Troubleshooting JTM30 - not passing signal

MachineTrout069

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Good evening all, happy Friday!

I was gifted a 1x12" JTM30 combo a few years ago, and it's performed well at home up until the last month or so. When my neighbor gave it to me, he said that the standby switch was "sketchy", so I replaced it and gave the boards a visual inspection while I had it open (no obvious overheating scars). I noted that the reverb wasn't working so replaced the tank with a new MOD unit, worked great until the amp stopped passing signal entirely.

Usually I use this amp with an ADA MP-1 preamp plugged into the FX return, bypassing the Marshall preamp, but I've confirmed that everything worked correctly when I plug into the input jack. Lately though, it's totally stopped passing any signal, no matter where i plug the signal in.

I get the expected "POP" and gentle hum through the internal speaker when hitting the Standby switch, so I think the power amp is working correctly. I tried a known-good 12AX7 in all three positions for the preamp tubes, so I think i've ruled out bad tubes and transformers. Tested output from the FX send and XLR direct out last week, got a background signal as if nothing was plugged in but no signal change when i rang the guitar strings. Did a lot of reading on these amps and saw that the FX loop is a common issue, so I hit the jacks with some Deoxit and plugged/unplugged a 1/4" patch a few dozen times, as well as left the patch cable in, to see if any of that helped. It did not.

Next step is to open it up again and start touching the preamp components with the oscilloscope, which I plan to do over the next week (i am an electronics engineer, but don't have a ton of experience fixing tube amps). I'm wondering if anyone else has seen similar problems with these amps, and/or if anyone has suggestions for components that are known to blow - basically hoping anyone can suggest a good place to start.

And of course, if there's a thread that I couldn't find that addresses a similar question, I'm happy to do some more reading! Thank you to anyone who has any suggestions!
 
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Gene Ballzz

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Good evening all, happy Friday!

I was gifted a 1x12" JTM30 combo a few years ago, and it's performed well at home up until the last month or so. When my neighbor gave it to me, he said that the standby switch was "sketchy", so I replaced it and gave the boards a visual inspection while I had it open (no obvious overheating scars). I noted that the reverb wasn't working so replaced the tank with a new MOD unit, worked great until the amp stopped passing signal entirely.

Usually I use this amp with an ADA MP-1 preamp plugged into the FX return, bypassing the Marshall preamp, but I've confirmed that everything worked correctly when I plug into the input jack. Lately though, it's totally stopped passing any signal, no matter where i plug the signal in.

I get the expected "POP" and gentle hum through the internal speaker when hitting the Standby switch, so I think the power amp is working correctly. I tried a known-good 12AX7 in all three positions for the preamp tubes, so I think i've ruled out bad tubes and transformers. Tested output from the FX send and XLR direct out last week, got a background signal as if nothing was plugged in but no signal change when i rang the guitar strings. Did a lot of reading on these amps and saw that the FX loop is a common issue, so I hit the jacks with some Deoxit and plugged/unplugged a 1/4" patch a few dozen times, as well as left the patch cable in, to see if any of that helped. It did not.

Next step is to open it up again and start touching the preamp components with the oscilloscope, which I plan to do over the next week (i am an electronics engineer, but don't have a ton of experience fixing tube amps). I'm wondering if anyone else has seen similar problems with these amps, and/or if anyone has suggestions for components that are known to blow - basically hoping anyone can suggest a good place to start.

And of course, if there's a thread that I couldn't find that addresses a similar question, I'm happy to do some more reading! Thank you to anyone who has any suggestions!

First> :welcome: to the forum!
NEXT> If the treble pot goes faulty/open, there will be no signal output! Working on these amps is not for the inexperienced and/or faint of heart, as the circuit board traces are beyond fragile! Be prepared to hardwire/bypass a broken trace or few! :eek: Below is a link the the most accurately "cleaned up" schematic currently available. Don't be misled by "helpful" folks piping up with suggestions under the assumption that the JTM30 and JTM60/600 are mostly the same amps. There are not many similarities, beyond cosmetics and moniker! It should also be noted that there are several solid state devices/transistors (for various purposes, including driver/recovery for the loop and the reverb) that are not specifically available. While there are electronically identical replacements, the ones currently available have opposite leg/pin configurations!


https://www.thetubestore.com/lib/th...shall/Marshall-JTM30-30W-JTM310-Schematic.pdf

FWIW> I have a JTM30, from '92-ish, in perfect working order and one of my favorite amps, although it can be unholy loud at a measurd 52watts with the stock tubes! Those OEM, SOVTEK 5881/6L6WGC are the only power tubes that should ever get used in this amp. Those tubes are not really a 5881 or 6L6 and the amp is non adjustable, fixed bias and said amp is designed around the unique characteristics of these tubes!

Have Fun & Welcome Again!
Gene
 

mickeydg5

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@MachineTrout069

Checking supply voltages come first.

Did you check HT T500mA fuse F102?
Is the heaters in the preamp tubes lite? Maybe check T6.3A fuse F101 also if not.
Also measure the resistances on fuses when checking. They should be fairly low resistance, usually less than 1 but above 0.

Since nothing is coming out of the power amplifier section the voltages at the power tubes may require checking.

Let us know what you find.
 

Matthews Guitars

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Look up the diagrams for the tube types. Know which pins are grids (inputs) and plates (outputs) and you can trace the signal through each stage with your o-scope. Just be sure you're using probes that can handle plate voltage and you're set to AC coupling. Inject a signal in the input jack and trace through. You'll soon find where the audio signal stops.

An open connection from any plate, cathode, or grid to the circuit will cause a no signal output condition.

Some Marshalls have bias resistors of the safety type that fuse open if bias current goes too high. They'll look perfect but will be open circuits if blown. You'll find them connected to pin 5 of the power tubes.
 

MachineTrout069

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First> :welcome: to the forum!
NEXT> If the treble pot goes faulty/open, there will be no signal output! Working on these amps is not for the inexperienced and/or faint of heart, as the circuit board traces are beyond fragile! Be prepared to hardwire/bypass a broken trace or few! :eek: Below is a link the the most accurately "cleaned up" schematic currently available. Don't be misled by "helpful" folks piping up with suggestions under the assumption that the JTM30 and JTM60/600 are mostly the same amps. There are not many similarities, beyond cosmetics and moniker! It should also be noted that there are several solid state devices/transistors (for various purposes, including driver/recovery for the loop and the reverb) that are not specifically available. While there are electronically identical replacements, the ones currently available have opposite leg/pin configurations!

FWIW> I have a JTM30, from '92-ish, in perfect working order and one of my favorite amps, although it can be unholy loud at a measurd 52watts with the stock tubes! Those OEM, SOVTEK 5881/6L6WGC are the only power tubes that should ever get used in this amp. Those tubes are not really a 5881 or 6L6 and the amp is non adjustable, fixed bias and said amp is designed around the unique characteristics of these tubes!

Have Fun & Welcome Again!
Gene


Thank you good sir! This is EXACTLY the kind of pointers i was hoping for - it would be really freakin sweet if it turns out to be the treble pot :D i did note that the board looks to have those delicate traces, which i assumed based on the posts i've read that noted that these amps were built in the Korg factory - well worth keeping an eye on. Luckily, my schooling focused on delicate circuits for RF and data comms so i'm prepared to jump some traces if i have to, but it's REALLY nice to have that heads-up.

Also really helpful to know that the replacement ICs will have reversed pins! I saw a few posts about the JTM60 being a very different beast, and the impedance on the output jacks being sketchy, but this is really good to know.

Finally, thank you for the link to the updated schematic! I'd found the other one with the missing traces, and had the requisite "WTF" moment about "well, where does the other connector go", but being able to see the proper one is very helpful.

I really hope it turns out to be the Treble pot.
 

Gene Ballzz

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A pop and hum does not tell me everything is appropriate.
Is that what you meant?

Sorry sir, didn't mean to be a jerk! You are right of course. Was just pointing out that something appears to make it to the speaker(s), as opposed to a fried output transformer or such.
Carry On,
Gene
 

mickeydg5

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Sorry sir, didn't mean to be a jerk! You are right of course. Was just pointing out that something appears to make it to the speaker(s), as opposed to a fried output transformer or such.
Carry On,
Gene
Not a problem and do not worry about it. It is good questions are brought up and what the OP said could confuse others.

Here is a simple explanation of the situation for anyone interested. Amplifiers have power supply systems which are usually energized and are connected to the output stage. Whether or not the output tubes or transistors of an output stage are working the power supply itself can inject some noise(s) at the speaker output.
So in turn any pop or hum just indicates that the power supply is either wholly or partially energized. It does not mean the output stage is capable of amplifying a signal correctly.

I hope that helps make more sense of this situation. :)
 

anitoli

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This is the proverbial time when an O-Scope would solve this issue pronto. If signal is going in but not coming out being able to see where it stops will tell you everyting.
 

MachineTrout069

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Sorry sir, didn't mean to be a jerk! You are right of course. Was just pointing out that something appears to make it to the speaker(s), as opposed to a fried output transformer or such.
Carry On,
Gene

Gene: No offense taken here, and hopefully none elsewhere either - my OP was probably not as clear as it should have been, considering that there were several shots of tequila required to get up the courage to ask ;) But everything here is absolutely fantastic. I will start with looking at fuses (feel kinda dumb for not starting with fuses :run:) and the Treble pot, and otherwise scope the signal from input to output if none of those are obviously bad.

I do think, based on preliminary testing, that i'm looking at the preamp - i tested the output from the FX Send jack and the XLR DI out to the input of my Focusrite interface, and only get the background hum that i would expect. The reason i think the power amp is OK is that the sound coming out of the amp is exactly the same as it was before it stopped working, when i'd fired it up with nothing plugged in, but Mickey is not at all wrong to note that PS hum can sound exactly like power-amp hum - so i won't rule that out, especially since i haven't been able to swap out power tubes (after all, those are a more expensive test when you don't have replacements laying around ;) ). It's interesting to see that the FX loop on this amp comes BEFORE the EQ, but that really just confirms what i'd seen from plugging the MP-1 into the FX return.

For now, i'll start with these suggestions, and let everyone know what i find in case it might help someone else at some point. But more than anything, i want to thank everyone for their quick, concise, and very helpful replies. For an old guy like me, it's really nice to remember how the "old school" internet works - there's more out here than just Facebook and Chapsnatch and Twizzer :D Please let me know if anyone has any additional pointers, but other than that, stand by for updates!
 
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