jcm 900 4100 help!!!!!!!!

[ALEXZANDEREPAIR]

My Jcm900 is in the dumps.Im getting no sound. Its getting power to the speakers so I assumed it was the preamp. I opened the sucker up and found that all four led lights were not lighting up. Something is wrong but what should i be looking for?

http://www.drtube.com/schematics/marshall/jmp52-02.gif

Yea im def getting sound from the effects return. (oh and yes I am being very careful regarding voltage. I like my sweet fingers )
So its def the preamp. Here are the symtoms. the only way I get sound is if i turn the volume up all the way. Im searching but not to sure what to be looking for

[Thorny]

The clipping LEDs on the board don't light up unless you really push them. Also you say they don't light up but you get sound out of your effects loop if I understand you correctly, so that does not make sense.

If you can insert a strong signal into your effects return and hear it fine your power amp is OK. Maybe that is what you did? But if you get good preamp signal out of your FX loop then I am not understanding what the issue is. Jumpering the FX loop can usually fix the issue and point to a dirty or bad jack in the FX loop.

Now if you are talking the channel LEDs not working, then you have a problem with the LEDS, the voltage used to switch them, or the devices used to switch them. But they usually are fine in that amp.

If you can trace the signal, trace it through the preamp tubes. Where is stops is where you start looking backwards to see why it stops.

Output transformers are a BIG issue in that amp and can cause very low output just like you mentioned. I just had to replace one last week in one of those. It had to be cranked up and had very low output. I bet that is your real problem. I would find a BETTER replacement for it than a stock JCM900 tranny if that is your problem. I bought a larger one from new sensor (this one had 4xEL34s). It sounded better and that tranny is more reliable. I had to modify it a bit to actually mount the thing but it was not too bad. Those stock 900 trannies suck in my opinion.

Hope this helps. And forgive me if I blew through that message without reading it closely enough and missed something.

[Thorny]

Oh yeah, the most obvious thing too - did you check or replace the preamp tubes? I assume you did.

[ALEXZANDEREPAIR]

thorny I think I didnt explain myself correctly
When I plug my guitar into the input of the preamp I get no sound but when I plug into the effects return I get sound.

[ALEXZANDEREPAIR]

Im gonna try the tracking back thing. But let me just put this out there because this issue has been driving me nuts and I refuse to give up.

THE FIRST PERSON THAT HELPS ME GET THIS DARN THING GOING WILL RECEIVE 50 BUCKS VIA PAYPAL

you guys have been at this alot longer than I have(schematics etc.)

please help me :icon_sad: :icon_sad:

Ive been looking over the schematic and Im gonna try the back tracking thing

[Enzo]

If nothing is coming out the preamp, then isolate the problem. First, verify the preamp is totally dead by connecting a cord from the send jack to another amp to monitor what's coming out, if anything. If signal comes out the send, and plugging into the return makes sound, then connect a spare cord from send to return - a sort of external loop bypass - and see if that restores the sound. If so, then the loop jacks are the problem.

If it is truly dead:

Go down the row of preamp tubes and see if they are all lighting up their heaters. If the heaters are dark, there will be no output. If they ARE dark, dark heaters means no heater voltage at the tube sockets. Should not be hard to find out why. Do all four power tubes light up? If only two light up, we might have a blown fuse. Ther are two fuses on the main board, and if one opens it can kill all the preamp tubes but only half the power tubes.

If the heaters are going, then go down the row of preamp tubes and check at each pin 1 and pin 6 for high voltage. If missing, then find out why - open plate resistor, missing B+ node etc.

The solid state op amps have their own power supply, two of them in fact, +/-15VDC. Is it present? There are a couple small fuses for that as well.

You can still have a bad op amp of course. So at this point I would apply a steady signal to the amp input and follow it with a scope stage by stage through the amp. Start with the tubes. They work within a sea of solid state. So look for signal at the grids of V1. Note the tube is direct coupled so the second grid is wired to the first plate. If ther is no signal there, check IC1, 4 or 6. And right immediately after the input jack is a resistor and cap. Make sure cap C5 is not loose from its solder or broken free.

Those 5201 op amps are special as they incorporate a switching function. Pin 5 is the output, while there are two sets of inputs. Look for signal at each pin 5 therefore.

And don't overlook the mundane - a broken control on the panel can shut you down just as well as some odd part failure. Inspect the solder under all the controls on the front panel preamp area. and look at the rear of each control to see if any are pushed out. Look for any pots cracked where the wafer leaves the pot body. This can only be done with the board dismounted from the front panel.

If signal gets to that grid, then is it coming out the cathode of the second half of the tube - a cathode follower? From there through the tone stack, through IC5 and on to the send jack. Pin 7 of IC5 is its output. You can't measure signal at the input pins. Make sure the treble control is not open.

No scope? You can still trace signal with your meter set to AC volts. If you are not sure the AC volts you are reading is actually the signal, turn the signal source up and down to see if the reading follows. Poor man's scope.

SInce you get sound from the return forwards, I won't go into troubleshooting the rest of the amp.

Larry lists the 4100 on his Schematic Heaven site, is it complete enough for you?

That ought to be enough stuff to get you started.