Marshall JCM2000 DSL100 Reverb Squeal

[nevetslab]

Well, I tried to unsolder the resistor R39, but while I did get it lifted up (to find it IS measuring 100k now), the top solder pad came up with it, and I lost the plate-thru to the bottom pad.

I have a damaged JCM2-60-00 that is an earlier version, NO IC1 on this.I do see R39, R40 and R41 solder pads connected on the back of the board. I moved the lead of the 100k to tack-solder to that of R40/R41 next to it, as that would restore the signal path. When I powered this up this time, now all I get on the output of the amp is HF leakage. I see no reason for that, looking at the PCB in the chassis, and the older damaged board (flame-out at the Heater circuit on end of the PCB). Kept it for parts, and first time I've had to look at it.

I have zero ohm continuity from the 220k resistor R37 to R39 100k, and with the other end tack-soldered to R40, I have zero ohm continuity from R39, R40 and R41 AND to Pin 7 of V3. So, the original circuit is intact.

And yet, all I'm getting for output is HF leakage. I'm so happy.

[nevetslab]

I've pulled the chassis apart, removed the tube PCB. Repaired the damaged solder pad, which only lost the top-side pad and plate thru. Unsoldered the lead from R39 and ran it back thru it's original hole, soldered it back into place. Circuit remains as it was, with my tack-soldering that lead to R40 and R41. All else looked fine, so I put it back together. Finally assembled, so connected the tank, the speaker, AC Mains and my input signal.

Powered up. Reset the controls as I had when it was working (and shreeking & feeding back). I have signal again, on both Ch A and Ch B. So, I boldly turned the Reverb volume up, and instant shreeking and howling. I guess that's how THIS model is supposed to work. I sure don't see what's wrong. I didn't have any signal before when I lifted out the lead of R39, and tack-soldering it to R40 and R41, which is still the circuit I have....I at least DIDN'T have shreeking and howling. Either-Or appears to be the choice?

I have to pull the other older DSL 100 apart to fix the open heater fuse, so I guess I'll see why it works and this one doesn't. Watch....it will be the older PCB that doesn't use the M5210 switching op amp. Sigh...........

[nevetslab]

I just got thru creating an Interconnect Cable guide for the JCM 2000 DSL100, using the one for the TSL model I made a few years ago. At first, I thought I might have an issue with the support circuitry that's on the tube PCB JCM2-60-02. that has three MPS A13's and lines going to TR1 and TR3, both J174 P-Ch FET's. but then, after I got this interconnect list created and then found a couple errors, I went thru the list and found two of the connecting cables in the wrong place, one being in this Reverb Send/Return circuit.

I made the corrections, and just powered it back up, after writing a first draft of this post. Had to remove the interconnect list and revise that, and now, I have the amp powered back up and the REVERB is working as it should. Two mis-connected cables causing all this shrieking and howling feedback!

Odd that there was no mention of this problem on the service note taped to the road case for the amp head! At least I now have a correct inter-connect cable list created out of the effort, and it's attached below.

jcm26002_ISS5.pdf

jcm26102_ISS4.pdf

jcm26202_ISS2.pdf

jcm26302_ISS5_Reverb PCB.pdf

DSL-100 Amp Head Interconnect Cables List .pdf