Marshall JCM 800 50W Split channel rev. Combo sounds like crap!!

[Steelwitch]

Hello! It’s been a while since I’ve posted but here I am. I have a JCM 800 50W on the bench that I’ve serviced before. Last time I replaced the filter caps with 2 50uf FT can caps and new voltage drop resistors, the previous work done in not related and has held up well. The customer brought it back with the complaint of not enough high end, it sounds kind of muffled. I found a shorted cap (C9) on the board related to the channel switching circuit I believe but it’s unrelated, no difference after clipping in a new one. One problem which might be related is grounding. Seems like even with nothing plugged into the input jack 60hz hum can be heard when turning up the tone controls and volumes. Grounding the chassis through my meter kills it. Now, this is one of the issues but not sure if I can say it’s related. On some level it’s all related, but all stages are conducting to ground the way they should. I can post a generic voltage chart if it will help you help me track down what’s going on! Thank you. Schematic attached..

https://drtube.com/schematics/marshall/4210.gif

[Steelwitch]

IÂ’ve localized the issue to boost channel only.. the reverb is not functional and some arcing from the reverb transformer secondary. Anyone have a datasheet for these? IÂ’ve never seen anything like this

[Enzo]

C9? I don't see part numbers on your schematic.

Your schematic shows a transformer part number. If those are not available, it seems to me you could mount a Fender transformer, and change the reverb pan to a 4AB part instead of the specified 4FB part. ANyone see problems with that?

[Steelwitch]

To clarify, the complaint about the amp is that it sounds muffled, and lacking high end. It’s possible arcing happened because there was no load on the txfmr while it was on the bench. I put another reverb tank on the bench and it does work. Sorry about that. My initial thought of it being a shorted transformer was false, which overall I’m happy about. I localized the problem to the boost channel only. After sending a 1k sine through the circuit I realized that the treble control does nothing. There are signs of grounding issues. Nasty 60hz hum when the MV is turned up. I’m gonna look for problems in the preamp. There may be a grounding issue somewhere in the tonestack, although I haven’t found it yet

[The Dude]

The treble control may not have much effect on a 1K test signal. Try a higher frequency. Yes, you could indeed have a problem there, but your test isn't proof of that.

[Steelwitch]

Ok it took me all day, but I think I know what’s going on here: it is the switching circuit after all. For the reason of some failed diode or transistor, maybe this cap, there’s bleed between the boost and normal channel. When all the controls on the boost channel are dialed down to 0, normal channel sounds great. Anyway, each channel introduces 60hz hum when not selected. Does that make sense?

[Steelwitch]

The treble control may not have much effect on a 1K test signal. Try a higher frequency. Yes, you could indeed have a problem there, but your test isn't proof of that.

Agreed, the pot itself is good but haven’t completely confirmed the tonestack is good. I think it’s a product of this channel switching issue

[drewl]

The pots and their pc board connections can go bad often on these.

Re-work their connections, test and clean the pots.
You're going to have to lift the board to troubleshoot the switching circuit anyway.

[Steelwitch]

https://vimeo.com/366561781

Heres a video of exactly what’s going on. Kind of an elusive problem and almost positive it’s grounding. I went through and tested almost every ground point that I could think of and got an ok reading.. what do you think? What should I try next? Barely getting any signal through the boost channel

[Steelwitch]

Doing a little bump on this

All the ground connections are seemingly good.. but when I touch the boost channel volume knob specifically, its conducting signal like an instrument cable would. Not sure what could have failed here. It’s taking me forever to find it!

[Helmholtz]

From the video it seems your amp chassis is not properly safety grounded. Please measure resistance between amp chassis and mains safety ground/earth.

[g1]

You have the plastic knob on or off the control shaft? Not sure how it could make noise if it is on as it is plastic. Is the nut that secures the pot to the chassis tight?

[Steelwitch]

Seems your amp chassis is not properly safety grounded. Please measure resistance between amp chassis and mains safety ground/earth.

0 ohms

[Helmholtz]

You have the plastic knob on or off the control shaft? Not sure how it could make noise if it is on as it is plastic. Is the nut that secures the pot to the chassis tight?

The interesting thing is that the noise disappears if he connects the scope probe's ground to the chassis.

[Steelwitch]

You have the plastic knob on or off the control shaft? Not sure how it could make noise if it is on as it is plastic. Is the nut that secures the pot to the chassis tight?

The knob in question is a replaced pot. It’s an alpha pot so no plastic shaft or insulation.. right now I have the board unmounted from the faceplate, but the result is the same even with the pot mounted