Marshall JCM 800 - "splatty" Lead channel

[earache]

Well I'll bet it's not a 330uf ceramic cap So that's not what the board or schematic is showing. And herein are the problems with making corrections from a remote location.

And you're right earache. Most of the images I saw were with the chip. So...

Does the designation on the board for that cap actually say 330uf?

Chuck H

So THATS part of my problem hahaha! The board has just reference designators (C4, R19, etc) on it’s artwork - no values. The schematic has values...hard to read values - and no ref designators. So effing hard to follow... you follow the physical traces and come to a part and discover that it’s sometimes not what you expect.

I gotta find someone with pictures of a similar unmolested version of this thing...is it appropriate to start a different thread requesting pictures of an Amp’s guts?

[Chuck H]

You're going to have to trace every pathway The photo provided by g1 should give some idea about how lead dress was handled. You may need to remove and mark such leads as keep you from lifting the board and use your meters beep short setting to confirm traces, but this IS absolutely doable. Have courage (clinched fist salute). There may yet be several differences between the 4210/2205 schematic and your amp. That's not uncommon on Marshall boards. It's not even uncommon for their boards that have designators and/or values printed on them to have different values factory installed. Sometimes very different values as well as uninstalled parts or jumpered parts and be totally stock. So anything the board says won't necessarily help much. I'm not a member of any of the Marshall forums. I'm sure a couple have some tech info. You should be a member of all of them, posting gut shot requests and asking about circuit info as it relates to the 4210 schematic. My guess is that the 4210 schematic is pretty damn close because I've seen it posted for the "no chip 2205" a few times now.

[J M Fahey]

It´s doable, just do it slowly, at your own pace.

Meanwhile use the amp with one or two good pedals ahead, on the clean channel.

PD: components are not placed at random,they are there for a reason.

So a 12AX7 triode will have 100k to 220k from plate to +250V rail, 820r to 2k7 from cathode to ground, and so on.

Component designations are just reference guides ... if they match schematic and PCB that is, but there´s nothing sacred about them, by any means.

You can use anything you like, but for consistency let´s use those printed on the PCB.

For example, look at the circuit section I uploaded showing Clean and Boost Channels, up to the mixing resistors.

Print this *large* (if possible use 2 sheets of paper) and under good light check your board,roughly following the signal path shown on chematic.

* from input jack to first grid you have a capacitor. ... what does the PCB call it ... C1? ... ok, so be it, you write C1 on the schematic.
Oh, so you find it´s not .047uF but .022uF? ... ok, YOU write .022uF by it.
It isn´t used? ... ok, cross it and draw a line joining ends.
Always using pencil, so you can erase mistakes.

You find a component not on the schematic?
track not on schematic?
Ok, where does it start, where it ends? add it to schematic.

Personally I use a white print correction pen to mark components, wires, connectors, etc. already checked and added to schematic, I find the white dot very visible, but more important, clearly non original, to avoid either double accounting or forgetting what I already annotated on paper schematic.

Once you have it done, upload a picture.

As a side note, it´s far more trustable than any random schematic , even a factory one, because this one really matches what you have.

[g1]

Well I'll bet it's not a 330uf ceramic cap So that's not what the board or schematic is showing.

Schematic says '330uF Now .022' .
Agree with you Chuck that this is going to require a lot of legwork by the OP regarding transcribing the schematic to the layout.

[J M Fahey]

I suggested above a slow but certain way to do it, doublechecking along the way and marking with a white dot parts already added to the paper schematic.

As of the 330uF caps, no big deal, they may not even be used .

This amp uses transistors as switchs to ground to kill channels.

Left and Center transistors are grounding 1M pot wipers, in principle no DC there so not *really* needed and in any case, value is very flexible.

330uF of course will ground *anything*; I bet even 2.2uF will be more than enough.
Not much smaller since a little unwanted Bass might get through, although only as a distant rumble.

Right transistor is different, since it grounds a plate which has >100V on it so electrolytics won´t do.
.1uF as shown is acceptable ; much smaller will let Bass frequencies through and to boot this channel has more gain, so it may be a problem.

But main task remains anottating printed schematic, so work late at night, wife-kids-pets asleep, goo9d easy Music set at Muzak loevels and a thermosfull of coffee.

Go to sleep when you feel tired - bored, in 2 to 6 nights you´ll have it.

Draw things as they are today, I fear we´ll find a couple crazy Mods, but in doubt fall back to Factory schematic..

[earache]

Gentlemen

First - what a great community this is! You can't know how much I appreciate your help.

I'm going to take this task on because it will be a terrific learning experience for me. I've already learned a lot about using a scope. Now I hope to learn about the common topography of tube guitar amps.

Please stick with me as I work my way thru...I may have a lot of questions, and some may really show how much of a noob I am!

Let's create a reference for other people who, like me, have some knowledge of amp repair, but need to move to the next level.

Help me narrow the field though - I only have to trace the parts that are related to the Boost channel, correct?

earache

[J M Fahey]

In principle, yes, since it´s the one showing problems.

Besides, since "Marshall users are mainly interested in Distortion", add to that the Boost channel is not much loved, if at all, and it becomes the target for much modding ... which in general is poorly made (and that´s an understatement).

So let´s first what you actually have there, specially what differs from original circuit.
As mentioned above, it can never have left Factory that way.

Absolute worst case, leave everything that looks original, including those vintage looking resistors, BUT pull jumpers/wires/capacitors/etc. which are clearly tacked-on, not having own exclusive use pads and tracks, simply to clear the field.

In any case you are NOT interested in poorly made Mods which do NOT sound good and even make amp unusable, so best is to go back to original.
Better lackluster performance than unusable amp.

IF you are leaving parts matching PCB and , hopefully , schematic, we are 90% there.

Some value may have been modded; say PCB shows a part soldered to proper pads, but value differs from schematic, say a 10uF cap was replaced by .68 or whatever, then write it on paper schematic so we see "original" ... what was Factory printed there, and "current" , whatever you find.

Having both values side by side will help us guess which one to leave.

FWIW I have listened to a couple YT videos about this (presumed unmodded) amp and sound was not ´orrible by any means.

Yes, it lacked the boiling fury of a Plexi driven balls to the wall ... but then many amps suffer the same problem.

[Pedro Vecino]

FWIW I have listened to a couple YT videos about this (presumed unmodded) amp and sound was not ´orrible by any means.

Yes, it lacked the boiling fury of a Plexi driven balls to the wall ... but then many amps suffer the same problem.

I´ve searched for videos of that model and the first fifteen or twenty that I found on youtube were the modern version. This is the old version. In combos the dog bone shaped metal plates that hold the chassis identify it. The sound (the video is terrible) is what I remember. The lead channel allows basically "clean" sounds with some compression at low settings but as soon as the gain rises the disaster begins.

https://www.youtube.com/watch?v=-oREJbneh1E

[Chuck H]

I agree that that amp sounds awful. So much so that I think that once earache's amp is restored and working "correctly" that would be a good time to consider improvements to the circuit. But not before square one is reached with a proper restoration to stock. That's the only way to keep things from becoming too complicated.

[Pedro Vecino]

So is. The original reference is essential.

[g1]

I think that once earache's amp is restored and working "correctly" that would be a good time to consider improvements to the circuit. But not before square one is reached with a proper restoration to stock. That's the only way to keep things from becoming too complicated.

The problem is if there are schematic errors (or maybe better to say discrepancies).
I had earlier said that the V1b DC voltage readings seemed wrong, and that he had a 820R cathode resistor where the schematic shows 2K7 (which I didn't think they would ever use). Now looking back at the picture from another unit (post #20) they both look to have 820R's there, while the schematic shows 2K7.
So, yes, you can put it to the schematic, but you may not end up with what rolled out of the factory (not that there is any viable solution to this problem).

[Chuck H]

So is. The original reference is essential.

Well right now the amp is unstable. If the amp is returned to original "working" condition it won't be. Then any modification that causes instability could be handled on a case by case basis. JM2C

But it does occur to me that I might describe the tone of that amp in the video as "splatty".

[Chuck H]

Well there IS another option...

What earache has is a cabinet, chassis with pots and tube sockets, transformers already mounted, tubes,.. I think everyone see's where I'm going with this. Gut that sow and make some delicious pork chops

[J M Fahey]

Well, my last post started with a similar suggestion about gutting it and building a "real" JCM800, the killer non-switching-channel one also known as "Master Volume".

But it´s 20X the job needed compared to repairing this one, which is at least usable either clean or with a couple pedals up front, and has the very high possibility of being abandoned halfway because of Job/Family/finance/whatever reasons taking all free time and then some, turning a usable amp into a boat anchor which will eventually be sold for parts so I erased that part.

So most sensible is to start by restoring it to original (which worst case be sold that way) or eventually modded the good way.

As-is... it´s neither.

[earache]

Hello All

I ended up by giving up on this one. I gave it back to the owner with links to all of the research and the schematics and notes I took.

He later told me that it has almost always been a problemic Amp. It’s been to several other techs before me over the last several years.

It was evident from the state of the cab and all of the fasteners that it has been apart and back together a bunch.

Thanks as always for your kind help and attentions!

Final thought, in the vein of what Chuck H has said - what possible kit or plan is this amp close to? What could it be modified/changed into that would work? And has anybody on this forum undertaken such a project?