Marshall 8080 white noise/gain/heat issues

[chazpope]

Hi, got this 8080 in a bad state, opened to find heater buffer R112 missing with wires cut off, replaced it and amp started to work - nice clean sound from power amp. No apparent reason if/why original R112 had failed.

Preamp also worked but with a huge amount of white noise that I traced to both sides of IC1 and indeed there was 9V DC on all its signal pins. Replaced IC1 and all white noise stopped, however sound on clean channel is now not clean - it sounds kind of crunchy and and on its 'crunch' setting there is so much overdrive that you can play SOTW without having to switch to overdrive channel You can't really get a completely clean sound out at all really.

So it looks like new IC has way more gain than it should, sound isn't great either.

My question is - is this normal to expect because I changed the IC? Should I be just trying new ICs or it there something else going on? Note what I put in was not a new IC, it's just one I had pulled form some old amp, didn't have a socket either. I won't get the new IC/sockets for several days.

Thanks
Preamp - http://www.drtube.com/schematics/marshall/pc0689.pdf
Power Amp - http://www.drtube.com/schematics/marshall/pc0689p.pdf

[g1]

Did you replace IC1 with the same number part or a sub? You should triple check your work in case there is a solder bridge or bad connection where you soldered the chip.
Verify that it is a preamp problem by running the pre out or FX send into another power amp, or running some other preamp into the FX return/power amp in of this amp.

[Enzo]

Or maybe your new IC is fine, and now that that stage works it has revealed another IC to be bad elsewhere. ANy other ICs with DC offset?

[chazpope]

The other IC seemed fine, and when I put it back together I saw that in fact you can get a clean sound as long as the pot is below 12 o'clock, after that is starts to distort a little. So I guess it is usable but it is quite different from other amps where clean means clean even if put on full. Next I changed IC1 with a new chip and there was no audible change - so I guess probably that's the way it is. I played a gig last night and the amp seemed to work well enough.

Bass knobs on both channels seem to not have much effect, the reverb is kind of shallow not sure these are features or problems though. Inside I hate the heater buffer solution - about 10W heat spent next to the high voltage cap. Also the +-15V circuit is cooking seriously - and by sheer conductivity it is also cooking the two 1000mf caps nearby.

I have an old 80W Peavey Bandit that does a similar job and now I wonder which of the two amps I should keep