Marshall JCM 2000 DSL 50 gain loss troubleshooting

[Volodymyr]

Hi folks,

I recently bought my first Marshall the JCM 2000 DSL 50.

I have spent a few days, playing around with it, switching between channels and mods, trying to pool all possible sounds from it.
Some moment, when I was playing on a red channel Lead 1, the gain suddenly disappeared, the volume dropped a bit and the sound became thin and lifeless, all lows went as well, sounds like a wah-wah.

At the same time, the green channel is still functioning as should, from a clean to punchy JCM 800-like tone.

Cleaning up the tubes and sockets, and trying new tubes - didn't help at all.

I also didn't find any blown, burned, or unsoldered components inside the amp, it looks new.

I'm not good at schematic reading but I was able to find the signal paths for both channels the red channel uses additional, V1b, gain stage, and the green one skips it
I highlighted it on a schematic

I have tested the relay that switches channels, and it works as expected, the signal switching from CON6 to CON4 (at least the multimeter shows some AC voltage at corresponding pins)

The optocoupler changes its resistance as well

My last idea is to check the tone stack, but what to look for?
So I'm out of ideas, does anyone have an idea where the issue maybe?

 

[Volodymyr]

can anyone help?

 

[Tatzmann]

Yes, i could. Problematic is that those amps
can have a crapload of problems that can be responsible for this kind of behaviour.

It would be best to hand it to a VERY good ampguy that can solve this problem.

(There will be a non-tech guy posting soon that says just buy a new circuitboard (presently unavailable!).

He thinks Marshall kicks out those (still buggered!, just ask around users here!) circuitboards until 2030. Only 250 dollars!

 

[Volodymyr]

Unfortunately found VERY good amp guy it's a lottery, I had a negative experience with some tech who created a mess of soldering in the amp and fixed nothing
I used to fix all my previous amps by myself, but this one is much more complex than a single-channel one like Orange tiny terror

I returned back to fixing my first marshall, I heated the amp couple of minutes and measured DCV and Amps on the first 3 tubes
I suppose the results on the V3B are so high because of the cathode follower

The V1 is JJ high-gain and V2 is Sovtek also a high-gain tube
The V3 is the default marshall tube

Do the results look normal for marshall?

Position	Plate voltage to ground	Cathode voltage to ground	To each other	milliAmps
V1A	188	2	194	1.77
V1B	262	2	267	3.45
V2A	315	3	329	3.36
V2B	291	3	297	3.84
V3A	252	2	259	4.32
V3B	444	266	175	4.25

 

[Vesperado]

I don't have a DSL 50, so all I can suggest would be (1) ensure all internal connections are seated properly: you may have to reseat and inspect for broken pins, pinched wires (2) replace suspected electrolytic aluminum caps as that amp is 20 years old, being mindful of polarities, (3) check your external connections for switching, it's possible your Red channel footswitch wire got pinched, (4) reflow footswitch jack with fresh solder, and (5) sleep on it if all else fails, chances are new ideas will surface which offer a solution.

 

[Franktvster]

Hi folks,

I recently bought my first Marshall the JCM 2000 DSL 50.

I have spent a few days, playing around with it, switching between channels and mods, trying to pool all possible sounds from it.
Some moment, when I was playing on a red channel Lead 1, the gain suddenly disappeared, the volume dropped a bit and the sound became thin and lifeless, all lows went as well, sounds like a wah-wah.

At the same time, the green channel is still functioning as should, from a clean to punchy JCM 800-like tone.

Cleaning up the tubes and sockets, and trying new tubes - didn't help at all.

I also didn't find any blown, burned, or unsoldered components inside the amp, it looks new.

I'm not good at schematic reading but I was able to find the signal paths for both channels the red channel uses additional, V1b, gain stage, and the green one skips it
I highlighted it on a schematic

I have tested the relay that switches channels, and it works as expected, the signal switching from CON6 to CON4 (at least the multimeter shows some AC voltage at corresponding pins)

The optocoupler changes its resistance as well

My last idea is to check the tone stack, but what to look for?
So I'm out of ideas, does anyone have an idea where the issue maybe?

Hello, what year is your amp ? I will try to help you

 

[Chad Brockman]

Dr.tube is still selling the bias mod for this amp. But you will be building the whole bias circuit off the main PCB and cutting traces and directly wiring to the tubes. This is where I would start because of the inherent problem with this amp is the PCB becoming
conductive leading to your bias running away. After that repair is done then a full cap job would be done because of age of amplifier. Trust me when I say caps just don't last forever think of it as a oil change for your amplifier. I have done this many times for the amplifiers that I have bought and it have fixed many problems with them that started to show up.

 

[Pete Farrington]

I heated the amp couple of minutes and measured DCV and Amps on the first 3 tubes

How exactly are you measuring current? I suspect you’re doing it wrong, it’s different to measuring voltage, as a current meter is a short circuit. eg consider how to measure the current your bedside lamp draws.

I suppose the results on the V3B are so high because of the cathode follower

The schematic has V3A as the CF?
To avoid such confusion, it’s best to refer to pin#, rather than A or B sections.
Whatever, 266V at the cathode is nuts.
Most of the other voltages seem excessive too.

 

[Volodymyr]

Hello, what year is your amp ? I will try to help you

Thanks
The amps are 2005

 

[Volodymyr]

Dr.tube is still selling the bias mod for this amp. But you will be building the whole bias circuit off the main PCB and cutting traces and directly wiring to the tubes. This is where I would start because of the inherent problem with this amp is the PCB becoming
conductive leading to your bias running away. After that repair is done then a full cap job would be done because of age of amplifier. Trust me when I say caps just don't last forever think of it as a oil change for your amplifier. I have done this many times for the amplifiers that I have bought and it have fixed many problems with them that started to show up.

I don't think this is a bias issue, I have checked bias is stable during a couple of hours of playing
also, the green channel works as expected, so there is a problem somewhere on preamp side

 

[Volodymyr]

How exactly are you measuring current? I suspect you’re doing it wrong, it’s different to measuring voltage, as a current meter is a short circuit. eg consider how to measure the current your bedside lamp draws.

The schematic has V3A as the CF?
To avoid such confusion, it’s best to refer to pin#, rather than A or B sections.
Whatever, 266V at the cathode is nuts.
Most of the other voltages seem excessive too.

For measuring plate voltage I used pint 6 and 1 to the ground
For measuring cathode voltage I used pint 8 and 3 to the ground

Then for getting the voltage between the plate-cathode, I used pins 6 to 8, and 1 to 3 for each tube

For current, I used the exact same amper-meter, dial on a 20m, measure between legs 6 to 8 and 1 to 3
Shouldn't the internal resistor prevent a shortage?

I just repeated the measures, the results are nearly the same, do the numbers say something to you?

 

[Volodymyr]

I don't have a DSL 50, so all I can suggest would be (1) ensure all internal connections are seated properly: you may have to reseat and inspect for broken pins, pinched wires (2) replace suspected electrolytic aluminum caps as that amp is 20 years old, being mindful of polarities, (3) check your external connections for switching, it's possible your Red channel footswitch wire got pinched, (4) reflow footswitch jack with fresh solder, and (5) sleep on it if all else fails, chances are new ideas will surface which offer a solution.

I checked every soldering on the PCB, and unplugged and plug again every wire, but nothing really helped
I suppose some capacitor or resistor has died but still looks new

 

[rockgod212]

pics of the pcb would help? so many things can go wrong in this amp. try changing the preamp tubes out with known good ones. FYI the green channel doesnt have lots of gain, the red channel has the gain. sound clips would help too.

 

[Pete Farrington]

For current, I used the exact same amper-meter, dial on a 20m, measure between legs 6 to 8 and 1 to 3
Shouldn't the internal resistor prevent a shortage?

The readings are nonsense. I hate that this will sound condescending, but you need to research what a current meter is and how to use one.
Again, think what would happen if you tried to measure lamp current using that method (hint - the meter would be a short across the mains and its fuse would blow).

I just repeated the measures, the results are nearly the same, do the numbers say something to you?

Just that they seem rather high. Sorry, I’ve no experience of DSL amps. I agree that the schematics are ridiculously convoluted.

What does the heater VAC measure, between pins 2 and 7 of any of the output valves?

 

[FleshOnGear]

For measuring plate voltage I used pint 6 and 1 to the ground
For measuring cathode voltage I used pint 8 and 3 to the ground

Then for getting the voltage between the plate-cathode, I used pins 6 to 8, and 1 to 3 for each tube

For current, I used the exact same amper-meter, dial on a 20m, measure between legs 6 to 8 and 1 to 3
Shouldn't the internal resistor prevent a shortage?

I just repeated the measures, the results are nearly the same, do the numbers say something to you?

I think Pete meant to reference the measured voltages to the pin number on the chart, so there’s no confusion between sections A and B. It’s just more precise, and it looks like you might have mixed up A and B on your chart.

All of your voltages seem quite high except for maybe the first one. It looks like a couple of the gain stages are biased nearly to cutoff, or maybe the tubes aren’t conducting, which is why Pete suggests measuring the heaters. This would affect both channels, though, not just the red one. It still doesn’t explain the symptom.

 

[Vesperado]

I checked every soldering on the PCB, and unplugged and plug again every wire, but nothing really helped
I suppose some capacitor or resistor has died but still looks new

Resistors typically don't go out, unless one gets fried, in which case it would be visible. It is possible some capacitor needs replacing somewhere in the signal path on the Red channel, possibly even a bleed to ground which might be shorting out (and might explain the loss of signal you are experiencing). Perhaps it would be good to run the amp, visit every channel, sweep every knob, press every button, and annotate all symptoms. That might elude to narrowing down the culprit in the circuit. Chances are if your Green channel is up and running normal, the suspect lies within the Red's independent components.

 

[Volodymyr]

The heater voltage for preamp tubes

For v1 and v2 is 7.4 DC
For the v3 and v4 7.3 AC

the power tubes have no pin numbers and are hard to reach not shorting something, will try tomorrow one more time

I uploaded a video with both channels, a creepy smartphone mic was unable to pick the lower frequency of the green channel so on the video they sound very close to each other, but in the reality, the red one is much thinner and has lower volume as well



I did some experiment
I swapped outputs from the tone stack PCB so switching on the green channel the signal goes straight to the second gain stage and switching on the red one, the signal skips it

The tone has changed a bit, but the overall picture is still the same, the red channel still has a lower volume than the green one
I suspect the issue is with the red channel on the tone stack board
I attached the schematic, pin 3 for the green channel and pin 5 for the red one
The photo of the board also shows both PCBs

 

[Vesperado]

I hear the bass reduction, but no perceivable volume drop from the video. At this point in time, I think you need to measure every component for tolerances for both channels, as the Green sounds bloated to mine ears. Sometimes changes occur on the production floor which are not annotated on the official drawings.

Considering your observations, I don't suspect one tube/valve to be of any concern here, it likely is a tolerance variation from a component or two large enough to cause the effect. That, or the previous owner did a modification to the circuit and kept it to himself. So you can try a few things. First, decode every value from every component from its number or color band. If you are unsure, you can lift one leg of a component and verify with a multimeter. Then, reflow every component on the board, including the pots and switches. By then, something should have disclosed itself to you. My guess is, that, a 470k resistor into a stage is off a little, or perhaps a 1M to ground; those have the most effect on perceivable signal strength and bandwidth.

Take a few days to cover the entire circuit, don't rush, and take plenty of breaks.

 

[Pete Farrington]

The heater voltage for preamp tubes

For v1 and v2 is 7.4 DC
For the v3 and v4 7.3 AC

Should be 6.3V, they’re 16% too high, the limit is 10%, you need to sort that.
eg what is your actual measured wall voltage, what mains voltage is the amp set for?

the power tubes have no pin numbers and are hard to reach not shorting something

Yes, it’s far too hazardous to use regular meter probes on output valve sockets. Use insulated, clip-on probe adapters.

 

[Volodymyr]

The amp is for US voltage, I just measured the socket it shows 135 AC, It explains where the 7.+ volts on heaters cames from