1.) There are three versions of the D-90 : 90 ,90A and 90B.The obvious differences are power tubes ( 6550 in original and a, 6L6GC in b) and power output 90 in original and a ,80 in the b.Don't know if there are differences in the smaller inverter/driver tube compliment. Basically -which variant do you have?
2.) Hum. How loud? Can only be heard with your head in the speaker or is audible from your listening position? You could try putting a ground lifter on the power plug - but since the problem is only in one channel - this probably won't do it.
3.) If you're going to work on this amp -you need the manual. Talk to Leonard at ARC and order one for $10 -you aren't going to get anywhere without it and can definitely make a small problem much worse by doing the wrong thing.Also, the cap bank on this thing stores more than enough charge to hurt you - even if the electric shock is only VERY unpleasant - there's a definite chance of hurting yourself jerking into something else in reaction.MCM makes some test probes that are the thing for servicing these amps -a sharp tip slides out of the probe - it's insulated until the very tip which is a sharp point you can stick into the solder TestPoints called out in the manual.Keeps your hands away from the nice hot power tubes.
Here's what we ended up doing to my D90B :1.) Replaced every single electrolytic cap in the thing . These amps are reasonable to work on - you have to decide how you want it to look when you're done. I decided I wanted to use 105C rated caps and ended up getting some caps that required drilling holes in the circuit board and building new traces with solder wick.Works fine - but my amp is no longer mechanically as rugged as it was when stock - don't think I'd want to ship it across the country after this treatment.This technique works fine and saved me about $350 on caps - so I'll live with the consequences.Don't just replace the filter caps -do the smaller 10uF etc. as well ( no brainier for me - one of the 10uF/150V caps had bulged and leaked).
2.) replaced every tube in it with fresh new tubes.
3.) Trouble shot a F#$%^&ng miserable intermittent problem for two solid afternoons .Turned out to be bad out of the box 6922EH ( emission problem that only showed up when you taped the tube while testing it) and a single Sovtech 5881 bad out of the box. 5881 Tube would be very noisy at start up - then quite down after 45-60 seconds of spiting and loud frying pan sounds - simply eliminated everything else and by substituting tubes -determined that a single tube was the source of the problem- replaced all power tubes with fresh recreated?Tung-Sol 5881's from Jim McShane (highly recommended).End of problem.
4.) Although about two thirds of this was spent trouble shooting the problems - have 15-18 hours of bench time into this amp( a nightmare).Arguably I have the kind of money into this project that basically takes a inexpensive used amp and pours so much money/time into it that it's like I bought it for new retail.
5.) Only have a few hours of listening in at this time - but am happy with the sound.because this particular amp has been gone over repeatedly top to bottom and side to side by three techs and all age/use sensitive components have been replaced - anticipate years of use out of it.This is by far the most powerful tube amp I've owned and really is sweet.
One final note - after lugging this beast in and out of the shop repeatedly -I've got a whole new appreciation for my Quicksilver Monblocks -way more practical design in terms of transportation ( and they sound great).
