The NOS Valves way to sonic bliss with a McIntosh C22 *updated*

[NOSValves]

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. Its 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 lets get on to the fun stuff!
Here are some before pictures of the top and bottom innards of the C22:

Top



Bottom




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 its 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 its 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 lets 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.


*I would like to give Paul "Parrot" credit and a thank you for editing the write up on this post*

[Painful Reality]

I must say you have nimble fingers for a troglodyte!

Nice job!

[NOSValves]

I must say you have nimble fingers for a troglodyte!

Nice job!

You and a few others are in rare form around here these days. I've really been enjoying the forum for a week or so.

Craig

[Painful Reality]

Yeah, I've shed quite a lot of pounds lately...

Nice and informative post about the McIntosh. Thanks for sharing!

Seriously, I envy your patience to work in these old amps.

[NOSValves]

Jeff,

Actually once the VRD's were designed and built the fun was gone out of that project now putting them together is just work for the most part. Although its great to hear from the customer when they fire them up. I enjoy working on these old peices its like doing some thing different for a living everyday.

Craig

[Duke Spinner]

Nice Job ....!

complete & thorough all the way ...

anybody that wants to keep as many old caps as original components ..

well, they are just gonna watch the amp die a slow death...

over 'n over ..

Gilbert, no doubt will be Real Happy when this comes back ....[]

[Timmikid]

Right on, Craig! Keep bragging! [] I really enjoy pics of insides (equipment ofcourse), even though it fills me with envy that I hardly have any clue what you are talking about. Thank Gods that I at least can hear well.[Y] Tim.

[mark1101]

Craig,

Nice. How does it sound? Why did you select the type of caps you did?

[NOSValves]

Right on, Craig! Keep bragging! [] I really enjoy pics of insides (equipment ofcourse), even though it fills me with envy that I hardly have any clue what you are talking about. Thank Gods that I at least can hear well.[Y] Tim.

Thanks Tim,

Were equals since people tell me I'm deaf LOL !!

Craig

[LarryC]

nm

[Colin]

those cheap Macs need so much work!

great pics

[NOSValves]

speechless

[LarryC]

nm

[NOSValves]

Studdering

[Gilbert]

Craig,

Thank you for being so diligent in your restoration of my mcintosh preamp. This write up is interesting to me, especially so, since it is my unit that's being discussed.

I purchased the preamp almost a year ago, and even though she played music and sounded pretty darn good (to my ears anyway),.... and even though the previous owner provided me with a reciept showing the unit had been serviced just 2 months prior,..... and even though the outside of the unit was cosmetically flawless, I knew she was sick inside.

The restoration work you performed on my first pair of MC60's made such an improvement, that I had to get you 2 more; And now the C22 has been reborn. I know she's gonna sing for me, and I will be in true vintage mac heaven at last. Thank you again.

[LarryC]

nm

[NOSValves]

befuddled

[Speedball]

So you do work on McIntosh....Mmmm, good to know that......

[NOSValves]

So you do work on McIntosh....Mmmm, good to know that......

Yep actually brand doesn't matter, Any brand tube Hi Fi gear is welcome like a said in the initial post it all works on the same principals. Heck not long ago I tweaked up a set of Heathkit amps a guy built on some nice wood chassis but just couldn't get rid of some gremlins with his multi meter. I fixed them in about 3 hours. It's all about having the proper tools and knowing how to use them.

Craig

[thebes]

Dear Trogy (hereafter known as Craig),

It has come to our attention here at Thebes-O-Rama, LLT (lLimited

Liability Twins) that as your mad-crazed scientists white lab

coated skills have become more refined, you have, how shall we say

it,:become more mad. Certainly eccentric geniuses must absolutely

be catered to, and I trust the funds we forwarded for the completion of

your new lab have met with your approval. We trust the padded walls are

comfortable and suffient to give you full reign over your creative

urges. I

t is certainly evidenced by your stellar work on the innards of

that poor Macintosh fellow. Although we are not particularly squemish,

the sight of such a diginified individual cut open, pinned back with

it's alimentary canal laid bare gave us pause.

Certainly it brougt us memories of that horrible day in 9th grade

science when that harlot Miss Beatrice brought in those damnable frogs.

It was our impression that the initial purpose of your post was to

demonstrate some restoration techniques to those young interns in the

arcane art of electrical repair. However, when certain novices made

inquiries as to the specific nature of the patients condition, "it" I

believe was the word used, you went off on one of those wonderful

indicators of true genius: the snappy comeback.

Perhaps we are wrong, or are simply misconstruing the whole incident.

However, you should be aware that Thebes-O-Rama LLT is a wholly

owned subsidiary of Thebes Cement Truck Company, LCN (la costra nostra)

and the aforementioned individual serves on the Board of that

august entity.

I find your efforts to resussicate that fine chap, Macintosh, to be

most engaging, and I certainly look forward to your upcoming

dissertation on that simian brute, what's his name again, oh yes: Citation II.