5150 combo with reverb problem

[Anti-Hero]

a friend brought over a 5150 combo with reverb built into it, The reverb unit looks to be working, i tested the ohms at each end the input was 60 the output was 200. When the amp is on you can hear the reverb on, but if you shake the amp you can hear the springs. I just checked the pot and its coming up at 5k, someone told me it should be 10k but i didn't think that was it. I also cleaned and checked the send return jacks to see if something was wrong there, but they seem fine to.

I just pulled out the pcb, and nothing looks wrong.

I am unfamiliar with this version of the 5150. Any help or direction would be greatly apprecitaed.

Thank you

[pecorporation]

5150 combo.pdf

Do you get those same resistance readings for the reverb tank through the lines to the tank?

[Anti-Hero]

yes i tested from the terminal at the PCB board, 60 & 200...

I'm wondering if it has something to do with a relay.

It just sounds like its off, on the input side

[pecorporation]

The next thing I'd check (or just good ahead and reflow) is the solder on the footswitch jack. Spray a little deoxit in there as well and work the jack.

Secondly, that jack controls the switching circuit for the amp and the reverb circuit. It engages Q4, a J174 FET, in a clamping circuit that mutes the input to the reverb pan. You should verify the voltage changes on the center pin of that FET when the reverb switch is engaged. If the voltage changes from a voltage to 0 volts, then the clamping circuit is working up to that FET. With the amp off, look for continuity across pins 1 and 3. If your meter beeps, I'd replace the FET.

[Enzo]

Q4 could be bad, shunting the drive signal, but I doubt the switching circuit that drives Q4 is bad. Q4 is driven by CLAMP, but so is Q1, dead across the preamp output. SO it would seeem to me that if CLAMP were screwed up, not only no reverb, but no anything.

CLAMP is just a momentary pulse that occurs at the moment of channel switching. Q4 does this brief mute. Q4 is not there to turn reverb drive off and on though.

The test for Q4 would be with power ON, measure resistance across it. If pins 1 and 3 are still shorted then, Q4 looks bad.