Solving the case of the missing output transistors

[Enzo]

Well, check the main rectifier diodes for shorts too.

[Ptron]

I think the it was just one of the standoffs that had swiveled out of position and was touching one of the rail traces. All seems good now with the new drivers.

I now have a seemingly unrelated issue with the reverb. It was fizzling when I first tried it, now it's gone. Using a secondary amp for signal tracing, I get a good signal on the input of the switching IC (U19, pin 7) but nothing on the out (pins 4&3). The reverb should be on with no footswitch correct? There is 15v on pin 2&7 and -15 on pin 5. Do these switches go bad? I'm not sure what part it is. The schem just says 604. I haven't pulled the board to read the part directly.

[Enzo]

The TL604 is an analog switch IC, but TI quit making them YEARS ago. PV, bless their hearts, madde a little workaround for it - a small board with a couple little ICs, and 8 long legs so you can solder it in place of the 604. Call Peavey and ask for the replacement for the TL604.

But really, just pull the chip and stuff a small jumper wire in its place - from the output of the previous circuit over to wherever it was headed. If the chip is socketed, then the little jumper can be shoved right into the IC socket.

Now the reverb will be always on like so many amps already are.

[Ptron]

Thanks Enzo, that certainly sounds like the thing to do.

[Ptron]

Well, I was saying all was good with the new drivers but what I hadn't done was reconnect the diodes CR4 & CR7. Anyway it turns out Q5 had a really low C to E resistance. I dropped a 2N3904 and it seems to be good now. Hope that's a suitable replacement.

I bypassed U19 with a jumper but left it in the circuit. It seems to work fine. Anyone see this as a problem?

[Enzo]

If I replace an IC with a jumpoer, I get rid of the IC, but if it works, leave it...

The limit xstrs draw base current off the emitter resostors of the outputs. 10 amps through a half ohm resistor is only 5 volts. The thing normally works on the base of the driver, so look into whatever max volts might appear there. Certainly no current is flowing. 3904 sounds OK.