Marshall VS100 Op Amp issue?

[allante666]

Hi. I have a Marshall VS100. Channel 1 (clean) works ok but sounds just a little wooly. Hardly noticeable and if you had a cheap budget amp that sounded like it you would be happy. channel 2 however whilst you can hear a little white noise when its turned right up, theres no sound through it, on OD 1 or 2. There is quite a draw on the -12V rail resistor R21 (42V one end and 26V the other end. This in comparison to -42V and -36V on the -ve resistor R20). Its not burning up but obviously running hotter than normal.
Im suspecting a dodgy op amp chip pulling on the rail but as theres several possibilities wondered if anyone had come across this one and could give me an educated guess which one to start with? The -ve rail is at -12V while the +ve is at 11.3v so that ties in with the big drop on R21.
And this despite the fact that the Marshall diagram shows the 12 volt regulators putting 15 volts out?? whats that about?

TIA
Andy

[Enzo]

That is simply a mistake in the drawing. 12v is rare, and the draftsman was so used to drawing 15v everywhere, he wasn't thinking. No big deal.

SO use your fingers, any of the ICs getting hot? Use a meter, any of the ICs have unwanted DC on an output pin? Use your scope or signal tracer, follwo the signal path and see where the input signal disappears.

[allante666]

Hi Enzo. Yes Ill be going through it with a meter and a signal tracer when I get chance, I just wondered if anyone had a pointer with the info I gave. None of them felt hot though. Ah Draftsmen, yes, I remember those! I almost became one when I left school. Just as well I didn't though Id be out of a job by now!

[olddawg]

Might try some freeze mist.. I would also suspect a failing 1uf or .47uf or such. The only way to really know is to troubleshoot as Enzo says.

[allante666]

a cap on the 12v rail you mean?

[olddawg]

Yeah... If you float the power supply pin of the suspected IC and the voltage is still being pulled down there's a reason. Here's another trick.... If you have an old Sony camcorder with IR (night vision) mode, it will quickly determine which components are warmer than others with a glance.

[allante666]

Damn I never knew that! I thought they only picked up the infra red light that they illuminated things with? I didnt know they worked a bit like a thermal imager? Wish I hadnt got rid of mine now!

[Enzo]

Another camcorder trick (and for all I know smart phone cameras) TV remotes and other similar remotes work on infra red. We can't see it but the camera can. SHine your remote at the camera and look at the image. You can see if the remote is sending or not.

[allante666]

Yes I knew about that one, however, it doesn't work with an iPhone for some reason, the earlier digital camera phones could see it ok. We used to also check TV remotes by holding them near a MW radio tuned mid band, you could hear the oscillator working.
Actually Ill take that back! It does work with an iPhone, just not as well as the old ones, it has to be fairly dark to see it.

[allante666]

UPDATE. problem sorted. I was looking in the wrong direction. My slightly low 12 volt was simply a slightly low regulator. New one brought it back up. the fault however was nothing to do with it or the op amps. I only noticed by chance that the LED that tells you which channel you are on (clean or OD) was switching between green or yellow. It suddenly occurred to me that it should switch between green and red. It was going yellow because the green part wasnt going out when you switched channels. This led me down the switching circuit route and I soon found TR1 on the pre amp board was leaking enough to keep the clean channel LED (and presumably the clean channel) switched on. Replacement provided a complete cure. I can only assume it was trying to use both channels at once, or similar.