JMP "Volume I" problem

[valhala92]

Hi,

Recently I bought a '74 Marshall JMP Bass, there's a problem with Volume I, when I close the Input I volume all the way to zero, I can still here the sound of the guitar, I took it out  and cleaned it and I also checked the ground wiring for the pot, it was fine, any suggestions for fixing this problem?

There another problem too, when I flip the standby switch to ON i can here some noises (it's like caps are charging), and when I flip it to OFF I hear a popping sound. (I changed the filter caps with F&T ones when I bought it and double checked the connections).

I really appreciate it if you help me,

Thanks.

[HotBluePlates]

Recently I bought a '74 Marshall JMP Bass, there's a problem with Volume I, when I close the Input I volume all the way to zero, I can still here the sound of the guitar, I took it out  and cleaned it and I also checked the ground wiring for the pot, it was fine, any suggestions for fixing this problem?

When you do this test, is Volume II all the way off?

According to the schematic, both channels share a cathode resistor in their first stages. So if you have the volume up on one channel while playing through the other channel (with its volume off), you could get some cross-coupling through the shared cathode circuit.

I wouldn't consider this a problem. I mean, how many times do you play through a channel with its volume on zero?

There another problem too, when I flip the standby switch to ON i can here some noises (it's like caps are charging), and when I flip it to OFF I hear a popping sound. (I changed the filter caps with F&T ones when I bought it and double checked the connections).

The schematic also shows the standby switch as being before all filter caps, and that it would disconnect the bias circuit from any voltage. Those 2 things seem to guarantee popping when flipping the standby switch, so it would seem again your amp is performing as the stock circuit would naturally.

If possible, you may want to move the bias supply to attach directly to the PT high voltage winding before the standby switch, like the JCM800 amps. I said "if possible" because I don't know if the bias circuit wiring is fixed by p.c. board traces, which would be a difficult modification for a beginner.

If you're able to do that and it still doesn't stop popping when flipping from standby to play, then you might add a 0.05uF cap to ground right after the rectifier diodes, as in the JTM45 circuit (where it's after a tube rectifier, but the function will still be the same in your amp).

[valhala92]

With both volumes on zero, I still can hear the sound in the other rooms of the house.

I'll try the cap on standby switch.

Thanks.

[HotBluePlates]

With both volumes on zero, I still can hear the sound in the other rooms of the house.

The you have to have a ground problem in the volume pots, or in the filter cap feeding the 1st stages. I don't see any other way the signal could bleed through the amp otherwise.

[Jack1962]

are these the original pots or have they be replaced ? check ALL your grounds , if it isn't a grounding problem replace these pots .

[valhala92]

They're the original pots, I took 'em out and cleaned them and checked the ground connections, and I also measured their resistance, they were fine.
Is there something else that I should check?

Thanks

[sluckey]

Filter caps.

[valhala92]

but they're new, and I checked their connections, they were ok.

[valhala92]

And one more question, how can I bring the filter caps before standby switch, so they get charged as soon as I power up the amp?

[sluckey]

OK, give this a try. Use a gator clip lead to connect V2 pin 2 to chassis. Does that kill the sound?

[valhala92]

yes, it kills the sound

[sluckey]

yes, it kills the sound

Great. Now use that clip lead to ground the wiper of the volume pots, one at a time. Does that kill the sound?

[valhala92]

When I connect it to the wipers, it doesn't kill the sound, but when I connect it to lug 1 of the volume I pot (the one that goes to 22n cap), it gets a little bit lower and bassy.

[sluckey]

Do you have a 20uF to 50uF cap to use as a test cap? If so, connect negative to chassis and use clip lead to connect positive to each side (ONE AT A TIME!) of the dual can that feeds the preamps. Does that kill the sound?

[valhala92]

Yes I have a couple of spare caps, I'll try it

[valhala92]

No, it didn't kill the sound

[HotBluePlates]

I'll ask the stupid question: When you did that filter cap test, did you have both volume pots all the way off?

[valhala92]

yeah, both of them were on zero

[HotBluePlates]

Is the amp 100% stock, or are there any mods?

[valhala92]

100% stock, except for V5 power tube that was changed, I checked the wiring, it was like the schematic.

[HotBluePlates]

At this point, I'm stuck without seeing the amp in person to see/hear the problem.

If both volume pots and the filter cap were working properly, sound couldn't get past the volume controls when both are turned to zero. You're saying they're working, so we're at an impasse.

How I'd check volume control operation:
Clip your meter's black lead to a ground point in the power supply, the the PT center-tap. Clip the red lead to the volume pot wiper. Rotate the control to 0 and observe the resistance measured by the meter. It should be substantially close to 0Ω (under 1-2Ω at least).

I'd do the same with the filter cap: clip the red lead to the - tab on the can and measure the actual resistance back to the PT center-tap.

It might be possible that there is coupling between wires from the input stages to something later, but it's unlikely. It would be more likely for that to happen from an output stage wire back to a preamp tube stage (but you're not describing feedback/oscillation, so this doesn't apply).

[valhala92]

I measured the resistance:
Volume I: 13.5 ohm
Volume II: 10.5 ohm
Preamp filter cap: 0.7 ohm

[HotBluePlates]

The volume pot grounds should read the same 0.7Ω that the filter cap did.

So check those grounds wires for the volume pots, resolder if need be, don't assume the soldering of the ground wires running from the pot buss to the board are good, etc.

Those are the reason the volume doesn't go all the way to zero.

[valhala92]

From back of the volume pots to ground, the resistance is 0.4 ohm, but from their wipers  to ground, it's around 12 ohm (I measured it again, sometimes it's between 1.7 and 6, but sometimes it's around 10-12 ohm !!!).
I used a wire for connecting the wipers directly to ground, but it didn't solve the problem? can it still be a ground issue?

[HotBluePlates]

You'll have to answer the question for yourself, as I am not there taking readings and making connections...

If you turn the volume pot to "0" and don't get 0Ω from wiper to ground (meaning a "real ground" as seen by the power supply; the ground buss could always have a poor connection to "real ground"), then you won't get to zero volume.

So now you have to figure out why you're getting the different readings & symptoms that you do. Once the wiper can get to ~0Ω to ground, your volume issue will be solved.