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

[jjman]

If all but one pot has been changed someone has been in there committing mischief. You may need to confirm if other mods have been done to be able to isolate the problems. The fact that the hiss/hum is only on the bright channel yet is not subject to it’s volume control seems like a contradiction to me based on the schematic. I wonder if someone may have cascaded the 1st tube as Marshall did on some other models.

Posting pics of the internals might reveal any mods.

[Tmay]

Ok here are the innards

Left side
http://i990.photobucket.com/albums/af25 ... 1252045509

Right Side
http://i990.photobucket.com/albums/af25 ... 1252045507

Another thing im unsure about is the power resistor on the power tubes. I think there 1K 5Watt, is there anything particular special about the ones being used or would any type of power resistor (ie metal film) work the same? The Power resistors seems to be connected from pin for 4 to 6 and as far i am aware pin 6 isnt used by the tube. The only schematic ive found of the 1987 which has the power resitors has them in series with the incoming connection to pin4 and pin 4 rather than whats going on with my amp. Is the way mines set up right?

[Structo]

Those screen resistors are fine.

It's common to use pin 6 as a tie point for the screen resistor.