Marshall JCM 900 4100, problem with the reverb

[chetbango]

Greetings, I have tried the search function on this site and have searched the web as well and I have not found an answer to my problem.

I recently acquired a 1993 Marshall JCM 900 4100 head from ebay. The PCB's are:
Preamp - http://www.drtube.com/schematics/mar...d0189-iss7.gif
Power - http://www.drtube.com/schematics/mar...0192-iss10.gif

The Output tubes are newer looking JJ EL34C's and the preamp tubes are all USA made 12AX7's. The caps appear to all be original, with '93 dates. The amp sounds fine, other than fact than the reverb only functions intermittently in both channels. The amp always plays, sounds quite nice really, it is only the reverb effect itself that doesn't always work. I removed the reverb tank and have found if I drop the reverb tank from a short distance (1-2 inches) or smack the tank, the reverb effect starts to work, then fades over a fairly short period.

The head had some issues at first, the pots needed cleaning and some previous owner had covered the blown fuses with foil to instead of replacing them. Everything worked OK except the reverb, even after I:
1) Sprayed and Cleaned the pots, they are no longer scratchy.
2) Replaced the fuses with correct value fuses (glad they didn't blow).
3) Re-tensioned and cleaned the preamp tube sockets.
4) Replaced the V2 preamp tube that controls the reverb.
5) Checked all the wiring and solder connections to/from and in the reverb tank.
6)I have cleaned the contacts and innards of the reverb tank, I have retensioned and cleaned the RCA connections to the reverb tank.

I don't know a ton about amp theory, but I wonder if there is a cap or resistor that must reach a certain value to allow the effect to sound. Could the smack raise the current, allowing it to "open".


Any ideas on what might cause this problem and any idea why hitting the reverb tank would 'fix' it? Any ideas on how to diagnose the problem, as far as checking particular circuits? I know enough about electronics to properly drain the caps, do some easy soldering, check circuits and use a multi-meter.

Thanks in advance for any ideas...

[g1]

Have you looked inside the tank? Sometimes the little wires (between the connectors and transformers in the tank) get broken inside their insulation. Or it could be the tank itself. Otherwise it could be a poor solder connection somewhere in the reverb circuitry.

[chetbango]

Have you looked inside the tank? Sometimes the little wires (between the connectors and transformers in the tank) get broken inside their insulation. Or it could be the tank itself. Otherwise it could be a poor solder connection somewhere in the reverb circuitry.

Well, I put an ohmmeter on the input and output jacks of the tank, the readinsg were both inconsistent and not in the parameters of what the tank's spec is. So I ordered a new accutronics tank for 16 bucks. It was cheap enough I figure it was the best move rather than trying to troubleshoot the old tank. Thanks for the reply, I'll let you know if the new tank does the trick.

[Enzo]

The resistancxe your meter measures is the DC resistance of the coil at each end. That is NOT the same as the impedance. The DC resistance will be much lower than the impedance. The hand meter test is bsically to see if the coil is openj or not. It will either be open or it will be OK. It will not be of "wrong" value. If neither end of your reverb pan is open, then it is probably OK.

From your description, my first suspects would be the cables that connect the pan to the amp chassis. I'd pull the litle plugs out, then push them back in and rotate them a little to rub up a fresh contact surface. And remember the cables have connectors at both ends.

[chetbango]

The resistancxe your meter measures is the DC resistance of the coil at each end. That is NOT the same as the impedance. The DC resistance will be much lower than the impedance. The hand meter test is bsically to see if the coil is openj or not. It will either be open or it will be OK. It will not be of "wrong" value. If neither end of your reverb pan is open, then it is probably OK.

From your description, my first suspects would be the cables that connect the pan to the amp chassis. I'd pull the litle plugs out, then push them back in and rotate them a little to rub up a fresh contact surface. And remember the cables have connectors at both ends.

Thanks for the info on the tank. It definitely is not an open circuit. I cleaned and retentioned the RCA jacks and plugs on the tank end. The Chassis end is soldered directly to the board. The solder looks solid.

[Jazz P Bass]

So I guess it is time to narrow down the problem.
Is it a 'send' or a 'return' malfunction.
The return is pretty straightforward as all that is needed to tough the tip of the return wire.
It should make the amp hum.
The send circuit can be tested from V2A onwards.
Do you have signal at the junction of R20 & R23?
Do you have an opamp output signal at IC2, pin 6?
Do you have output at IC3, pin 1 & 7?

[chetbango]

So I guess it is time to narrow down the problem.
Is it a 'send' or a 'return' malfunction.
The return is pretty straightforward as all that is needed to tough the tip of the return wire.
It should make the amp hum.
The send circuit can be tested from V2A onwards.
Do you have signal at the junction of R20 & R23?
Do you have an opamp output signal at IC2, pin 6?
Do you have output at IC3, pin 1 & 7?

Awesome advice, exactly what I was looking for. Thanks for the specifics detail on what to test. Unfortunately, your advice wont help me. Fortunately it is because I went out and spent all of 16 bucks on a new accutonics tank. It arrived today, I hooked it up and now the head sounds wonderful! It actually sounds pretty lush for not being a fender. I am so stoked, the amp roars and it has a nice wet reverb now!
Tthanks to everyone for their help, I am going to go play now...so stoked!