Blues Deluxe, signal loss between preamp and power amp

[carlg]

I've ended up with a mid 90s Blues Deluxe that's losing signal between the pre amp and the power amp and I'm having trouble following what's going on in that area of the circuit. Is anybody here familiar with them?

The signal disappears on me at R20 - on the preamp side I've got good signal, on the other almost nothing. Unfortunately there's a lot connected to that bad side. There's a master volume that's only active in drive mode, I've ruled that out because the problem exists in either mode. There's a buffered "preamp out", I've disconnected that from the preamp to rule it out too. Then it's got the mother of all switching jacks here labelled "power amp in", eight pins on this jack. A couple op-amps are connected here too. One drives/recovers the reverb, one I'm not sure about (U1b), but I suspect it's to buffer a signal someone might inject with the power amp in jack. It doesn't look like it's supposed to be involved in normal operation. There'a also a jfet connected to this point from the channel switching circuitry, can't tell what it does either (Q1) but it's directly connected to the signal path so it's a suspect.

The previous owner reported something about intermittent reverb before it went quiet so these op-amps might be suspect. Not sure if the reverb driver going bad could pull the signal down this much though. My other suspects are the mystery jfet and the big switching jack. Any help is much appreciated...

Schematic

[Enzo]

REmove Q1, does the amp wake up?


Q1 is a mute. Its job is to very briefly mute the amp when you switch channels. Thos prevents big pops and noises when channels switch. If it fails, or the gate drive fails, the amp remains muted. If removing it fixes the amp, replace Q1.

Well, first, check the gate voltage on Q1.

For that matter, with the amp running, measure resistance to ground from the dead end of R20 - the point Q1 would be grounding. See if it is something low like 200 ohms or less.

[carlg]

A mute! Well that sure would do it. From the bad end of R20 it was 33 ohms to ground, and I see zero volts on the gate of Q1, -0.4 on the other side of the diode C1 on the gate. So I removed Q1 and we're back in business. Is this a failure of Q1 or a failure of something else to drive it?

Thanks Enzo, that saved me a lot of fuss ruling out everything bit by bit. I remember your name from the old ampage. Good to see some of the same guys are still around.

[Enzo]

Q1 is a JFET. By their nature, JFETs are ON until turned OFF by a voltage at the gate. With zero volts on the gate, Q! can;t turn off, so it stays in mute.

Q1 could be defective, and shorted, so it is grounding off the gate signal. WIth the Q1 removed, is there now voltage on the hole for the gate leg? Or the gate drive could be missing. In that case, look back at teh channel switching/footswitch circuits. I don;t have the print open at the monet, but ther is a point where at the output of an op amp it says somethinglike +14v for onr channrl adn -14v for the other. From that point there is a cap over to the input of another op amp, which drives Q1 gate. So check the impit pin for reference voltage and output pin for voltage. The op amp could be working but the voltage never gets as far as teh JFETs. In any case, it should be straighforward troubleshooting.


And if the amp switches channel without pops, crackles and other racket, just leave Q1 out.

[carlg]

Sorry I didn't mean to be lazy there I was just excited it was working again and posted befor I'd really thought it through. Q1 is definitely dead. There is a pretty loud pop the first time you switch channels after power up. After that it's very minor. I don't have a replacement handy so I've given the amp back for now, I'll replace Q1 next time I see it. Thanks again.