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

[faman777]

About that PI test.... did you mean I should solder the 1M pot after the PI and just before the power tubes and if so should I isolate the pot on the power tube side with temporary coupling caps as well? Essentially a post phase inverter master volume?

Sorry for the need to clarify, I wasn't sure what I'd be checking after I did it the first time and realized I had put the 1M pot on the PI side of the coupling cap coming from the master volume. duh.

[Chuck H]

Yes. After the PI coupling caps, before the power tube grids. No need for the additional coupling caps.

[Gtr_tech]

Sounds like shorted turns in the output xfmr to me. I've seen (heard) this problem several times and got to know it quite well. Even tho the output xfmr tested fine, there was an ugly "blatty" distortion at higher volume settings. Replacement output xfmr cured it.

[Jazz P Bass]

You need a scope to nail this one.

[faman777]

gtr_tch, did you listen to the clip at the link in several of my posts above? (page 36)

it sounds fine at first, you can hear more hiss as I turn it up several times. It still sounds ok then at almost a minute in, I turn it up just past the breaking point and it sounds squashed with ugly distortion. Then I turn it down again and it's fine.

I have suspected the OT from the beginning, but I don't have a scope and since I've never seen one do this, I've been hesitant to throw that expense at the owner without knowing for sure if that would cure the problem..

Chuck, Haven't had a chance to do the volume pot thing yet (other work). that's on my to do list.

[km6xz]

I was following this thread reading the advice and progress but when you mentioned in the last post(#20) that it was not yours I felt compelled to intrude.

Wouldn't the owner be better off having the problem repaired before more throwing wasted money on parts that did not need replacing? His total cost would be a lot lower if a competent tech with the needed test instruments had looked at it from the beginning.

Without a basic set up of test instruments, troubleshooting and rebuilding an amp that belongs to someone else is a very questionable practice, unethical if you are charging. It would not be an issue if the amp was yours and what you did to it would no ones business but yours.

This case is one that calls for a tech who can diagnose the problem and fix the real problem before any more playing around and changing parts needlessly occurs. If the owner is a friend, you might lose a friendship, and if a customer, from whom you expect to receive money for the work, you could be sued.
Until you have a basic competent setup, don't work on other people's gear.

[faman777]

Reality Check?

Thanks for expressing your concerns, KM6xz, but the amp already needed the maintenance of a cap job and the power tube sockets which were baked and chipped.

I have not charged him for anything other than parts and since he's already had other local techs work on it without success, I told him I'd do the maintenance on his spare amp to see if that fixed the problem. No harm, no foul ...

Of course any useful suggestions would be appreciated as well. As for my lack of qualifying tech equipment, times are hard here in the states. Worse for some.

it's OK... really... every thread needs a reality check now and then, but don't worry I won't lose a friend or make an enemy here or there or anywhere...

but if I'm able to fix it for him, I might learn a thing or two and help a guy that couldn't afford to send his amp back to the guy who takes six months and charges him too much for nothing. And he will appreciate that.

[Krwkka]

Wow! I just listened to your clip! I don't know what it is but it sure is an interesting sound.

[Euthymia]

I'm with Gr_tech.

It's rather unusual (in my shop, anyway), but I have seen similar symptoms that turned out to be the output transformer secondary arcing. I had a hard time with it until I had removed the OT to swap in a temporary replacement and with the original OT still connected, I cranked it up and a spark jumped from the transformer's frame to the amp's chassis. Ai-yi-yi.

If you have access to another OT to swap in, try that.

[faman777]

Thanks for the input Euthymia, I do have access to another OT.... and I'm not afraid to use it.

I won't be able to get to it until next week, but I'll post results when I get done.

[Amp Kat]

I used to have one of these and used to cut out alot and it was tube sockets so you may want to consider the preamp ones also if you haven't changed them. Sounds like it's either losing a ground when you turn it up that vibrates it loose or a capacitor discharging. One thing you may try is isolating the problem to either preamp or power amp. If you can inject a tone of anykind around 1volt RMS to the PI coupling cap that would be like a return in so if the problem persist you know it's in the power section. If you have no tone source generator then take the signal off of the 1st coupling cap and lift it taking the signal path away from the chain and wire that to the PI bypassing the rest. To check the preamp you could take the signal off after the PI Coupling cap and wire that to a 1/4" plug and send that to the return of another amp. If it does it still the promblem is from the PI back.

One more thing is these chassis rust very easy so it doesn't take much to get them rusting espescially in the South.