McIntosh C20 dilemma – rebuild or …

tcdriver

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I recently acquired a McIntosh C20 preamplifier. This example is in fair to poor cosmetic condition. None of the knobs are original, the front panel is a bit worn and the wood case is in need of refinishing or reveneering. My original thought was to install it in my music system, see just how good it sounded and then decide whether or not to keep it or pass it on.

When I bought the unit I was told that there were no tubes in it. Since it was in the case I was unable to verify that information until I got the unit home. Much to my surprise, after I removed it from the case, I discovered that there was a full complement of tubes. Removing the top and bottom cover revealed that all the capacitors (black body with colored stripes) and resistors (mostly ½ W, 10%) appeared to be original. The only components that appeared not be original were the diodes used in the filament supply.

Using a Variac, I slowly brought up the voltage over the course of an hour. I noticed that one of the power supply capacitors was getting warm, so, I started measuring voltages. Voltages were low all around and I determined that the input filter capacitor (C1 on the SAMS) was bad. The bad capacitor was a dual section twist-lock specified at 40uF, 40uF @ 350V. Rooting around my OOS capacitor supply I found a capacitor that would fit and after testing it on my Spargue Tel-Ohmike TO-6 and finding it good, I installed it in the preamp. Voltages were up, but, still below spec.

Further testing revealed that the last section of the second twist-lock capacitor (C2D on the SAMS) was bad (leaky). This time I could not find a suitable replacement, so to verify the operation I temporarily kluged in a single capacitor to replace that last section on the quad section. Voltages were better but still low in the phono stage. Further testing revealed that the root cause of the remaining low voltages were the result of leaky coupling capacitors (SAMS C11 and C51).

Here is my dilemma; if I am ever going to hear this preamplifier work as it should, I will most probably have to replace every single capacitor. This will take a little bit of money and a fair amount of time. On the other hand removing the original components will most probably reduce the resale value and alter the classic sound of the preamplifier.

I would really like to hear from other McIntosh C20 owners. What should I do? Have you replaced the paper capacitors? If so, did you use modern technology (polypropylene or polyester) capacitors or did you replace the capacitors with NOS or new paper capacitors? What is the best way to replace the capacitors? Does one desolder them from the terminal board lugs or does one cut them out and attach the replacement capacitors to the lead ends? Do you remove the terminal boards from the chassis or try to work on them in place? Do you recommend replacing all the resistors as well or not?
 
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Here is my dilemma; if I am ever going to hear this preamplifier work as it should, I will most probably have to replace every single capacitor. This will take a little bit of money and a fair amount of time. On the other hand removing the original components will most probably reduce the resale value and alter the classic sound of the preamplifier.

I would really like to hear from other McIntosh C20 owners. What should I do? Have you replaced the paper capacitors? If so, did you use modern technology (polypropylene or polyester) capacitors or did you replace the capacitors with NOS or new paper capacitors? What is the best way to replace the capacitors? Does one desolder them from the terminal board lugs or does one cut them out and attach the replacement capacitors to the lead ends? Do you remove the terminal boards from the chassis or try to work on them in place? Do you recommend replacing all the resistors as well or not?

Well, it is cosmetically not good and does not work, so I don't see how it has much resale value and certainly does not have the classic sound (since it does not work at all). So I don't see anything to preserve here.

I would think a potential buyer might be either a collector or user (or both at once). A collector might want to just put it on the shelf to admire, like a museum piece, and would want one that is near mint cosmetic condition and with all original parts.

But if the buyer wants to actually use it, of course it will need to be repaired.

So you need to decide what you want to do with your C20. You can either trash it, shelve it, try to sell it as is, or try to restore it. If you restore it, you may want to try to make it look better - find a set of original knobs, restore the cabinet and clean up the faceplate and chassis as well as possible. Then you will need to restore it electrically.

My guess is if you restore it and try to sell it, you probably will not recoup your costs - unless you can get it looking mint. So you need to decide if it is worth the time and money to restore this for you or someone else to use.

I have 6 old McIntosh items (about 1970 or older) that I have collected. I don't have a C20 but I do have a C22. I want the old Macs to be cosmetically excellent, to sound as good as possible and to be reliable. I don't really care if the internal components are not original. So most have been to Terry DeWick's bench for "audio upgrade" which includes new caps and resistors as needed. It also includes correcting any other issues and clean up.

In my case, I want the best sound, so I prefer good polyprop caps. Others might want to find caps similar to the original to get the sound as original as possible. I this case, I would not use old caps, I would try to get new ones that sound similar to the originals, some how.

I have restored a few items myself over the years (Dyna ST-70s, a pair of MC30s and and Audio Research SP-3A-1 preamp). All got good polprop caps and precision metal film resistors. For the pre-amp, I replaced all the signal-path caps with polyprops, all the resistors with precision metal film (why not), the volume control with a stepped Alps pot, and several of the RCA connectors. Other controls and switches were cleaned. I did that one for a friend, but wished I could have kept it as it sounded really great. This type of restoration, along with replacing power supply caps will make these great old, well built items like the Macs and ARC good to go for 2 or 3 more decades - or longer.

Good luck and have fun with it.
 
The striped capacitors in the C2o are commonly called bumblebee capacitors, all of the larger value ones .47, .22. and .1uF are bad or will soon be bad. This type of capacitor has a bad reputation for high leakage with some cracking open (40%+), even NOS ones when found will have leakage, plan on replacing them, Nichicon QX series or Illinlis MPW will do a fine job. The C20 is a good preamp that some prefer over the C22, with the 20 being a bit more musical with a bit more bass punch.
 
If you have letters wiped off then you probably have the C20 series that McIntosh is making faceplates for again. Check with them.
I have the early with the glass you cannot wipe letters from. Some work was done to it, but still was not right. Took the faceplate off and sent it off to Terry. Ups can't break it if they don't have it to break. :D
Anyway, Terry got her fixed up good and it is in service with my MC60's and MR67. It may be 50 years old stuff but it is so sweet and warm. :)
If you are worried about collectability, the picky Hong Kong guys just keep the old parts to give the next owner in case they want a museum piece. I'll be dead when mine leaves my house so it doesn't matter. :smoke:



Carl
 
Thank you all for the very thoughtful replies.

TSmith8605, you make a lot of good points that I agree with. And, you get to the heart of my dilemma; what am I going to do with this unit? Because the cost to rebuild it will make it more expensive than it is valuable, it certainly does not make any economic sense to repair it to sell.

My inclination, if I do rebuild it, would be, to update the capacitors with modern ones.

Dewickt, Thank you for the capacitor references and additional information.

Carl, Thank you for your insights. If I do rebuild, I will keep the old caps. It is interesting to note that when I looked on ebay recently to see what had sold and for how much, I noticed that almost all C20’s had the original “bumblebee” capacitors still in place.
 
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Thank you all for the very thoughtful replies.

TSmith8605, you make a lot of good points that I agree with. And, you get to the heart of my dilemma; what am I going to do with this unit? Because the cost to rebuild it will make it more expensive that it is valuable, it certainly does not make any economic sense to repair it to sell.

My inclination, if I do rebuild it, would be, to update the capacitors with modern ones.

Dewickt, Thank you for the capacitor references and additional information.

Carl, Thank you for your insights. If I do rebuild, I will keep the old caps. It is interesting to note that when I looked on ebay recently to see what had sold and for how much, I noticed that almost all C20’s had the original “bumblebee” capacitors still in place.

If you have the time and money and can get a face plate and knobs to make it pretty, I think you should restore it for your own use. I think the results might well be worth the effort and you'd have one sweat preamp. (If you have the money, but not the time, send it to Terry.)
 
I have ordered the power supply electrolytic capacitors and already replaced a couple of the coupling capacitors. I just see how far I get on this project.
 
Update

It has been a while since I last wrote about my C-20. I have replaced the twist-lock and "bumble-bee" capacitors and refinished the case. I have not replaced the knobs for now. I want to see how much I like this preamplifier before I go that last step. I plan to install the C-20, along with my MC-225, in my system this afternoon to give it a listen. Thanks again for all the suggestions and thoughtful comments.

Here is a picture with the “odd” knobs.

C-20ontable.JPG
 
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It has been a while since I last wrote about my C-20. Here is a picture with the “odd” knobs.

I had a chance to see this unit @ Tim's the other day as he was generously troubleshooting a Scott integrated for me.

The picture does not do it justice; the cabinet really looks good :yes:
 
Walnut Cabinet

I had a chance to see this unit @ Tim's the other day as he was generously troubleshooting a Scott integrated for me.

The picture does not do it justice; the cabinet really looks good :yes:
I agree that the photo does not do true justice to the cabinet. McIntosh appears to have used very good walnut veneer. When finished correctly there is a lot of depth to the grain. My woodworking skills are rudimentary. The cabinet was really scratched and scuffed up when I got the C-20. I sanded it down and then applied some Walco Danish Oil Finish (Dark Walnut).

I am also camera challenged. I have yet to figure out how to take a decent picture of stereo gear. Maybe it is time to read the manual that came with the camera. The picture below may show a little more of the cabinet.
C-20Cabinet.JPG

those knobs actually look good!
I can take no credit for the knobs. They came with the unit when I bought it. All things considered, I too think that they look pretty good, although, not as good as the originals.

I have now set the C-20 up in the system with the MC-225. They seem very happy playing together. Right now I am playing Mingus Ah Um and it is sounding pretty darn good.
 
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My C-20 is sitting idle now. When last summer came, I realized that running tubes made my listening room too hot. In addition, there is an intermittent noise that has to be located and banished. The C-20 is in the fix-it queue.
 
My C-20 is sitting idle now. When last summer came, I realized that running tubes made my listening room too hot. In addition, there is an intermittent noise that has to be located and banished. The C-20 is in the fix-it queue.

Ahh.. heat is certainly a factor. It would be safe to say that the C20 would be your "winter" preamp, or rainy March days like the past weekend. Given two units equal in other areas, I'd take the one that runs the coolest. That makes the case for the C33/34 monitor amp which is used here quite a lot. Does the noise start after the amp warms up? Everything grounded ok? Nice unit BTW.


-Gregory
 
Did you ever find original knobs for it and replace the coupling caps?
I never looked for the original knobs. I got used to the look and kind of like it. Yes, I did replace the coupling and power supply capacitors.

The coupling capacitors (polypropylene). Sorry for the out of focus picture:
McIntochC-20019.jpg


… Does the noise start after the amp warms up? Everything grounded ok?
The noise I referred to is an intermittent crackling sound. Sometimes the unit will play for an hour or more without the noise and other times the noise will come on right away or after a few minutes then go. It sounds to me like a resistor going south or maybe poor contact. The intermittent nature of the problem makes tracking it down a lot more difficult.

Too HOT for the summer time:
SystemC-20.jpg
 
I have had, in the past, a LCR module for the tone circuits go intermittent causing a random popping noise....
 
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