Peavey Combo 300

[goldtop5]

I have a Peavey Combo 300 bass amp that kicks the signal in and out when you hit a hard note on the bass guitar. Almost like it's going in to protect mode. Any ideas?

[The Dude]

Bad power amp in switching jack? Jumper preamp out to power amp in. Does that fix it?

[Enzo]

Loose speaker connection, either at the speaker or the amp end. Failing speaker - maybe broken tinsel wire. Intermitting cutout on a patch jack. Broken solder connection. And so on.

Play some music through the amp, and whack the top with your fist. Does that make it cut out? If so, something is loose.

[goldtop5]

I forgot to mention. I've connected a test speaker to the external speaker output and it did the same thing.

[Enzo]

Do the other tests. Does whacking it make it cut out? Whacking won't hurt it, it just helps expose loose connections.

Run your test signal into the power amp in jack. Does it still cut out?

And the other way, run a cord from the preamp out to some other amp. When the 300 cuts out, does the signal to the other amp also cutout or does it continue?

On the power amp board, in the middle near the speaker connections, there is an ugly brown paper coil. Wiggle it, is it loose at all?

The wires to the speaker plug onto molex pins on the power amp board, are those pins snug?

[KCman]

If Enzo's tests narrow it down to preamp, A dirty input Jack or bad solder joint on it could cause this. Plug into the other Jack and try to replicate the problem. (Unless booth jacks are bad, that would be my luck)
My order would be, power amp out test, preamp in test, then I would wack it (cause by then I would be a bit ticked, ha)

[goldtop5]

I've tested the amp while the chassis was out of the cabinet. Whacking the chassis does not cut the signal in and out. I've plugged in to the power amp input and it did the same exact thing. I've used contact cleaner on all the pots and jacks. I've checked all the solder connections and retentioned the all the molex connectors. It will play normally at low volume, but when you turn the volume up a few notches and strike a hard note on the bass guitar, that's when the signal dropout occurs.

[Enzo]

OK, so you have isolated the problem to the power amp or speaker. And it is not mechanically sensitive

Does it do this when connected to a different speaker?


Apply a steady signal to the power amp and probe all the parts with a wooden chopstick or other insulated thing. Any response? You check my big coil yet?

On the PA board - and verify yours in indeed the 400BH - there is one small 8-leg IC, U2. Remove that, noting which way it sits. That have an effect?


CR13 is the dual diode thermal compensation. It is on the power transistor board, but connects via a two pin molex back to the main 400BH board. Playy a steady signal into the amp and monitor it on a scope or AC voltmeter at least, at that dual diode. I'd probe the molex pins myself, but collector of Q9 would do. You should see the output signal there. I prefer music to a sine wave for this, but if turning up a sine wave triggers the symptom, so be it. Turn up the signal until it faults. Did the signal go away at the dual diode too?