Marshall JCM800 50W Combo

[earache]

Hello

I have this Marshall JCM800 50W combo that isn't as loud on the "Boost" channel as it should be.

I also had an issue with the reverb being static-y, buy solved it with a spray of DeOxit to the footswitch jack. I cleaned all the other jacks and pots with that stuff while I was at it.

I thought the issue might be with a weak preamp tube, but I swapped in a known-good one at each location with no difference.

What gives? Schematic attached



TIA
Earache

[g1]

Did you check DC voltages at pins 1,3,6,8 of V1 and V2? Please post.

[m1989jmp]

Hello

I have this Marshall JCM800 50W combo that isn't as loud on the "Boost" channel as it should be.

I also had an issue with the reverb being static-y, buy solved it with a spray of DeOxit to the footswitch jack. I cleaned all the other jacks and pots with that stuff while I was at it.

I thought the issue might be with a weak preamp tube, but I swapped in a known-good one at each location with no difference.

What gives? Schematic attached

[ATTACH=CONFIG]49112[/ATTACH]

TIA
Earache

Isn't as loud as it was before or that's your opinion?

Clipping diodes in that circuit really mess up the tone and cut some volume too.

[earache]

G1 The voltages as follows:

V1
P1 242.
P3 1.83
P6 84.
P8 .55

V2
P1 260.
P3 2.94
P6 207.
P8 1.5


m1989jmp Re: "Isn't as loud as it was before or that's your opinion?"

It isn't my amp but it seems that the owner's claim is plausible, as similar loudness settings on the volume knobs on both channels have the Boost channel being quieter. Otherwise, the amp isn't noisy as far as crackles or static.

[m1989jmp]

G1 The voltages as follows:

V1
P1 242.
P3 1.83
P6 84.
P8 .55

V2
P1 260.
P3 2.94
P6 207.
P8 1.5


m1989jmp Re: "Isn't as loud as it was before or that's your opinion?"

It isn't my amp but it seems that the owner's claim is plausible, as similar loudness settings on the volume knobs on both channels have the Boost channel being quieter. Otherwise, the amp isn't noisy as far as crackles or static.

That's my experience with every two channel amp with clipping diodes in the overdrive/boost/distortion channel signal path.

I may be wrong, but there's a possibility you'll find nothing wrong with the amp. Try bypassing the diodes with alligator clips and something like 470k to ground, the channel will become much louder.

[earache]

So these voltages also look ok?

[The Dude]

They look "OK", but the amp works, "kind of" so we are looking for something "kind of" off. If you follow that strange logic, I think V1 P6 is slightly low. I could be wrong, but I'd check the plate resistor for that plate.

[m1989jmp]

So these voltages also look ok?

V1 pin 6 seems low.

[Pedro Vecino]

That amp is the first version of Split Channel. His lead channel is probably the worst in Marshall's history and can be confused with a broken amp. Marshall stopped making it after a year and a half and redesigned it completely.

[g1]

So these voltages also look ok?

Not necessarily, but a few things first.
m1989jmp asked a good question earlier, did something change for the owner, or was it always this way? The channels are different, so you can't expect to get the same volume by setting them up to the same numbers. Can the boost channel get loud enough using different setttings?
Now, about the voltages. V1 pin6 & 8 look suspicious. But I'm not sure you have the correct schematic. Does it have the IC for switching like this schematic?

(I'm suspicious about the first schematic because I don't see any measured plate voltage resembling what should be seen for a cathode follower)

[J M Fahey]

similar loudness settings on the volume knobs on both channels have the Boost channel being quieter.

That by itself is not a problem and in fact quite common.
"Knob position does not mean any specific power output"
Feed 100mV 1kHz to input, set all tone controls to 7 , master to 10, and slowly rise channel volume ... can you make power amp clip?

If you can (and I bet you can) there is nothing "broken" .

When I looked at schematic found it goofy clumsy, and those diodes adter the cathode follower are a mistake, but too sleepy now (and doubly stupid because of the flu and fever, Winter hit us hard and suddenly) to suggest corrections.
But please check sensitivity.

[earache]

If this channel is a known poor design, are there common mods to “fix” it?

[earache]

G1 - the amp has the chip, i believe a CA3046

[J M Fahey]

If this channel is a known poor design, are there common mods to “fix” it?

Maybe.
Personally it´s the first time I see it and am amazed by its kludginess.
When I saw "JCM800" in the thread title I expected the classic "1 trick pony" we all know, a very simple amp, which does just 1 thing, that 80´s metal tone, VERY well.
But when I enlarged the schematic, I saw without much analysis a couple WTF? things; the main one adding diodes after the classic cathode follower stage, the core of the most Marshalls distortion, so they negated its action. WTF?
And in general they tried to do very many things at the same time, which in general is not good.
Let me see it while I sip my morning Tea, might suggest something.

Is this for you or for a customer?
For personal use it might pay to spend a lot of time modding it all over the place and, worst case, ripping the board off and building something else, nof for a customer you´ll never be able to charge for all the time you´ll soend on it, in nthat case address his specific request and be done with it.
Or it will be an endless time, money and effort sink.
Ok, the kettle is hissing, see you later.
NOTE: maybe somebody knows a specific set of Mods, or some discussion page or Forum about it; in that case, please suggest that.
I will only *look* at the schematic , can´t promise anything.

[Pedro Vecino]

G1 - the amp has the chip, i believe a CA3046

Then it must be the modern version. As G1 said before, the voltages you have measured (V1, v2) do not match the old version. The first thing in this case is to identify it correctly.
Check it with the scheme of the new version.