Marshall M1989 Popping Noise

[patlaw]

The complaint is that the audio dies after playing the amp for a long time. I left it on for a long time. When I came back to it, the 0.5a HT fuse had blown. After replacing the fuse, I powered it up. The amp sort of works, but there is some popping noise in it. I was unable to observe any arcing before I turned it off. I'm seeking suggestions for troubleshooting the problem. Should I remove the output tubes and monitor the earlier stages with a scope for the transients? All of the tubes are new (replaced by the owner before I got it.) The complaint was the same or similar.

Here's the closest schematic I can find. This amp is apparently an organ amp.

[drewl]

Yes, remove output tubes, check bias supply and high voltage supplies for excessive ripple.

[J M Fahey]

Scope amp from output to input to see where does popping get injected or produced, then focus on that area to find the cause.

It is obviously present on speaker out since you can hear it.

[patlaw]

Scope amp from output to input to see where does popping get injected or produced, then focus on that area to find the cause.

It is obviously present on speaker out since you can hear it.

I hope to have some more information this weekend. I'm finally able to correlate the parts with the incomplete schematic. It takes a while, but then the light comes on in my head. The sorta close schematic has UK values instead of US values, like the amp has.

[g1]

The sorta close schematic has UK values instead of US values, like the amp has.

You mean like a 6550 version?
And model # is M1089 ?

[patlaw]

You mean like a 6550 version?
And model # is M1089 ?

6550, yes. Model M1989.

[g1]

Ok, the thread title says 1089, so that won't help. Maybe someone can fix it. (edit: fixed, thanks!)

Maybe this schematic will have the proper values? For US versions, usually just the power amp has some changes. Is the preamp part of the schematic you posted correct?

[drewl]

The layout is the same, just the bias feed resistors are changed for 6550's.

My '73 worked fine with EL34's without those changes.

Years later I swapped the 150k feeds to 220k while tweaking it to be exactly like a '69 Plexi.

[patlaw]

Well, it looks like one of the (new) 6550s is bad. When I take the amp off of standby, one of the tubes glows (gray not red), puts a loud noise in the speakers, and eventually lights the light bulb limiter. The popping was probably caused by the tube going bad. The amp has a matched pair of new Tung-Sol tubes from Tubes and More, but the amp has been in storage since the tubes were installed, so the warranty has long expired. The owner has the old tubes, so I'm going to get those to see if the problem is solved. I moved the glowing tube to the other socket, and it glowed the same way. At least the problem is not a cold solder joint on the tube socket or a bad socket.

[patlaw]

The old tube solved the problem, so far. I have to let the amp run for a while to make sure. Antique Electronics said they can't sell me a single matched tube with the same measurement (47), but they have a pair of tubes with the same spec. At least that way, there's a spare that the owner will probably never use.

If you buy any tubes for stock, be sure to run them a few hours before you put them on the shelf. The owner could have saved a few bucks by running the amp a while with these tubes.

By the way, after removing the nuts holding it in, how hard is it to access the bottom of the PCB? Is there enough slack in the wires to get to the bottom of the board?

[patlaw]

Can anybody help with the question on removing the PCB?

[g1]

What do you need to get at?
I think worst case is you have to disengage the pots. I don't think I've had to, you can usually raise & tilt the board enough to get at it, just be careful not to burn anything with your iron.

First I would take some pics, so you can get the lead dress back the way it was before.

[drewl]

Nah, remove pots from front panel and the PC board nuts and it should flip right over.

[patlaw]

There's a resistor I'd consider replacing. It's seen too much heat even though it's still reasonably in spec. It's not worth a major operation to get to it. Thanks for the suggestions.

[patlaw]

It's bias time. As is, with the 6550EH's, the idle currents are 27 mA and 31 mA. The plate voltage on each tube is 359 volts. These numbers yield PD of 9.7 watts and 11.1 watts. The maximum dissipation of this tube is 42 watts. Half of that, assuming cold biasing, is 21 watts. Things seem way low here. Am I calculating the numbers correctly? The plate current is measured by measuring the DC resistance of the power transformer from each side to the center tap and then measuring the voltage across the winding from each side to the center tap.