Crosstalk channel bleed trouble shooting

[Wrongdog]

Okay - i have stared at this damn amp too long, trying everything I know to get rid of channel bleed.

Are these the things and approximate order of diagnosis:

Bad pre-amp tube
Ground issue at pots
Bad V pot(s)
Coupling caps
OT

Someone mentioned power tubes in a thread - why?

It is a Marshall major, loud as a Champ with both volumes off. Both Channels
interact with each other, output volume is low (loud as a concert or pro say), TMB are affected by volume interactions. It is the earliest design - it is a 1968 plexi. Here is the schematic:

http://www.drtube.com/schematics/marshall/200w.gif

This thing is making me nuts! It is the simplest preamp with that crazy KT88 power section basically nothing fancy. HELP!

[pdf64]

Check (by tacking known ones across them) the V1 cathode bypass cap and the last power supply cap (which feeds V1). Peter.

[Wrongdog]

Did the first (plus subbed in all coupling caps) but not that filter cap. Hmmm two last are original and the 1st filter caps were re-done. Thanks I will try that.

BTW This thing takes up my whole work area on my cluttered bench and I want it outta here! I have to put a power lifting belt on just to move it!!!

[Wrongdog]

Put a new set of Valve Art KT88's in it today from CE Distribution last night. Two were microphonic right out of the box. Get what you pay for I guess. I will see what CE sez'....

[Wrongdog]

CE distribution gladly let me return KT88's. Great outfit!!

[Mars Amp Repair]

the 52uf decoupling filter cap (with the 100K 2watt resistor across it) in the B+ line of V1 & v2 if weak or open can allow signal to bleed through to the next stage. Not sure it would make it blast, but I've seen many older amps that this cap gets weak & stops doing its decoupling job. The symptom is with the volume fully off, you still can hear sound. Just a thought. g

[Wrongdog]

Crazy as it may seem this amp does not even have the decoupling stage!! Here how the chassis is except the one I have does not have that canned cap at that 10K 2watt resistor and appears it never did?!!? Check it out:

http://www.fkh.dk/major/

Even the one in the amp pictured looks added in . The later ones have the can where the power filter caps are:

http://www.fkh.dk/71major/

I did find a major problem. Someone replaced a resistor and did not trim the end, stuck it through the turrett so far that it was a dead short on the chassis! It was not apparrent until I lifted the board up. That was tough to see due to the short lead dress to the pre amp tubes and no slack to lift the board and see the hidden bone head mistake.

Another trouble shooting rule to stick by - always check the last techs work. Not that you are better than them but where there is a change there may be trouble.

I still have cross talk but when I subbed in some caps it did cut it a lot. I am awaiting some parts (JJ 50/50uf filter can caps etc) and will get into this further in a few days.

Thanks for your help I think we are on the right path.

[Wrongdog]

Here is a picture of the one I am working on:

Marshall Major 68 on Flickr - Photo Sharing!

Notice there is no screw holes where the internal style can bracket should be and being an early model there is not a hole punched for a can cap like the later ones. What do you think about this???

[Wrongdog]

Okay - I re-capped the middle filter stage, replaced the volume pots (neither would get close to zeroing out and added the missing decoupling cap section it was supposed to have originally. It is now working fine. It is NOT a very good sounding amp IMHO. I had a 68 Traynor Bass Master II in from the same guy and it blows away the Martshall as far as tone goes. It is almost as loud too, but who needs THAT loud...

[Mars Amp Repair]

I used to have a Plexi Major when I was a kid...I used it for bass tho it was a guitar head.

Don't know if anyone mentioned it here, but I had to add a 'brite' cap across the 2nd channel volume control to get any appreciable hi's out of it...and I was playing a Gibson non-reverse Thurnderbird at the time with RotoSound Round Wounds! It definitely lacked in the tone department. I think I added like a .01 or something...been a long time on that one.

I made a 'Y' cord & bridged the 2 inputs together. I sounded pretty decent after that.
Is the presence control working properly...or doing anything? Could be a source of issues in tone, too.

glen

[Mars Amp Repair]

BTW WrongDog...dig the 'Lost in Space' spaceguy icon. have the DVD set. wonderfully cheesy stuff..gfromthatera, too!

[Wrongdog]

Glen - thanks for all your help. I put that cap back in that some one else had installed to the II channel - it helped a lot. The Y chord is the only way this sounds okay to me. the previous owner put white out marks on the controls and he must have always bridged the channels from the looks of the marks.

The presence has to be on 5 to get the eq to function at all. I subbed in a new cap but saw little change so I put back original. I left all that was original in the amp that I could that was not needed to be changed. It was a '68 and all.

I am perplexed by that missing filter stage! This was the earliest US version. These were one off amps - maybe they spaced?? Maybe it was the 1st US one to hit the states??! I put the cap right where it should have gone but just siliconed it in since the owner didn't want the bracket and holes drilled.

Ya! Lost In Space! Thats me. I am 53 and that was a fav. My quote is "Beer and loafing in Las Vega" - get it?! Hunter Thompson meets Will Robinson...I gotta get a life.