Mcintosh C28 is Flaky

[spacecowboy]

I've had a lot of problems keeping my pre up and running for long periods of time. My knowledge on repairs is limited and the thing works sometimes. I have used deoxit several time now and that does seem to fix whatever issues I have, but after about a month a new malfunction will appear. The strange thing is that when this happens I will use my other amp until I get around to fixning it (using more deoxit). And when I am in the mood to after a month or more to listen to that kick ass sound I plug it back in, and voila! It works as it should for a few days. My question is, does this indicate that something more complicated like a malfunctioning capacitor is to blame? Maybe the caps are not able to hold their charge for extended periods of use?

If anyone can inform me on what the symptoms of bad caps are I would be grateful. Otherwise I can keep trying to locate any oxidized connections I may have missed in the previous attempts. Or any tips are appreciated.

Most recently the problem has been a silent right channel with a strange staticy kind of sound even when no signal is present. This is a major problem for listening in stereo but also effects the mono output.

 

[c_dk]

The most common cause for your described intermittant problem is corroded ferrules that crimp the shields to ground drains. Slit, solder, unsolder lead and install new heat shrink wrap is the PIA solution.

The multiple metals in contact with each other set up a galvanic cell which leads to the corrosion and intermittant issues that seem to change as you look at them. This was a common issue that showed up at clinics and would be left to us to correct because of the time it takes.

A crackling noise is also a common symptom of a failing semiconductor, not usually a cap.

 

[spacecowboy]

Is this something that a novice like me would be able to repair, or should I find a reputable service to hire in your opinion? Are there any diagnostics I could run? And finally are the Mcintosh clinics still around? It would be awesome if there was a lifetime warranty for these kind of repairs but that is far fetched.

Thanks

 

[TR6man98]

Space cowboy,
C-dk wrote about this repair few times but one of the post was particularly good with detailed pictures.
Try google it or maybe he or someone else would post a link to that thread.

I am also interested doing that as I have C28, which has no issues so far (I own it some 20y) however I'm sure improvement could be made.... And it seems to me I'll be keeping it for awhile.

Good luck

 

[jlovda]

A crackling noise is also a common symptom of a failing semiconductor, not usually a cap.

Maybe I was lucky. A friend has a C28 that had terrible crackling sounds. I changed as many tantalum caps as I could easily reach and the noise completely disappeared. There are still a few in there but I guess they are OK for now.

 

[c_dk]

The typical C28 preamp is 40 years old.....a recap now just makes sense but is not a cure all.

DOB first made us aware of the ferrule/ground drain issue back in '79 or '80 at a amp clinic. This issue started cropping up as corrosion between the dissimilar metals started happening in more and more units that came into the clinics he did around the country.

If under testing it proves to be noisy transistor how many do you replace. I mean they are 1970s devices, there are quieter replacements available designed in the late 80s and 90s.......how far do you go?

While a EE might find the grounding scheme via the shield drains "elegant" I think a star grounding scheme retrofit now makes sense even though from a production standpoint I understand the old design.

The S/N design goals of the 70s were not as lofty as today's expectations......how much time, that means dollars do you spend to eck out another 5 to 10 dB.

Lastly techs hate callbacks, complicated 40 year old preamps are callback traps waiting to happen. Has to make many techs gun shy to do repairs on them.