Gilbert's McIntosh C22 Rebuild

This McIntosh C22 stereo preamplifier was sent to me by Klipsch forum member Gilbert. Gilbert has owned the preamp for a year, and it had been serviced just two months before he purchased it. The technician who worked on it had given it a clean bill of health. Even so, Gilbert noticed a few obvious issues with its operation, and sent it to me for the NOSValves treatment.

Just thought I would share the experience of rebuilding this excellently built piece of audio equipment. I really like doing work for Gilbert because he and I have the same way of looking at this old gear. He just tells me to do what I would do if it were mine. I'm a firm believer that 45 year old capacitors, whether they are PS filters or coupling caps, can't possibly be doing the job McIntosh, Scott, Fisher, or ____ designers intended them to do. It’s important to realize that these old tube components may indeed be working to some extent, but it's almost a given that they are not working up to their full potential.

McIntosh tube gear is by far the easiest tube audio gear to work on, at least from my point of view. The perception that a technician has to be a McIntosh specialist to work on McIntosh gear really gets under my skin! Tube gear is tube gear: It all works on the same principles, especially when dealing with preamps and amplifiers (tuners are another story).

The first thing to be done, that I'm not going to be able to post pictures of, is to disassemble the entire face plate (I did this portion of the work before I decided to do this write-up). This is required for cleaning the controls and switches on the C22; in most gear you don't have to take this step but McIntosh glass is really delicate and it's not worth the chance of damaging the finish or lettering with accidental overspray from contact cleaner. I'm a firm believer in drowning the controls with Caig D5 to really get them clean! After this is done I clean out any residue with compressed air and re-assemble the face plate.

So let’s get on to the fun stuff!
Here are some before pictures of the top and bottom innards of the C22:



Okay, time to get to the main work. In testing the electrical condition of the circuit, the first thing I noted was that the voltages at the three reference points that McIntosh provides for the power supply in the service manual, that these voltages were down from 14 to 27 volts.

First reference point was 296V should be 310V
Second reference point was 272V should be 290V
Third reference point was 241V should be 268V

The next thing I did was to replace all the diodes. For the B+ I used Fairchild brand Stealth Soft, fast recovery diodes (what the heck, they really don't add much to the cost). For the heater supply I used a basic bridge rectifier rated at 4A 400Piv. This did nothing to correct the voltages, not that I expected it to, since the original were SS type diode that don't degrade over time--they either work or don't work, no in-betweens.

The next step is to rebuild the power supply filtering. For this I ordered in the original type, twist lock cans from CE Manufacturing. These are built on the original Mallory tooling and are of great quality. Plus they look original and aesthetics count, especially with classic equipment. With the C22 or C11 there are three cans in the power supply. Two of these cans are no problem replacing, but the last can is a problem since it’s no longer available. This last can has three connections: two are filters for the heater supply DC voltage and the third is used merely as a connection point for various components (no filter connection). For this can I use separate radial lead capacitors, which you can see in the photos, and leave the can in place--but disconnected--to keep the original look intact. There is one more section that is part of one of the other cans that is also for the heater supply; this is also replaced with a radial lead under the chassis. For the two B+ filter cans I used a 4-section 50uF each @ 350V. This is a substantial upgrade over the factory filtering, which never hurts. The original McIntosh design has 210uF of effective filtering, while the new upgraded design has 375uF. Does this make a huge difference? Nope, but why not beef it up when you can at no real extra cost to the customer? And it actually will help stiffen the bass and quiet the preamp to some degree.

Here is a photo before replacement

Power supply after rebuild
(I did reuse one axial lead cap since it had already been replaced recently and tested as perfect on a cap analyzer.)

Now it’s time to fire up and test the voltages to see what good I have done.

First ref. point now 305V should be 310V
Second ref. point now 288V should be 290V
Third ref. point now 262V should be 268V

Much better! I tested the cans after removal (which is required for testing; in-circuit is worthless) on my Sprague cap analyzer. I found five of the six sections for the B+ to be leaking some voltage to ground above the minimum suggested tolerance. The sixth section was completely dead.

Now let’s move on to the rest of the circuit. One the problems that Gilbert experienced with the preamp was the rumble filter would cut a channel completely out when engaged. Not a big deal since the switch in this day and age should never be used anyway, at least if your turntable is any good at all. If you look at the shielded wire in this picture you will see it has obviously been tampered with, and under investigation I found the shielding only traveling an inch or so into the jacket. The solder joints on the surrounding components were obviously worked on at one time also. I went through and ran a new shielded wire and replaced the coupling caps that the signal runs through when the switch is in the off position. The rumble circuit, like all the filters, loudness control, and tone controls on the C22 are handled with PEC devices (Packaged Electronic Circuit). Fortunately there was no problem with the actual PEC, since a replacement is not available. When I fired up the preamp, the problem was gone. I am not sure which part of what I did actually fixed the problem, but it is fixed nonetheless. 

Rumble filter area before

Rumble filter area after (Rick, does that shielded wire look familiar?)

I call these next four capacitors “little buggers” since they are in one heck of a fun spot to get to. They should be replaced since they perform an important function. Two had already been replaced but for the sake of making them match, I replaced them all. I don't care for the 25-cent yellow jobs anyway!

Little buggers before

Little buggers after

Here are all the main coupling caps, before and after. These caps were leaking some voltage and had become resistive, which creates a similar effect to having the High and Low filters in the “on” position.

Coupling caps before

Coupling caps after

So what do those tell-tale voltages measure now?

First reference point is 309V should be 310V
Second reference point is 292V should be 290V
Third reference point is 267V should be 268V

All voltages were taken with the unit powered at 117V with my Variac, which is the spec noted in the McIntosh schematic and service manual.

I also ran sine wave and square waves through the preamp--they look equally good.

Thanks for reading and I hope you enjoyed it. This may help a few DIY'ers out there also.

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