Peavey 5150 combo pre lead low volume

[pontiacpete]

I have a strange problem with this amp. The rhythm channel works fine, but when I switch the channel to the lead the volume drops to almost nothing. And when I switch back to the rhythm channel that too has an extreme volume drop, almost nothing. If I pull off the connector to the small, return./send foot pedal PCB and put it back on the volume to the rhythm channel returns.
Anyone every experience this problem?
Thanks in advance.
pete

[Enzo]

I would suspect the muting circuit is latching up.

Try removing Q1 from the board and see if the problem disappears.

If I am correct, whenever you change channels, the mute pulses to prevent loud noises during the switching process. When you start things are OK, but once you change a channel, that pukses the mute, which may not be able to recover.

[pontiacpete]

Thanks Enzo, the removing of Q1 worked. Should I leave it out or is this a sign of another problem?

Also, now I'm finding that the reverb isn't working. I pulled the RCA connectors off the tank and when touching the ends there is no popping sound.

[Enzo]

Look on the schematic - there is another muting FET on the reverb. Without looking, is it Q4? Remove that. If I recall, both of those run on the same control line, so if one shorts they both are affected. It may be that Q1 was OK, but being held muted by a defective Q4. They are cheap, just replace both.

And if you get no loud pops or sqwuaks when you switch channels, then you could just leave them out.

[pontiacpete]

I replaced both of the J174, Q4 and Q1 before I started this thread, so I think they were both good, but the removing of Q1 brought the lead channel back. Removing Q4 didn't cure the reverb problem. I'm thinking that Q7 might be bad. With an in circuit diode test it's showing it's shorted, but I don't have a replacement, a J231. As far as the tank goes, it might be bad but with a resistance check, I get continuity on each of the transducers,,, I realize it still could be bad. I hope to get one tues.

Now I notice another problem, both 'post' pots, lead and clean have DC on the wipers. I changed C29 and no change there. Is the absence Q1 the cause of this?

OK, I'm getting somewhere. I checked the CR1-4 with the diode tester and they tested good. Then I clipped my DVM to CR2 and turned the one of the post pots that had the DC on it and noticed it going up to a bit over a volt. Changed all 4 diodes and no more DC on the pots.

Now the reverb

[pontiacpete]

This amp is really confusing. Now it's back to no volume , 2 days later. The only thing I did was take out Q4 again to see it that would get the reverb working like was suggested and now no volume.
It's hard to believe that pulling Q4 would put me back a square one. Hmmmm.

[Enzo]

The Q1/Q4 thing is a pplace to start. Many times that is all it needs, but sometimes the cancer has spread.

If they were working, or at least all the rest of the amp were working, then pulling Q1 and Q4 should have NO effect on the amp other than the loss of that momentary mute. They both operate from the CLAMP signal so their gates are wired together, so they can intereact. A leaky one can affect the other.

With them removed, make the amp work 100% before returning them. If you used a replacement part, PLEASE make sure hte leads are right. Take an ohm meter to the leads. With the part all by itself, two of the leads will have a low resistance between them, like 100-200 ohms maybe. The third pin will seem isolated. The isolated one is the gate. I ALWAYS check to make sure, since there is no standard for JFET pinout. That gate should act like a diode to the other two.

At each of those JFETs see the pair of diodes to the +/-15v rails? Make sure none of those has shorted or gone leaky. For example CR1 CR2 are by Q1. If one of those goes leaky, then they put DC on the post controls? Is there DC on R28? And do similar checks over by Q4.

Now we know JFETs must be turned OFF by a voltage at their gate. so check for gate voltage, if it is missing, then the Q is just doing its job of shunting off signal.

Look at the CLAMP circuit. The CLAMP line is held up by R55, but a leaky triac CR9 could ground that off. So could a leaky Q12. SO if there is no gate voltage on the JFETs then we find out why over in the CLAMP.

And lesson to learn, and we all do, is just because you pulled Q4 and the symptom changed doesn;t mean Q4 is involved. The act of removing the board and changing the parts, wherever they were, can cause a poor connection somewher else to fail. SO it is possible that the issue was not Q4 itself, but the act of changing it.

[pontiacpete]

Here are some voltages

R55 on one end has 22vdc and on the other 20vdc
R28 has 0vdc on both ends
CR1 thru 4 have 0vdc on the ends closest to the FETs and on the other there is +/-23vdc

I have been following the other recent thread with similar issues so I checked J8 and there's 9.2vdc

Does this mean CR9 and/or Q12 are leaky?

[Enzo]

If you have 9v on J8, that explains no reverb, but not the whole amp going quiet.

You have no leaky DC through those diodes, good. CR9 and Q12 are not bad because they have not grounded off the CLAMP voltage. I don't know if the CLAMP voltage is making it all the way to the JFET gates though, check.


Does the amp function OK other than reverb with Q1 and Q4 removed?

You need to get the reverb drive back to normal. I'd be checking the four transistors rthere, and CR8 CR11. And the two 47 ohm resistors.

Once the amp consistently works in all ways WITHOUT the two JFETs, THEN we verify there is voltage where the JFET gates would be. If there is, then we verify which pin is the gate on the JFETs themselves and try installing a couple with the gate legs in teh gate voltage holes on the board.

[pontiacpete]

Thanks for the help, Enzo
There is 19.8vdc at the gates of Q1 and Q4.
I'm getting sound out of the send and return jack but not out of the main input.

I did some in circuit tests of the transistors in the reverb circuit and it appears Q6 is shorted. Might have to wait a few days for that one.

[pontiacpete]

finally getting back to this. I changed Q6.
I found that R28 measures as short but when pulled it's 2.2M.

[pontiacpete]

Set up the pre amp board this morning so I can run it outside the chassis with a lot wires in place of the 8 prong connector cable. Dont' know why I waited so long to do it this way. Now I'm able to get readings off of the preamp tubes and scope the board.
Sure enough one of the small connecting jumper wires had snapped from taking the board in and out of the chassis.
Fixed it so now I have sound but something weird is going on, the post and rhythm channels are working BUT when in the post ch mode the rhythm volume has control of the volume. Strange.
I cleaned the board up because of a soda (or something) spilled on it and changed K1 because there was so much gunk underneath it. Perhaps I should put the old one back in.
I can't see how the post vol control is now being diverted to the rhythm vol control.

As for the reverb, after changing Q6, J8 has -3.4v, J9-11 have 1 to 2vdc. Not sure if the reverb works . I would think there should not be any voltage on those pins.

Any ideas? Anyone
thanks

[pontiacpete]

I put Q1 and 4 in and when switching channels the lead ch volume is very low, then when switching back to rhythm ch that goes very to a very low volume. Power on and off quickly brings rhythm ch back.
So I decided to leave Q1 and Q4 out until I can get it working

[Enzo]

Good plan. When that happens, you need to check what is going on at the gates of those JFETs. If the turn-off voltage disappears, they will remain ON.

Where is R28?

[pontiacpete]

The gates have 20vdc on them.

R28 is a 2.2M connected to ground on one side and the other is to the relay K1B and CR1 and 2, it has 1.4vdc on both ends/ which doesn't make sense since one side is ground.