Audio Research D-90

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).

Relay had crudy contacts 0n mine - burnished contacts,Cramolined and ordered a new one - which I plan to leave taped to my chassis to remind myself that this is something to replace next time I have the covers off ( which i hope is a year or two).
BTW -ARC has made up a replacement cap plus mount for these amps that is pretty cool - they've attached a new and very probably superior cap to a sled that fits right into the holes in the board the old caps mounted to - mechanically this has to be a better way of doing it than the way we did mine -downside is cost - $60 per cap ( X 8 in a 90B).This will probably look more "stock" and also be a lot more rugged in terms of shipping the amp - not important to me - definitely a YMMV situation.
One of the reasons I purchased this particular amp was I knew it's service history - sold new by a friend of mine - owned by a lawyer who used it - but didn't tube roll - sockets where in good shape . Hate replacing tube sockets - so not much interested in units that have been rolled a lot.
Houseforsound - contact me if you'd like my old caps - I will keep one as a sort of reference - but you could have the other 7 if you want them - remember - these are almost 30 year old caps - they're not treasures. Even if tested with a standard cap tester - you really aren't going to know if they're "good". If you know someone with one of the big time ( big bucks) cap analysisers (spl?) (Sencore)- that would be able to tell what kind of shape your (or my) caps are in.Standard cap testers basically will tell you if the caps shot - but not much else.
Also - look into a solder sucker - you're going to need it to get those caps out of the board.

Yup -they're twistlocks. Another thing to look at is what color are the caps - if blue - they are probably CDE's - these are a known problem with this era ARC - they probably aren't blue simply because the odds are they would have failed if CDE's - grey caps are 400uF @ 400VDC - I think the sleded caps ARC sells are 450uF's - but don't remember the working voltage? Replacing the caps with the ARC supplied caps will be relatively straightforward - toughest part of the job will be de-soldering the old caps - you'll need to have the wife hang onto the chassis and pull the old caps out as you de-solder the old ones and suck the solder out of the plated thru holes they go into.This is one of those tasks that really does require two people to do and it will probably be handy to have her hold the new caps onto the board when you solder 'em in.The problem with trying to do it by yourself is a tendency to overheat things while trying to get every single atom of solder out of all the holes ( four if I remember right per cap). She would also come in handy when you are biasing the amp - she can adjust the bias pots while you hold your probes into the test points ( do get the MCM probes - well worth $40 especially with 6550's which are taller and wider than 6L6 series tubes -gonna be harder to get at the test points.BTW - in a minor whine about this amp -it bugs me that every other set of bias pots adjusts opposite in rotation - clockwise is increasing bias on one pair of tubes - next pair is counterclockwise for the next pair of tubes.
BTW - if doing it again - I'd probably go with the Russian Tung Sol 6L6GC's instead of 5881's in a 90B. Your power tubes do need to be matched pairs - not necessarily matched quads. My 90B came with semiworn out 6550's in it when i bought it . Lesson here is that with used tube products - never assume what's there is what should be there - a lot of old tube gear has glowing glass plugged into 'em that passed only two tests -1.) No fire 2.) makes sound .
House -shoot me your name and address via e-mail and I'll mail the 7 old caps off to you for the cost of the postage- while the price is right - very questionable given the age and inherent quality of these caps if they are worth bothering with.
Would strongly suggest waiting until you have the manual in front of you before resuming work on this amp.Really - talk to Leonard at ARC.

Checked -caps on my D90B were 400uF at 450V - not 400V as I posted above.