I have a Marshall JCM 900 100W tube amp head from 1990 that likely has an issue in its Reverb Drive solid state circuit.
JCM 900 Preamp Schematic from 1990
/\ The schematic of the JCM 900 preamp especially the "reverb drive" section referred to here.
Troubleshooting Progress
While the amp is on and feeding it a 1khz sine wave, an oscope successfully sees the sine wave up to and exiting the V2a preamp tube and can also be seen after the reverb section of the circuit following the dry path.
However, the sine signal is lost below the R23 resistor at the start of the reverb circuit (totally fine above it). Above it is a 400mv peak/peak sine wave. I would expect to see at least a 200mv sine wave below R23.
- While the amp is off/caps discharged, in circuit continuity and ohm checks using a DMM on every component in the reverb circuit pass/expected (cannot test the caps but none beep with continuity which is assumed good).
- While the amp is on, measuring DC below R23, the DMM is registering about -300mv.
- While the amp is on, measuring DC right of R28, left of R26, above C17, left of R27, right of R26 all measure roughly -14.25V.
This leads me to assume that "maybe" either R23 or C16 have issues and should be replaced, but since I am not a guitar amp tech guru, before I start desoldering components and adding even more variables/fail points to this issue, I am seeking some understanding and suggestions.
Questions
❔ Should I only desolder components, test them out of circuit and then get them back in as the only option to pursue?
❔ Can/should I see if R23 is the issue by temporarily jumpering another resistor of same/lower value on top of it in parallel?
❔ Can/should I see if C16 is the issue by temporarily jumpering another capacitor of same/lower value on top of it in parallel?
❔ Can/should I do either of the above by snipping one or both ends of the resistor or capacitor and then jumpering or "top soldering" the left over leads with new components and avoid temporarily having to disassemble the preamp board to under solder them?
❔ Is there a way to replicate this issue with a simulation? See my attempt at an LTSpice rendition of this part of the circuit below.
❔ Is it correct that if the reverb drive circuit was working I should be able to see a > 50mv sine wave through its components on a scope but doing so with DC coupling on the scope since the op amps have 12-15VDC on all of those lines?
❔ What affect does having the reverb tank being connected or not play into any values I would read?
❔ Would acquiring a newer/digital scope provide such a difference that I should seek that first?
Additional Info
Above is an overview of the LTSpice I attempted to create to provide a simulation of this circuit.
The reverb tank is not an exact replica as it has a much higher input impedance, the reverb pot is fixed R, and I took liberty excluding the channel switching IC7 M5201 and replacing that with a single TL701 (see the actual real link schematic above).
With this simulation, I achieve the expected results below and above R23 and above R20 in the dry area.
While the simulation does see -12V in the op amp areas of the drive circuit, it produces a very small and very distorted ac result which leads me to assume the simulation isn't great.
Thanks to Adrian Immler's post "spice reverb simulation?? This is now possible!" for providing a LTspice of a similar reverb tank.
For extensive additional background leading up to this point, please see this post:
Marshall JCM 900 Reverb Issues
I am aware that it is not ideal and borderline/crosses forum rules about not posting two threads about the same topic; however, when I posted to the valve/tube area I did so since a similar reverb thread was in the solid state area and I have a tube amp and other research implied that one of the more common faults with reverb were related to pre amp tubes, so made the assumption that group was appropriate and now see that forum is more specific to hi fi tube amps.