Troubleshooting Marshall Mark II 100W Lead Master Vol. Model 2203

[faman777]

This 100Watt head is fitted with a quad of Svetlana 6550 power tubes, was in need of a cap job and the power tube sockets had started cracking and breaking due to heat and age I suppose. It broke down finally but not completely... it still sounds ok up to about half way up on the master volume. (the owner uses it with master at 2 oclock and preamp dimed)

Replaced all six dual 50 microfarad 500V cans and two 10 microfarad caps that are part of the bias supply. Also replaced the power tube sockets and upped the 1K screen grid resistors to 8 watt type that I already had. Replaced the power switch because it had a dead light. All that was just a matter of maintenance and it was in need of those things already.

After completing all the maintenance, the amp still sounds fine up to about noon on the master and then the ugly distortion starts again... sounds like arcing to me and I'm not sure what else could cause it, but where? I don't see anything happening anywhere that's visible. Inside one of the transformers??? It's just confusing to me that it sounds so good up to a point (and it's a loud point) and then it starts cracking up like that.... if I back it off a little it works fine.

As far as I can tell this amp is original in every way circuit wise

I don't have a scope so troubleshooting is limited to what I can do without one.

Does anyone have any suggestions or relevant similar experiences with cures????

Thanks in advance for any and all helpful suggestions.

[Chuck H]

Have you given any consideration to the preamp tubes?

[faman777]

Yes, I've replaced all the preamp tubes with new ones, even after they all tested good on my tester. I also replaced the 6550s with a matched quad of 6L6GCs just in case it was a power tube issue... re-biased after each change and the problem was still there.

I've checked the values of all the resistors in the amp , even lifting one side in several cases to get a good reading out of circuit.

I've never had an amp with a problem like this.... is it even possible for this to be a transformer problem when it works flawlessly up to a certain (very loud ) volume???

I dated the amp to 1979 and it's got a coating of corrosion over the outside like it got wet or over-spray of some sort that corroded the top of the chassis..... enough to cause several of the standoff screw heads to break off.

The internal wiring is all tied up in the original fashion with plastic and little rubber bands.... I'd hate to have to tear it all up just to test transformers, but I guess I may have no other choice. Then again how do you test for specific transformer problems when the amp is already working?

[Chuck H]

You could try this...

Temporarily solder a 1M pot across the downstream side of the PI coupling caps. Then you can turn down the power amp and determine if the problem happens in the PI with the master up half way.

My guess is that it won't but it's easy to do and could reveal something.

With the cossosion problem I might think that you have a ground issue. What you describe sounds like a parasitic oscillation. A bad ground can cause this. With the amp having worked fine for many years and no changes to the circuit or layout the attention has to be turned to what may have changed. There is oxidization on the amp such that screw heads are corroded away. Perhaps check the pot cases to make sure they are grounded to the chassis.

If you moved any leads during the cap replacement, put them back. Of particular importance is the NFB lead that goes to the presence control. It should be as far from any preamp leads as possible.

If you have a signal generator (or any low V AC source like a wall wart transformer or the like) you can test the OT. Then you'll KNOW if it's bad instead of guessing.

[faman777]

Thanks for the suggestions, Chuck! I did consider the corrosion problem to be a factor and double checked all the pots and ground connections.... oddly, the corrosion is mostly limited to the topside of the chassis.

I will test the isolated PI section as per your suggestion.

Not sure how a low voltage power supply test of the output transformer would reveal a problem since it works great up to the point of crack-up (wouldn't a low voltage test be the same as playing the amp at low to mid volume which it does perfectly well?)

It's really difficult to describe the sound it's making so I will make a clip of it tomorrow (when I can turn the amp up that loud) and post it on my website. It's not really the kind of parasitic oscillation I've heard when building an amp from scratch and trying to get the negative feedback to the correct value (although it may be... I tend to turn that off very quickly too). On some of those amps I actually chose to lower the gain as well, but this one never had a problem before and I want to leave it stock.

I'll post the link here in a few hours...like 14 maybe. I'm on central time USA, which is GW+6 I think.

Thanks again

[g1]

Two problems I have seen similar to yours in these amps:
1) Bad connection at lug/rivet of filter cap (faulty cap) - not likely as you have just replaced yours.
2) Bad connection from power tube cathode to ground. This one also had corrosion issues and the ground lug to chassis had to be cleaned up plus the solder connection to the ground lug was corroded but it was very hard to see the problem.
Worth a triple check anyway, good luck.

[faman777]

I made a file and posted it on my website at the following

page36

Just click the text as directed... the schematic for this amp is there too.

Since listening to it, it now sounds like some kind of oscillation to me too, but why would it just start doing this without any kind of gain modification... it's apparently worked well for years then it just started doing this noise (before the cap job and before the power tube sockets were replaced). I've thoroughly checked all grounds and redone them twice looking for any stray corrosion or anything. Lead dress is unchanged from the original.

[faman777]

Checked the PI section and found excessive voltage on the power tube side(plates of phase inverter before the .022 coupling caps) ....

The schematic says there should be 80VDC and 90VDC on the plates of the PI tube and I'm measuring 190VDC and 200VDC on pins 1 and 6. All other circuit values seem to be within spec or close.

I'm measuring about 455VDC between the choke and the two 10K resistors but my schematic does not show what voltage would be normal at that point.

I also took one end of the choke out of circuit to get a DC resistance reading and it was 112Ohms which I guess is correct.

Using a purely resistive 120W dummy load for the output and a test signal from a keyboard going into the input, I measured up to about 10 VAC across the dummy load at noon on the Master Vol with the preamp dimed. As I turned it up past that point the AC voltage across the dummy load dropped quickly to less than 4VAC.

[Chuck H]

Your PI plate voltages are correct. They shouldn't be 80V and 90V. More like 180V and 190V. Perhaps there's a "1" ahead of those figures that your not seeing.

I'm still thinking it's an oscillation caused by a ground fault. Perhaps recheck the grounds for the preamp filter caps. It may help if you post the schematic your looking at in case it's for some reason different from the circuit we all know.

What is the bias current?

[faman777]

Thanks for the clarification on the voltages , Chuck.... The schematic is posted on the page with the sound clip(link below). the 80 and 90VDC labels were in a box which could easily have had a one in front of them which was obscured by the box edge.

I have about 460volts on the plates and bias set at 55ma(per tube) right now.


link to sound clip and schematic: page36

[Jazz P Bass]

Isn't 55ma per tube a little excessive?
The schematic shows -62Vdc for a bias voltage.
What do you read on any pin #5 of the output tubes?

[g1]

Also looks like a 1 missing from the voltage for V2 left side shown. (should be 170, not 70 volts).

[Chuck H]

55mA should be fine for 6550's. Not for 6L6's or EL34's though. I'm thinking, You've been checking grounds and you replaced the filters with no change in the problem. You also plugged in all different tubes with no change in the problem. I'm still leaning toward a ground fault that just hasn't been detected yet but you may want to test the coupling caps for leakage. Also, try the isolated PI thing. Also, as suggested, test the voltage at each power tube grid.

[faman777]

Just finished replacing all 6 coupling caps.

The voltage reading on all tubes at pin 5 is -42VDC with no signal.

Testing revealed no change in the problem.

According to my schematic I should have 250VDC on the plate(pin 6) of the high gain input or first stage and I measure 197VDC.

Oddly enough pin 1, the plate for the second stage, has nearly the correct voltage according to my schematic which is 220VDC and I measure 218VDC at that point.

[Chuck H]

I don't know who put that voltage figure in there but it's goofy. With a 2.7k cathode on the same +V rail as the following stage with a 10k cathode it's certain that the first stage will be drawing MORE current at idle. That will cause more voltage drop across the (equivalent) 100k plate resistor. So the first stage plate voltage must be lower than the second stage.

I'd say that your voltage readings are correct so far regardless of the info on the schematic.

Have you done the isolated PI test yet?