Marshall JCM800 2205 Problems

[gibson17]

Hello All,

I have a Marshall JCM800 2205 50 watt head. I had posted on here some time ago about a drifting bias and the problem was cured. What I did was replace the trim pots, all capacitors and filter caps. What is happening now is the volume fades out as if a power or preamp tube is bad, so I tested all tubes and they were fine. I know the real test is in the circuit so I figured I would go ahead and retube everything. So I have the same problem with multiple sets of tubes. Any ideas on what the problem may be? I look forward to your responses. Thank you for taking the time to read this and have a great day!

[Jazz P Bass]

The only way to know where (which circuit) the problem is in, is to monitor specific points.

Divide to Conquer.

I would start at the output tube grids.
Measure the Vac signal level & see if it drops out there.

If it does, then you have to work your way back towards the input jack until you get to the point where the signal remains healthy.
The problem will be after that.

[52 Bill]

Agree with Jazz, isolate the problem first then fix.

Have you ruled out the basic things like dirty contacts on the break jacks or speaker jacks, etc.?

[Mark Black]

In addition to the above a true 'fade' in volume makes me think immediately of a heater problem, so one easy test might be to observe in a dimly-lit area to see if one or more tube heaters quit glowing.

[gibson17]

Thanks for the replies. I check things in reverse #4, then #3, then #2. I am not very familiar with testing off the pins. Any tips on post #2? I usually use a bias probe if an amplifier does not have internal test points like say in a JCM2000. I am a tinkerer who does things sometimes out of necessity. I think I have enough knowledge to be dangerous. I am at least smart enough to discharge caps before working inside anything powered.

[52 Bill]

There is an FX loop on this amp start by trying a spare patch cord from the send to the return jack. Any help?

Next try and send a signal into the return jack, any sign of the problem? Now take a signal from the send jack and plug it into another amp, any sign of the problem?

If you can isolate the problem to the power amp or the preamp, it will save you from checking everything.

[gibson17]

Hello all, I have been pressed for time so I have not had a chance until yesterday to tinker with the amp. I did notice the amp was making a bit of noise when it was sitting idling with no input... seems odd. Anyhow, I ran the tests as you all instructed me. We'll start with the tubes. I was getting 475vdc on pin 3/4 of both output tubes with the exception of one of them reading at 472. I was also getting -45 on pin 5 on both tubes which I believe is my bias setting?

I also looked at the speaker jacks, DI jack and effect loop. Some of them appeared to lose their spring, and I thought eureka, but noticed that they do not ground out against anything with nothing plugged into them. They are the plastic jack types so I cannot see if they are worn or oxidized, I applied a little de-ox it anyhow.

I did the tests on the effects loop with a jumper and the amp seemed a but more alive. I then put a signal through the effects loop and it worked fine. I also sent the signal to another amp and there was no change. I removed the loop in between each step and played it for a bit and there appeared to be no change. I played the amp for about 3 hours with another guitarist and the amp did not show the volume drop. The problem is intermittent and sometimes appears after 10 minutes of playing and sometimes it takes hours. Thoughts/advice?

[Jazz P Bass]

You pretty much identified the problem as the loop jacks.

The Return jack is the usual culprit.
It's not that it 'grounds out' the signal.
A fully functioning jack must 'Pass' the signal.
Corrosion or lack of tension will interfere with the signal.

It may be that the DeOxit & retensioning fixed it.

If in doubt, run a permanent short length jumper cord from the Send to the Return.
End of that issue.

[g1]

Leave the patch cord in the loop between send and return. Does this cure the problem (for more than 3 hours)?
If the problem is gone, try de-ox of the return jack again. Or replace the FX return jack if cleaning doesn't work.
If you still get the problem intermittently with the patch cord in the loop, then the return jack is not the problem.

[gibson17]

Ok, will test it with the patch cord for 3+ hours and see if any symptoms show up. Is there a specific way to retension the jacks? It seems like a throw away plastic jack as there is no way to get into the input jack like say a switchcraft where everything is exposed. Lastly, the amp has developed a bit of a buzz with no input. I replaced the input jack when I was replacing the capacitors as the original jack was noisy when nothing was plugged into it and if I pushed one of the leafs down to ground it out the noise went away. Perhaps I am having a grounding problem somewhere as well? Thank you again for all the help, I really appreciate it.

[g1]

Doesn't it use jacks like this?

[gibson17]

Yes it uses jacks something close to that. From my experience there's no way to retension those. I've tried pushing in the middle and using needle nose to try and squeeze the leaf but have never been successful. Either no tightening has been done or I've destroyed the jack. Is there a trick/tool or something I'm missing?

[The Dude]

Best to just replace them in my experience. As a temporary fix, insert a plug so that the jack contacts lift from the switched contacts. Carefully bend the ends of the jack contacts downward slightly so that when the jack is removed they have more downward pressure on the switch contacts. Again, I wouldn't call this a permanent fix, but it does work............sometimes.