Marshall JCM 800 - "splatty" Lead channel

[earache]

Hello All

I am looking at a Marshall JCM800 Model 2205 ("old style", no chip) which suffers some problems. Mainly, the Boost channel sounds very gated and "splatty" with very low output.

It has been worked on by someone previously, who replaced the dual-gang gain pot and the volume channel pot on the boost channel. They also replaced some of the box capacitors with combinations of radial caps in series...looks like they didn't have the right value or couldn't wait to order the right parts.

I humbly request help with two things;

Would someone have a schematic or service manual for this amp that has the capacitor values listed by reference designator? The replaced caps are at
C8 C9 C36 C38 and C39, and the schematic (attached) doesn't list them that way.

Additionally, where's a good place to start with the issue on the Boost channel?

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

[J M Fahey]

Problem with "messed up" amps is that we donīt really know whatīs there.
It will probably match no known (or unknown) schematic.

I suggest a longer but safer way to solve both problms: part designations may change but they basically still must be there for the amp to work.

Print a large copy of the schematic, even better if you print it twice the size, in two separate sheets which then you tape together, and go along the board checking parts one by one and annotating the schematic with new part designation and whatīs actually there.

Then take a picture of the corrected schematic and upload it here.

So, for example, you will find that "the cap which goes from V1a cathode to ground" which MUST be there, is labelled, say, "C4" on the printed schematic but "C2" on the PCB and instead of ".1" is actually ".22" or whatever.

Do it at least for the entire Clean channel, from input jack to input of V3a

Does the Dirty channel work fine?
Can you set its gain pot (the dual one) down and get reasonably clean sound there?
Or itīs also splatty gated?

[Mick Bailey]

Gated and splatty with low output could be down to one of the preamp grids not having a resistive path (grid leak) to ground. Check the resistance from V2a grid to ground with a cable plugged into the unpowered amp. In the original schematic the grid leak was formed by the 1M Ohm pot track and it should still read the same.

Capacitance values to me don't account for the symptoms. I would leave them alone for the time being and pinpoint the specific fault before replacing parts.

[earache]

Mick Bailey

The problem with this schematic isn't only that the reference designators aren't listed, the values themselves are very hard to read in some cases.

With a lead plugged into the unpowered amp, the resistance from pin 2 of V2 is 88K...until I turn the"Gain" knob, and the value follows the pot turn. 88K is with the pot at it's minimum.

Resistance from pin 7 of V2 to ground is like 4.8M

P2 of V1 resistance reading, taken as above: 7.4K P7 of V2 reads 75K

The Boost channel is the channel that is "splatty". The Normal channel works ok, mostly...the amp may need a speaker as well.

I am suspicious of the dual pot that was replaced. Also worthy of suspicion are the replacement radial caps.

[Justin Thomas]

Looking at that job, I'd suspect that first...
Maybe that single Mallory 150. That has no business in a Marshall. Unless you're trying to Fenderize it.

Justin

[Chuck H]

Just trace the circuit and rewire it as per the preferred stock circuit. You really shouldn't need R# value designations for that.

[Mick Bailey]

That's pretty bodged circuit board. Have you looked at the copper side to see if any tracks have been cut? I think it's down to what's already been suggested - trace out the connections and identify the components on the schematic. At least get to that stage and then post back here any questions on values when we know the positions. In cases where there is an unknown value that can't be read of the schematic I either go back to first principles or see what other Marshall amps use in the same location.

If the clean channel is working you at least know that the supply voltages are close. The boost channel only has a handful of components, but if the circuit has been rewired then you need to establish what's been done.

It would also be worthwhile measuring the plate and cathode voltages on the drive channel and posting back the results.

[Pedro Vecino]

These sets of electrolytic capacitors (five) seem to compose 320uF per pair. They seem to replace the original capacitors associated with the operation of the mutes in the switching circuit. Itīs easy to identify their functions following the scheme.
In addition to the Mallory capacitor, there is a resistor (R12) that may not be original and the same with the other on the second socket. There is also a reddish brown plastic capacitor next to the transistor on the left that may not be original. It would be convenient to check their values in the schematic.

The area is delimited and turning the board you can check which welds are original and which are not.

In any case, itīs the first Split Channel model, manufactured for about a year and a half and its boost channel (pre-drive equalization included) sounds very bad.

This can be confusing to determine if it sounds like it should or not.

[J M Fahey]

In general "splatty" which Iīd also call "chopped" means a stage which canīt pass signal for some reason until it exceeds a certain value, like a very poor noise gate.

Typical is stage being horribly biased, so itīs basically cutoff for normal signals; strong ones can get through by sheer volume, and very distorted at that.

In principle measure cathode and plate voltage at each triode involved as a quick mode to confirm/reject that.

But this amp also includes new actors: the switching transistors present everywhere; a poorly biased or shorted one can also act as "terrible noise gate" .

Since said transistors ground selected parts of the circuit through electrolytic capacitors, just for testing I would lift one of their legs so Clean channel is forced "enabled" and check whether we now have proper sound.

Or we have a horrible soldering somewhere along the path, we have no real soldered continuity but "metal poorly touching metal" and only strong sound can jump that barrier.

OF COURSE, a scope can show that quickly, thatīs why Techs have them.

[drewl]

The later version with a bad switching chip can cause this same problem, that's how I got a 2210 cheap.

So it may be something in the switching circuit.

[J M Fahey]

Transistors may be separate ones or an array inside a DIP package, electrically are the same: a transistor which shorts signal to ground when base is biased positive.

[m1989jmp]

Hello All

I am looking at a Marshall JCM800 Model 2205 ("old style", no chip) which suffers some problems. Mainly, the Boost channel sounds very gated and "splatty" with very low output.

It has been worked on by someone previously, who replaced the dual-gang gain pot and the volume channel pot on the boost channel. They also replaced some of the box capacitors with combinations of radial caps in series...looks like they didn't have the right value or couldn't wait to order the right parts.

I humbly request help with two things;

Would someone have a schematic or service manual for this amp that has the capacitor values listed by reference designator? The replaced caps are at
C8 C9 C36 C38 and C39, and the schematic (attached) doesn't list them that way.

Additionally, where's a good place to start with the issue on the Boost channel?

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

Given the fact that a lot of people don't like the sound of that amp even when 100% working, I suggest you pull the PCB out and make a nice turret board 2203 or 2204 with some bells and whistles since there's a lot of space inside that chassis.

[drewl]

Or just tweak it and it sounds great like I did.

[earache]

m1989jmp

Not my amp to mod

[earache]

Pedro

You sound like you are extremely versed in the topology of this amp. Do you have a gut shot of the circuit board?