Marshall JCM2000 DSL100 Reverb Squeal

[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

[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]

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]

Are you on the second unit now, or the original unit?

I did verify I had working Reverb on the second DSL100 that I borrowed from our Rental Depot. Still mystified not being able to get signal thru IC2 to appear on the output of the amp, as we can always do on Fender Amps via Reverb Return circuit. Separate issue.

[nevetslab]

Um, any chance the reverb return IC2 is turned to the off position by TR1?

NO....IC 2 wasn't being turned off by TR1, though I did get that alternate action with the foot pedal when plugged in/engaged. I saw solid signal out of IC2 this morning.

[g1]

Are you on the second unit now, or the original unit?

[Enzo]

Um, any chance the reverb return IC2 is turned to the off position by TR1?

[nevetslab]

Looking thru the maze of CONN connector cables that's enough to drive you nuts without an accurate guide to lead you correctly.....I see the Source to the Reverb Board JCM2-63-00 is CONN 8. That comes from the plate of V2 Pin 1 (4th tube stage), decoupled via 22nF C23, fed via 220k R37, thru jumper R43 to CONN 8. The Return from the Reverb board CONN 2 is sent to CONN 8 on the Preamp PCB JCM2-61-00. That feeds both REV A and REV B pots and on to IC1. Output of IC1 is sent back to the Tube PCB via CONN 9. It arrives to the Tube PCB JCM2-60-00 at CONN 7. It is fed to the Grid of the next tube stage V3 Pin 7 via R42 2.2k & R41 4.7k. It is summed with the signal from the plate of V2 pin 1 & it's circuit using a 100k R39.

What I'm reading between CONN 8 and CONN 7 on this tube board is 20k. I see a 100k resistor, along with the 2.2k and what looks more like 47k (R41) but that might just be pigment of the ink.

So, the problem of this horrid screech and howling may be the oscillation caused by what I'm reading on the tube board. I'm about to attempt unsoldering R39. I'd rather NOT have to gut the chassis to extract the tube board to do that right. Might be better to just cut R39, then tack-solder a known real 100k resistor. Maybe just cut one end, and measure again to see if the reading changes?

So much for my earlier statement that the Tube board probably isn't involved. Horse pucky.

[nevetslab]

OK...new day, and everything works. I hope. I stopped off at our Rental Depot on the way in, fetched an assumed-working Marshall JCM2000 DSL 100...one of our older units from the Asset code. Brought it into the shop, set it up, removed the rear grille panel, reached in and unplugged the RED Reverb Return Cable and plugged in my Burst Pink Noise source cable. Powered it up, and within a few moments, noticed the middle left power tube wasn't glowing. Hmmm....Powered back down, swapped left with middle left, and the problem didn't move. What wasn't glowing now is. Heater fuse, no doubt. AND, further justification to resume PREVENTATIVE MAINTENANCE on everything on the shelves! Ready for rental service, indeed!

Anyway, Switched out of S/B, tipped the cabinet up enough to see the control panel and dialed in what I had on the other amp, then turned up the Reverb Level. Nothing. Turned the source all the way up (source is patched into HP 353B Patch Panel, which has a step attenuator and an output xfmr), dialed up 0dB Atten. Still no signal....well, very faintly in the background with the Reverb level turned up full.

OK....WHY wouldn't I be able to get test signal thru this path, when we can get the very low level Reverb Tank signal thru this first gain stage, then pass it along to the next one, which follows the FP Reverb level control. Oh, I do love a good mystery.

[g1]

And issue 6 is in post #12

[Enzo]

Oh crap, sorry. This system hates most of my files anymore. Try this

[nevetslab]

Just for reference here is issue 5. The zeners are gone and the input circuit is a bit different

I can't open this file for some reason. I get an "invalid file specification" message

[Enzo]

Just for reference here is issue 5. The zeners are gone and the input circuit is a bit different

[nevetslab]

Your schematic says R2 is 47 ohms. Mine says it is 22 ohms. Yours is issue 4 mine is issue 5.

Of course those are the schematics. The PCB had 47 ohms in it. Interesting that we now have 150 ohms, 47 ohms and 22 ohms as the candidates for the value. I haven't yet checked to see that I can pass signal thru the Tank drive circuit. (though I was getting burst pink noise passing thru up to where it began shrieking & howling). While I didn't get signal appearing thru the amp when I injected signal directly into the Reverb return amp's RED RCA jack, I didn't check to see if there was signal coming out of the M5210 circuit (now using JRC equivalent). Hadn't thought about the possible cracks in the PCB, nor issues with the wire passing thru the ferrite beads on the input to the Reverb board. More fun for tomorrow.

[Enzo]

Your schematic says R2 is 47 ohms. Mine says it is 22 ohms. Yours is issue 4 mine is issue 5.