exploded.

Hah! Busted emitter wire.

No, the wires are just fine.

I finally tried the meter on the 0.33 ohm emitter resistor. It read open on the 20M scale. This was a 5W wirewound resistor with no visible heat damage at all. It looked like it had never overheated. Apparently it just opened.

Never saw that one before. I've seen overheated - nay, nearly exploded! - emitter resistors and other cement wirewounds, but never just quietly open.

The lesson? Trust nothing to be OK until you personally meter it.

[Jazz P Bass]

I think the 'Enzoizm" "never think of a reason not to check something" could apply here.
Good catch 'why is the one cold?'

[drewl]

I just always make it a point to check all the emitter power resistors lately.
Good job, and remember where you put the thermal compound this time.

[Gtr_tech]

I've seen this quite often in SS pwr amps. I check them all on the bank of output devices that caused the amp to fail without question. I think a high current in a short amt of time is what causes the resistor to burn open without visible signs of overheating. It never gets a chance to heat up enough to discolor the resistor or the board under it if its mounted close enough.

[Enzo]

I'd have to say it is unusual for an emitter ballast to go open without its transistor going first, but I do see it.

But I agree with Gtr, I always just run down the row of resistors and check them all. They can pop like fuses, but they can also die mechanically from vibration or other shock.

[R.G.]

It was completely new to me -quite likely because I don't repair anywhere near the number of amps you guys see. Experience *always* counts.

[Enzo]

Tell that to Nixon-Lodge.


Hey, everything is new the first time you see it. That's what I like about this gig: 50+ years of soldering and I STILL learn something new every day. Seriously.

[woodyc]

Another way to do it is to put a diff probe on all those resistors while the amp is on so you can actually see the magnitude and phase of the current blips. The stock clips on tek diff probes make it super easy. I try to avoid measuring resistance in situ when possible (since you usuallly have to discharge caps and think through what you're actually measuring.)