Home » Audio » General » McIntosh C20 tube preamp, sound inequal...
McIntosh C20 tube preamp, sound inequal... [message #26625] Sat, 23 June 2007 09:02 Go to next message
kraken is currently offline  kraken
Messages: 6
Registered: May 2009
Esquire
Hi,
I never lissen any other C20 for comparison. But here are my constatation on mine:
(for example: Jimmy Hendrix voice on left speaker and his guitar on right)

...the voice is omnipresent. Some instruments look (too) loud (as the guitar) and I suspect other frequences are quite missing.
When I turn the "mode selector" to "reverse".. here come the voice! but the guitar sound decrease too much and the snare drum sound just appeared.

As you see it's seems that some frequences are available on one side (reverse switch on) and other frequences are there when "stereo" switch on. I cannot get full sound even reverse or stereo and of course both position can be set on.

My technician and I just dont understand. Somebody from HotGlass Audio made a bad job inside (just replace all bumble bees by japanese commun stuff and made lot of cold soldering) but even after my actuel technician replace all again (by auricap) he cannot find what's going on with this frequencies mixed up problem.

If this tell you something please advise. I can't get original C20 papers (visible) from McIntosh.

Thank you

Check each stage, does output = input with gain? [message #26627 is a reply to message #26625] Mon, 25 June 2007 17:11 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

Has your technician done a swept sine test on both chanels to get a response curve? Has he checked for distortion? I think I'd check those two things first. You may have a problem that's easier to find with measurements than listening by ear. This kind of test should be done first end-to-end, then at each amplification stage in the circuit. You will eventually find a point where the signal changes from what is present at the inputs, and by narrowing down each stage one at a time, your technician should be able to find the cause.


Re: Check each stage, does output = input with gain? [message #26630 is a reply to message #26627] Fri, 29 June 2007 12:07 Go to previous message
kraken is currently offline  kraken
Messages: 6
Registered: May 2009
Esquire
Hello Wayne

Thank you! According to this good old logic approach we found out a problem at section "mode selector" (reverse, stereo, chanel A, B) and "Balance". That's it!
But even these two control knobs were same construction thru 3 generations of C-20, McIntosh used two different ways of wiring them and we tried to understand why (I have pictures of that). A black wire instaled in three point and in between Mode and balance is there... or not, depending of the C-20 serie.
I read on the net there was "new born C-20 balance problem" that was set in the sixties (false or true?) in accordance to McIntosh communication.
It's the only section of this C-20 that was not untouched when I bought it. Many traces of soldering...
The thing is, I dont have any close-up, any revealant picture of internal mode + balance control and wiring. Everything seems ok to my eyes.
I really dont know what I should try. But still...

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