McIntosh C22 Unsolved Problem - for techs/smart guys/EEs

Hello, I have a classic Mac C22 that the right channel goes out.  It has been to several techs over the years and never gets resolved (in their defense they have trouble replicating it).  I have done enough control tests to know that: it is not cables or the associated amplifier or speakers, balance control, etc.  I am wondering if someone can suggest what my problem might be based on some of the tell tale signs.  All the caps have been replaced by Audio Classics and it has had a clean bill of health a few times.  Here are some symptoms and clues:
1) The right channel "goes out" (down in level from left channel in varying degrees - about half volume all the way to nearly inaudible, but you can always hear at least a faint signal, and it can be "scratchy").
2) It goes out (or attenuates) either before the system is turned on or while it is playing.
3) If I turn the whole system off, the right channel (still being fed from power remaining in the tube amp) snaps back up to volume comparable with the left channel
4) When returned from servicing, it may be fine for up to 6 months before symptoms appear again.
5) Symptoms may go away for a short time but quickly come back
6) Unplugging the unit (and detaching and reattaching all connections) has on occasion returned the unit to normal functionality. 
7) The unit has worked (and not worked) without changing out any tubes, and it has also exhibited this condition with two completely different sets of tubes.
8) On a couple occasions (but it rarely works) moving the volume knob back and forth quickly a couple times seems to make the right channel pop back in to normal volume.

I know it's hard without being able to examine the unit but I thought it was worth a try - perhaps someone knows something.  Thanks in advance!
Does the C22 have a three prong power cord? What I mean is that , this could be a ground problem. I kept thinking it was a piece of my tube gear as well..(it wasn't)  Are you using multiple components plugged into a few different wall outlets?? If so, combine them down to one, using a cheap power strip that is grounded (three prong). (I had a very similar noisy sound with channel spikes) Good luck.

From what you describe it sounds like the trouble is with the volume control !
Sounds like a flaky tape monitor switch.  Next time it goes out, switch the monitor in and out a few times, see if that wakes it.

I've also seen an intermittent tube socket do this, but on that unit, the pins were visibly discolored and didn't grip the pins well either.
Definitely a bad connection somewhere. I have seen similar symptoms with cracked circuit board traces, cold solder, bad switches and dirty volume pots. If the Mac has circuit boards (not hard wired/discreet) my money would go on a cracked trace or solder connection - they are sometimes nearly impossible to see.  The techs have probably checked out the rest. It doesn’t sound heat related since it fails from a cold start.

Thanks all, I'll try the tape switch.  These units never had 3 prongs, only two, made in 1960...
I have a C-20 with a similar problem, except my L-R balance is always terrible. (Instead of pointing my balance control straight up toward the 12:00 position, I have to point it to about 2:00 - 2:30).  I have also had it in a few times and have had the caps and a few other parts upgraded.  But the L-R balance is still way off.
I also suspect it is the volume control and have made arrangements to have a new, upgraded volume control installed.  I don't know if they make upgraded volume controls for the C-22 but I could ask while my C-20 is in for the service.
I'm scheduled to have the service work done later this month or early October (depending on how fast work gets done on current projects).
If you like, I can keep you posted of my results, and inquire about your C-22 as well.
PS If you decide it's the volume control and live near Chicago, I can tell you where NOT to go. 

I would say to replace the potentiometers in the volume and balance, like toolbox indicated. These potentiometers are basically variable resistors that electrically control the gain of the signal going through the circuit. They can go bad and can also cause intermittent issues, like you have experienced.  The bass/treble potentiometers could possibly affect the circuit too, but I would do the volume/balance first.
What about tapping the unit? Does that ever help? If not, try tapping, or flicking the tape, volume and balance controls.

Follow up - so I shipped off the unit to Audio Classics as a last ditch effort (I live on the opposite Coast) knowing if anyone would know, they would.  I got it back a few weeks ago, and it has been working withhout issue.  As always, they did a great job; I felt like I was getting a new piece of gear in the mail.  OK, so what they said was that in the last five years or so they have found a measurable phenomenon with the RCA jacks in a bi-metal corrosion issue.  If I get this right, the jacks were made with a combination of nickel, aluminum, and perhaps some other metals.  The engineers presumably never anticipated this stuff being used  50-60 year later.  With extensive use, friction, etc., and corrosion the overall wear to the metal(s) changes the resistance.  They said that the symptoms I were having were typical of this problem.  Not sure I got that correct, but bottom line they changed all my RCA jacks with new ones (as well as some other stuff to bring the unit up to specs) and it works great !  Thanks to Audio Classics.  
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:

(I have no other connection to Don outside of being a really happy customer).

Tim G.