Marshall JCM900/4100 PS and channel switching problem

[jaz10]

Hi folks - thanks in advance for any help.

Here's my dilemna. This is a JCM900/4100 100 watt dual reverb head. The power supply in the preamp section isn't putting out +/- 15vdc and I'd think that's why the channel switching isn't working. I'm getting +/- 29vdc on one side of R33 and R34, but on the other side, only +/- 2.9vdc where it SHOULD be +/- 15vdc. Those resistors seem to check out fine, as well as BR1, C31, C32, C21, C22, D3 and D4 -- all in that ps area.

I'm stumped -- If anyone has ANY ideas I sure would appreciate any tips you've got.

Thanks in advance.

[52 Bill]

If the voltages drop across R33 and R34 you either have two bad resistors (not likely but possible) or bad 2 Zeners or 2 bad filter caps or a shorted IC that is drawing too much current and pulling the voltages down.

I'd check all ICs for heat and or voltages on pins where there shouldn't be any.

[jaz10]

thanks

Bill -- Thanks for the reply. I've had a hard time finding anyone that knew anything about this.

I've tried replacing all the above - everything in that ps area, Br1, C31, C32, C21, C22, D3, D4, and R33, R34. Still No +/- 15vdc, even lifting one side of R33 and R34. I also suspect something else somewhere down the circuit is pulling it down, but can't seem to pin down where to try, or even where to lift to check for voltage midstream - it's NOT there when you lift one side of R33 and R34. I saw a post somewhere that I can't find, where someone else had a voltage issue there, and it ended up being what he called a 'coupling' cap, but didn't say where it was.

Any ideas where to look for a dc 'drain' or short related to that voltage. I was hoping the problem was a more common fix.

Thanks again

Jim

[52 Bill]

Jim:
There are 8 ICs in that pre-amp. Most are M5201 switch chips, some are dual op amps. Any of them are suspect. Check for dc voltage on inputs or outputs.

The reverb driver section will be stressed the highest, so I'd start there with IC3.

Are you sure you installed Zeners D3 and D4 correctly?

[jaz10]

Bill - Double checked D3 and D4 -- zeners installed OK - They seem to check OK, but I don't know if they are the EXACT zeners that came out of it. I also did a 'finger' test for heat on the MS5201's - nothing abnormal. Haven't checked the 5201's yet for voltage where it should NOT be, but it looks like the only place that should NOT have dc voltage is pins 1 and 8, but the schematic I have is blurred.

Hey let me ask you this -- can you think of a spot to 'lift' somewhere after the ps, that might at least show me +/- 15vdc on R33 and R34, and isolate the 'problem area' PAST the ps itself.

thanks again

[jaz10]

All the 5201's have 2vdc on pin 1 -- The output is pin 5, not 8 I figured out - only mv there.

[52 Bill]

All the 5201's have 2vdc on pin 1 -- The output is pin 5, not 8 I figured out - only mv there.

Pin 1 is the control voltage for the switching, supplied from the switching circuit around TR1.

Inputs are on pins 2 & 3 for section A and 6 & 7 for section B.
Output as you noted is pin 5.

+ and - voltages to pins 8 and 4.

[jaz10]

Sounds like I need to check 2,3,6 & 7 for DC drain on the 5201's, because I didn't see much coming out of pin 5 on any of them.

[Amp Kat]

I've seen the footswitch jack fail and cause these symptoms. Make sure it is grounded properly or just change it if you can to be sure.

[jaz10]

I got some weird dc on ic2 and ic3. Pulled ic3 - no change. Pulled ic2 -- got some good dc on r33 and r34 now -- it's +23vdc and -18vdc -- and that's not right, but it's better than 2.9+/- vdc --- and ic2 and ic3 are out of circuit -- the switch light kicked in also. I'm gonna try changing both those chips and see what happens.

[jaz10]

Bill - If you are still out there -- I replaced IC2 and IC3 - I've now got +17 and -14vdc -- still not right but closer (was an even +/- 2.9vdc) -- BUT now there's also no output out of either channel A or B - just a bad hum at '0' with no preamp control at all -- although the channel switch light did kick in. I had channel A when I started - just no B. Now the voltages are closer to where they should be -- but no output at all, except a bad hum ! Changed tubes - no change.

Any ideas ?

[jaz10]

I've seen the footswitch jack fail and cause these symptoms. Make sure it is grounded properly or just change it if you can to be sure.

AMP KAT - thanks - but it's not the jack -I had channel A, but no switching to B, the dc voltage was almost non-existent - the switch light didn't work when changing channels and B did not kick in. I changed ic 2 and ic3, now I've got closer voltage (+17 and -14vdc) - but no output at all, except a bad hum.

[Enzo]

You replaced a couple opamps and the power that had been dragged down came back up. Now whatever regulates the +/-15 seems to be missing on the + side. I know you replaced the zener - didn;t you? - but how is it allowing +17v?

Isolate the problem. Plug a signal into the front and connect the FX send to some other amp for a listen. is there signal there or is the hum all you get? Likewise, plug a signal into the FX return jack, does it come out the speaker? For that matter, rock the amp back and forth to crash the reverb springs - leave the reverb up half way - do you hear the loud reverb crash out the speaker? AT each point in the circuit, the signal is present or not, and at each point, a signal injected proceeds to the speaker or not. This is classic divide and conquer.

In any case if more or less +/-15v exists, enough to try to run the opamps, check the output pins of each opamp for large DC offset.

[jaz10]

ENZO - thanks for the input. I'm not near the amp right now -- but that's what I was going to do next. The following is from memory - I probably should have noted that I substituted 2 'general purpose' opamps in ic2 and ic3 (U794 I think) - because I didn't have handy any TL071's or the 1484 -- so that's another thing that may be screwing things up a bit -- If you know that substitution is OK -- please let me know that.

ALSO -- interesting to note -- that when I had ic2 and ic3 completely out of circuit, that's when I first got some good dc at R33 and R34 -- but it was uneven at that point as well -- with both ic's out of circuit (I think it was +22 and -17vdc at that point).

So if you could clarify the general purpose opamps as OK - then I can move forward on testing for input at those points and dc at the output of ic3 (but I think I already did that and got zero), and look closer at the zeners.

About the zeners -- both you and Bill have mentioned the D3 and D4 zeners. so I replaced them again just to play it safe - As far as I've always known - a zener is a zener - so unless I've got that wrong -- they should be OK -- The schematic called for a specific zener -- but I never knew there was any difference -- so maybe I'm wrong on that point and maybe I've got 2 different ones in there causing some kind of inbalance in the ps dc.

Anyway -- thanks again for the input. I'll be working on this again tomorrow.

[jaz10]

OK -- Today I changed the diodes at D3 and D4 again, and left the 794 general purpose opamps at ic2 and ic3 -- I now seem to have my +/-15vdc, with the channel switching apparently working. The dc drops a bit when I switch to channel B -- so I don't know if that's normal or not -- but I think the preamp is now OK, although I could be wrong -- here's why >>

I tested the signal from the efx send to another amp -- and the preamp now seems to be working (no master volume control - just Gain), channel switching, eq and all.

However -- I now still have the problem with a hum out the speaker output, with no input source coming through.

Getting closer -- any ideas where to go now.