Marshall Super lead 1959 Problem

[COLMMCSHERRY]

Hi, i have a problem, when my guitar is plugged into channel 2, volume 1 effects this channel, But if plugged into channel 1, volume 2 has no effect.
I guess channel 2 is bleeding into the channel 1 pot some how. Both channels are slightly distorting... but intermittently... Plus there seems to be a constant low level buzz happening, Thanks in advace for any help!

[Jazz P Bass]

Marshall 1959

Did you try changing out V1 tube?
WTF ! Who drew this thing?

[COLMMCSHERRY]

Hi, yeah changed the V1 tube, but makes no difference what-so-ever... Ha ha, thats how marshall do schematics these days! lol

[Jazz P Bass]

Leaky Capacitor

Me thinks that you may have a leaky capacitor.
Check the pot side of C3. There should be no Vdc.
Volume 1 will be a little tricker to catch, what with six capacitors.

[COLMMCSHERRY]

Ok i have checked the pot side of C3 to ground, No Vdc, also checked points around vol 1 to ground, No Vdc..... Or should i be checking across C3 and the other caps?


By the way, have a great and peacefull christmas!!

[J M Fahey]

1)

WTF ! Who drew this thing?

Marshall did
2)

I guess channel 2 is bleeding into the channel 1 pot some how.

Yes indeed, their cursors are connected together by 470K resistors, *very* interactive.
If so, why the difference?:
Because the Ch1 resistor is bypassed by a 470pF capacitor, to give it extra treble.
If you move V2 end to end, V1's attenuation varies around 3 dB, not very noticeable, *but* if you plug into channel 2 and move V1, you not only change its gain by about the same amount, but you also in certain positions you kill most hi mids and all treble, which is *very* noticeable.
The slight hum and buzz: "welcome to Tube World"

[COLMMCSHERRY]

Thanks for the response, the buzz is not your average tube hum, the usual tube hum is there, with other tube amps i have used, when you switch standby you hear a slight pop and you can hear your hum..... When i take this amp off stand by, there is no sound till about 4 seconds later.... If i take out v2 the buzz goes away.... could the 50uf+50uf in the peramp stage be causing the problem?

[J M Fahey]

Hi. I distinguishez hum (low rumbling) from buzz (sharp and spikey), yet both come from the 50 or 60 Hz line.
Mainly from grounding problems, one of the most difficult parts of amp building, even for the factory itself..
I don't think you have a filter cap problem.

[COLMMCSHERRY]

Cheerz dude..... This prob has me baffled!

[bluesfreak]

I think the buzz u r referring to is some stray interference from the PT into the OT, something I've noticed on a lot of Marshalls. In most cases I was able to resolve it by giving the iron of the tranny good contact to the chassis by e.g. removing the laquer on the mountings and/or adding a star washer to the mounting bolts...
For the interaction between Ch#1 and#2 => thats normal to some degree but I also noted in the past that sometimes the switching contacts on the Input Jacks can cause strange behavior when corroded...

[COLMMCSHERRY]

Well the buzz goes away if i take out v2, i replaced all the caps in the tone circuit..... maybe its a failing anode resistor (100k)... although it checks out ok on my meter....


When plugged into Channel two, with channel two volume at zero, Channel One volume control can raise up too loud levels but awful sounding, and crackling....

But the other way around when plugged into channel one, with channel one volume at zero, channel two volume has no affect whem raised up.....

Merry Christmas to all.....

[guitician]

My guess would be that the input jacks are not shorting like they are designed to. What happens when you move/wiggle a plug in the ch.1 jack while plugged into ch.2 with it's vol. at zero? I've noticed this with my Cliff jacks on my amp.

[Jazz P Bass]

Shorting Jacks

You could also insert a cord in the jack and jumper tip to ring.

[COLMMCSHERRY]

Tried the jacks.... wiggling them and using a shorted jack plug.... problem is still there..... think ill put a signal in before the phase inverter to make sure the power amp isnt the cause of the voulume spikes/crackles....