For any of you with Mac C-20s, here is the result of my "last-chance" repair performed by Don Sachs.
I had tried four different repair shops over the past five years (three of the repair shops were listed on the MacIntosh Laboratory Inc. web page). The last two repair shops identified the volume control as the source of the main problems, a lot of hiss and very bad L-R balance. Neither shop could repair the volume control. One of them tried and made the problem worse.
One service Don Sachs offers is he is able to replace the original volume control pot with a new pot by Gold Point, so I shipped my C-20 off to Don. This is part of an email Don sent, outlining the work he preformed:
"What you are hearing is the result of three changes. First, the Gold Point solved all of your balance issues, and possibly the noise. Plus, the stepped attenuators will always sound very clean and clear with pinpoint imaging. The second thing you are hearing is a much better power supply and filament supply. The diode change makes the filament supply quieter, plus I used more capacitance in there for the same reason. This contributes to a blacker background. I used two really good F and T caps in the power supply to replace the big can that was on top. They are probably better than the cans. Finally, I added an extra big 20 uF film cap as the last cap in the power supply and changed the three or four resistors from the old carbons (which should never have been re-used) to modern metal film ones. The new resistors are quieter, which again leads to the blacker background, and I was worried some of your noise may have been an old carbon resistor. Finally, that film cap makes the power supply much faster, so transients stop and start faster.
The unit should sound less bright because I took out those mediocre Solens (French) caps in some key spots, including the output capacitors. They are now Russian K40 oil caps. They are only $6-10 each depending on value and sound smooth, and rich, yet detailed. So that is pretty much why you hear what you hear.
Anyway, glad it works well and you are enjoying it. Should be very reliable since the whole power supply is new.
Below is the letter I had previously sent to Don, which outlines my results:
I just wanted to tell you how happy I am with the repair/upgrade that you performed on my McIntosh C-20 tube preamp. My two huge problems; #1) a growing, loud hiss, which was starting to drown out fade-outs and fade-ins on songs, and #2) the unit was so imbalanced (left-to-right) that I had to set the balance knob at 75% left/25% right.
As you suspected, the culprit was my volume control. The addition of a new Gold-Point Volume Potentiometer cleared up both problems so much so, that my preamp is now quiet as a church mouse and perfectly balanced. I’m so very impressed because I’ve had this preamp in to four different McIntosh repair centers and none of them could fix the problems. The last one made it sound much worse - to the point I was worried that I would have to give up hope on using the C-20 again.
Just as a recap, I have a quadraphonic system and utilize two C-20 preamps. I have owned both C-20s since the mid-70s, (what you call “good old friends). The front channels handle about 75-80% of the music in my set-up. The really good C-20 sits just before the front channel amp and the C-20 you worked on, just in front of the rear channel amp. The front channel C-20 has always been the slightly better preamp and so, when I had both upgraded four years ago, the front C-20 got the fancier French capacitor upgrade, while the preamp you worked on got a very good capacitor upgrade. For instance, my front channel C-20 is in excellent sounding shape, (sporting a line-level S/N of -93 dB vs. the original McIntosh spec of -85 dB).
Thanks, not only for your excellent work replacing my Volume Control, but also for the additional, low cost upgrade suggestions concerning some of my diodes, resistors and four capacitors – changes which I had you make.
I am not sure where all the improvement comes from, but after only 30 hours of burn-in, your C-20 now sounds better than my previous front channel C-20. Your C-20 will now be my good, front channel preamp. While both preamps are now church mouse quiet, the C-20 you worked on has an expanded soundstage that is out of this world! It’s a little wider, more pinpoint accurate, and noticeably deeper soundstage. All the more impressive since it’s the other unit that has the fancy, French caps. Your C-20 helps retain the nice, warm tube sound of my music system, while still providing very accurate musical detail plus the snap and pop of percussion and plucked strings that I enjoy so much.
Once again, thanks for all the great work.
If you wish here is Don's web address: http://www.dsachsconsulting.com/CitationRestoration.html
(I have no other connection to Don outside of being a really happy customer).