Need Help Troubleshooting Solid State Bass Amp

[ckeene]

I inherited a broken Ampeg SVT 400T bass amp. It's one of the transistor models from the late 80s. The preamp section works ok and signal passes fine through the direct outs. The problem is there's a 10A fast-acting fuse in the power section that keeps blowing. I removed all 12 of the power transistors, powered it up and the fuse stayed intact.

I guess my next steps are to get a schematic (already on order), and to individually test the transistors. (The transistors seem to be 15022 NPNs and 15023 PNPs)

It looks like I can build a tester pretty easily using these instructions:
http://www.kpsec.freeuk.com/components/tran.htm

Is it a good idea to just replace every transistor as a matter of course? Are there any other areas in the amp I should look at as potential culprits?

Thanks!

-Chris

[Nate Dort]

The quickest way to check power output transistors is for internal shorts across any of the pins. There shouldn't be any direct continuity across any combination of pins you probe.

[ckeene]

The quickest way to check power output transistors is for internal shorts across any of the pins. There shouldn't be any direct continuity across any combination of pins you probe.

Ok, I have two that are shorting out (a PNP and an NPN) across all combinations of leads I test.

So that's a crude way to test, right? I assume once I get the schematic I'll want to go in and check out supply and bias voltages before just popping the transistors back in and powering it up?

[Nate Dort]

You definitley don't want to pop new ones back in if something else in the circuit is going to cause them to blow again. Those things are a few bucks each. I'm guessing the two that are broken were paired together in a push-pull configuration. One probably went out, causing the other to draw too much current, overheat, and go out. Why did the first one blow? That's the question. I've seen them spontaneously blow for no reason (old age probably) and all that was required was a removal and replacement. I've also seen out-of-spec resistors cause excessive current draw, which causes overheating. It would be really handy if the schematic you get shows typical voltages at various points.

The quick and dirty continuity test at least tells you how many to order, rather than ordering enough to replace all of them. I'd get an extra or two of each, just in case.

[RodC]

Generaly speaking, and adding to what Nate said, they mostly short C - E. (I can only rember finding 2 power transisters shorted B - E or B - C)

Here's where I like a good ole Analog meter, much faster to check (Simpson 260 Hell yeah) if you meter B-C one way should yeild around 15 ohms, the other close to infinity. Same way with B - E. If you know the layout, I always shoot for C - E, but more often I have to deduce the B by trial and error. After you have done it for 25+ years its pretty fast.

EDIT: if you only have a digital meter (yuk) put it on diode check or it may not provide enough test voltage to trigger current flow in a good transistor. We're all about analog here right??

Most of the time they are taken out by abusive overdriving and or a bad/shorted speaker.

Most transistors can be checked for shorts in the circuit, if you find a short, desolder it to confirm. (Transformers can give you a false short for instance)

Check any other smaller transisters in the same circuit, one of them shorted out that drives the outputs can take them out.

[ckeene]

Ok thanks for the advice on this. I've worked on tube amps enough to generally troubleshoot, but I don't have any tech experience with transistor amps.