Marshall 1987S Hiss/Hum Problems

[Tmay]

Hello all, I’ve finally decided to have a go at fixing my old Marshall which has been plagued with problems from the day I bought it, but sadly my knowledge and experience with amps is woefully inadequate so any help would be greatly appreciated. At the moment the problem is hiss/hum and intermittent volume swells (which may be a loss of high frequencies which give the illusion of a loss of volume) The hiss/hum is only present on the high channel and is not affected by turning up the volume, it is however affected by adjusting the treble, middle and presence controls. If I play through the low channel the sound will intermittently drop in volume/lose high frequencies.

What I know is defiantly wrong with the amp is the pot values, the 1M controls are right but all others a 470k which isn’t. I’ve checked the preamp tubes by tapping them with a pencil and one of them is defiantly passed its best, but when I tried to test them by pulling them out one by one to see if the hiss would stop the sound cut out completely. Is this what is normally supposed to happen? From I’ve read I should be able to pull them out and the amp should still work, but this is defiantly not the case, there is no sound at all.

Heres a link to the schematic
http://www.thinlizzy.de/Bilder/1987Schematic.gif

Any ideas on what might be wrong would be a big help.

Cheers,

Tom

[Enzo]

I tried to test them by pulling them out one by one to see if the hiss would stop the sound cut out completely. Is this what is normally supposed to happen? From I’ve read I should be able to pull them out and the amp should still work

Think about it. The signal enters the amp and goes through the first tube, then the volume controls, then through the second tube, then the tone controls, through the third tube and on to the power tubes. if we removed one of those tubes, how would the signal get past the empty socket.

yes, it is normal for the sound to disappear when you remove the tubes. But if you are chasing internal noise, you CAN pull tubes that come before the source of the noise and the NOISE will continue, but no sound will come out from the signal at the input jack.

You have two power tubes, if you remove one of them, the amp will still make sound. WOn't sound good, but it will function. What you may be thinking of is power tubes then. if you have four, some people remove two of them.

If you have hiss that doesn;t respsond to the volume controls but does to the tone controls, then i am led to the second tube. Am I correct that turning the volumes both to zero leaves the hiss still there? Try a new tube there, or swap it with one of the others just to see if things change. If it is not the tube, I then suspect one of the resistors associated with that tube. Resistors can get noisy.


Intermittant level problems could also easily be a bad tube or bad tube socket.

ALL the other pots are 470k????

[Tmay]

I did think it was odd. There is still a faint hiss when the volume is at zero and if is turned up slightly the hiss kicks in and is there after unaffected by the volume. The second tube was the suspect one when tapping, i'll get a new set and hopefully that will be it. Yes all other pots are 470k, no idea what the story is there.

[bluesfreak]

Volume cutting out on one input can be caused by the jack's switches being dirty, a known Marshall Problem, especially when they have FX Loops. I would give them a good clean with a glass brush and some DeOxit. However, since your tonestack seemes to be modified drastically I would evaluate if you can revert it back to its original values of 250k, 1M, 25k but ensure that the surrounding components are still the correct one as well (33k Slope Resistor, caps). Maybe you get already rid of the hiss by then...

[Mars Amp Repair]

Another thing to consider on an older amp is the decoupling filter caps. Not sure if it would cause hiss, but definitely can cause weird bleed=through issues when volume controls turned down.
That would be the dual 50uf cap at the top of the schemo with the 10K/1w resistor inbetween them. You could just jump that with a known good 50uf ~450V cap (or whatever the rating on the cap is). Lots of dangerous voltage there, so be careful.
Agreed that the modded tone stack could also be the route of all evil here...glen

[Mark Black]

Hiss + hum + volume change + Marshall makes me think of high frequency oscillation.

A scope on the output would reveal this very quickly, but in the absence of that a decent quality DMM set to read AC volts might as well (depending on the bandwidth of the DMM).

Just a thought...

[Tmay]

I rebuilt the tone stack and replaced the tubes but the problems still there. What would cause high frequency oscillation?

[AmpFix]

Bad grid stoppers?

[bluesfreak]

or no Gridstoppers at all? Check also the Layout, maybe somewhere Preampsignal and Pwoeramp signal do run in parallel?