JVM Marshall

[Valvehead]

Working on a jvm 210 Marshall

I cant find a schematic online all of a sudden-------anyone have this ?

The amp works perfect on the "clean" channel.

the Overdrive ch has almost no volume. Its Not a tube . These amps have 3 modes per channel .


The volume pot for the od ch was very loose so I replaced it. No change.

I can hear all the relays and amp switching properly thru all the modes

cant find a problem yet---tested the following diodes -

Clean ch.
D15 5v on pos (cathode) / 0v on neg
D22 5v / 5v
D23 5v / 5v

OD channel
D15 5v / 5v
D22 5v / 0v
D23 5v / 5v

any ideas on where to look ?

sorry but at the moment i cant find a scem

[Pedro Vecino]

Here you can find the schematics:

https://github.com/a-wai/ampfactory/...e/Marshall/JVM

[strato56]

Do you have OD-signal at the FX-send ?
Master Volume switching occurs after that point, so maybe that could narrow it down.

[Valvehead]

amp works fine going into the fx loop return

[Pedro Vecino]

Do you have OD-signal at the FX-send ?
Master Volume switching occurs after that point, so maybe that could narrow it down.

I do not know if I understood it correctly, but the logical thing is that the channel 2 signal does not appear in the send. The volumes of each channel are before the loop. If it were associated with a switching problem, I would check if there is a signal in the gain and in the volume of that channel. At least to try to isolate the possible.
From V8b to the power stage the circuit should be common to both channels, including the two general master volumes.

[Valvehead]

Another thing i just noticed...the volume pot for the OD channel wont kill the signal for that channel on 0 .

[Valvehead]

I do not know if I understood it correctly, but the logical thing is that the channel 2 signal does not appear in the send. The volumes of each channel are before the loop. If it were associated with a switching problem, I would check if there is a signal in the gain and in the volume of that channel. At least to try to isolate the possible.
From V8b to the power stage the circuit should be common to both channels, including the two general master volumes.

Do you see CON 201 or 202 anywhere? Im trying to find the exact point where the 2 channels meet, and maybe trace back from there

[Pedro Vecino]

CON7 in Circuit.pdf goes to CN5 in EQ.pdf (clean and overdrive gain)
CON8 in Circuit.pdf goes to CN4 in EQ.pdf (clean and overdrive volume)

[Valvehead]

OK--tested tonight, ran 1k test tone into input...i checked ACV at all the outs of the pots on both channels . The only one that stood out was the out of the treble pot on the OD channel, everything else was very close.
The out of the treble pot on the clean ch is 15vac (red mode)

The out of the treble on the OD channel is 5vac (red mode)

Volume pot for OD ch is new.

the Clean Vol has 14.6vac out---channel on / red mode
the OD volume has 0vac --channel on / red mode


both Treble pots test the same with ohmeter

am i on to something here?

this is the only way i can signal trace..unless theres a better way ?? I dont have a scope .

maybe on the OD ch (EQ scematic) , r42 /r43 is bad? maybe RL1b ?

ill have to pull the pot pcb again to test

any ideas guys ??

[Pedro Vecino]

Check the signal levels on both channels on pin 3 of the CON7 (this is the joint output after the gain of both channels).

[Valvehead]

Check the signal levels on both channels on pin 3 of the CON7 (this is the joint output after the gain of both channels).

Clean Ch. / red mode 0.6vac

OD ch / red mode 1.27vac

[Pedro Vecino]

Apparently switching to the gain controls works.

With chanel 2 activated:

You could measure the signal level in R94. That point should correspond exclusively to the lead channel (as I interpret it).
Then on the v7a grid. This confirms if rl3b works or not.
Then in r129.
Finally on the grid of v7b. This confirms if rl4b works or not.

[Valvehead]

Apparently switching to the gain controls works.

With chanel 2 activated:

You could measure the signal level in R94. That point should correspond exclusively to the lead channel (as I interpret it).
Then on the v7a grid. This confirms if rl3b works or not.
Then in r129.
Finally on the grid of v7b. This confirms if rl4b works or not.

r94
clean 93vac
OD 31vac

v7a
clean 0.62vac
OD 31 vac


r129
clean 3vac
od 31 vac

[Valvehead]

I was able to pull the pot pcb out today . Fixed a few cracked joints . That didnt help .
When switching between od and clean ch , i can feel Rly 3 and 4 (on the tube pcb) clicking , but not Rly 1 or 2 on the pot/eq pcb . i confused as to what relay to look at due to the 3 modes per channel.

the tube pcb has rly 3 and 4

and the pot pcb has relay 1 thru 6

[Valvehead]

on EQ pcb ,

i have +5vdc both side at Rly 4

Rl2 +5vdc in OD channel , 0v clean ch.

Rly1 0.2vdc and +5dc

Rl3 has +5vdc and -5vdc

probing with a 1k test tone , at all the pot outs , i only get full volume at the Clean CH. GAIN pot ...nowhere else in the tone stacks

in OD channel , i hear it barely, at the GAIN pot only