c33 diagnostics

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I have some McIntosh gear that I shipped cross-country (I'm in Portland, OR), and I'm having trouble with the C33 preamp now that I've set everything up. I used boxes from Audio Classics and they seemed to work well, but I know these units are complex and sensitive, so I wonder if something has come loose along the way. It's also possible these problems existed before I shipped it.

I'm not handy or daring enough to get inside and start working on the C33, but would like to get some idea of what might be wrong before I take it in for service, and possibly be prepared for what kind of repairs I'm looking at.

I found this thread and it had some good info about diagnosing the problem. I went through and compared each position on the mode selector, and also tested out the main output vs. the secondary outputs 1 and 2.

The most notable problem for me is the phono stage: I get sound in only one channel, unless using mono. Mirrored problem in each phono input. But then I also noticed that almost every input has some problems on the left channel if mode is set to "L into L&R". See my attached summary if you want more details. Not noted on my table: The front lights mostly don't work, but I assume that's unrelated and new bulbs are needed.

I opened it up just to see if anything is obviously loose or missing, but quickly realized most of the phono amp is not easily accessible. I'll attach a few pics looking into the unit, if they are helpful.

Thanks in advance for any interest or advice! This gear has been in my family since it was new so I'd love to get it back to operating at full potential.

 

[twiiii]

Call Pearl Audio they are down town across the street from a park near the river as I remember. They are great people and I bought three Mcintosh pieces from them over the years.; MX 110 MVP 861 and my first MC 207.

 

[TSmith8605]

I have some McIntosh gear that I shipped cross-country (I'm in Portland, OR), and I'm having trouble with the C33 preamp now that I've set everything up. I used boxes from Audio Classics and they seemed to work well, but I know these units are complex and sensitive, so I wonder if something has come loose along the way. It's also possible these problems existed before I shipped it.

I'm not handy or daring enough to get inside and start working on the C33, but would like to get some idea of what might be wrong before I take it in for service, and possibly be prepared for what kind of repairs I'm looking at.

I found this thread and it had some good info about diagnosing the problem. I went through and compared each position on the mode selector, and also tested out the main output vs. the secondary outputs 1 and 2.

The most notable problem for me is the phono stage: I get sound in only one channel, unless using mono. Mirrored problem in each phono input. But then I also noticed that almost every input has some problems on the left channel if mode is set to "L into L&R". See my attached summary if you want more details. Not noted on my table: The front lights mostly don't work, but I assume that's unrelated and new bulbs are needed.

I opened it up just to see if anything is obviously loose or missing, but quickly realized most of the phono amp is not easily accessible. I'll attach a few pics looking into the unit, if they are helpful.

Thanks in advance for any interest or advice! This gear has been in my family since it was new so I'd love to get it back to operating at full potential.

I would start by getting some contact cleaner and cleaning all the controls and switches - the switch contacts may have oxidation on them that can block or partially block signals. Recommended cleaners are Caig Deoxit D5 for switches and F5 for potentiometers.

See this thread:
https://audiokarma.org/forums/index...guide-to-using-deoxit-revisited.207005/unread

Regarding the phono, have you satisfied yourself that whatever source you are using to drive the phono inputs is working in the left channel - the problem might not be in the preamp?

Welcome to AK. Good luck and let us know how it goes.

 

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Regarding the phono, have you satisfied yourself that whatever source you are using to drive the phono inputs is working in the left channel - the problem might not be in the preamp?

Thanks, yes I actually hooked up another turntable for comparison. And I can run my usual TT through an external phono preamp without the left channel problems (using aux or tape inputs on the C33).

I appreciate the deoxit link.... That should be more useful than if I just opened it up and started spraying around!

 

[perryinva]

The C33 is a very easy unit to work on. Loss of a channel that isn’t due to one of the front panel pushbuttons (which is the most common) is typically a switching FET transistor. I have 2 C33s that I completely rebuilt and each one had a different single FET that caused a problem in one channel. I detailed the fix on AK in a thread.

 

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The C33 is a very easy unit to work on. Loss of a channel that isn’t due to one of the front panel pushbuttons (which is the most common) is typically a switching FET transistor. I have 2 C33s that I completely rebuilt and each one had a different single FET that caused a problem in one channel. I detailed the fix on AK in a thread.

Thanks, I'll check it out!

In the meantime, I tried to do some deoxit cleaning with d100 and faderlube — I found it hard to access many of the pots besides the EQs, but I was able to work on the mode selector. I couldn't do much with the phono preamp since it is boxed in, and I'm hesitant to remove it just yet. Also I didn't remove the front glass, perhaps that'd give me more access.

The good news is that the cleaning really seemed to help on most inputs and the secondary outputs. Bad news is the phono stage is acting the same as before. It's strange to me that the problem is exactly mirrored between Phono 1 and Phono 2.

But all in all I'm happy with the deoxit results especially considering that it's my first time trying. Cheers!

 

[perryinva]

Here is the link to what FETs I used to replace the originals. That thread was before I had done my 2nd C33 and it turns out it was a muting transistor. It’s a possibility yours is just permanently in Mute mode. The other was in the tape monitor section. Both have been superb since those recaps; the best preamp I have ever owned.

Every input on the C33 is switched via an FET, per channel. The front panel knob simply switches the correct FET pair on to the rest of the desired pre circuit. There are two identical boards, one over the other in the left rear, that house most all the switching, one for each channel. So It could be the FETs that switch the phono sections on that one board. The “easy” diagnostic for a tech is to simply swap the two boards and see if the issue travels to the other channel. At this age, all the electrolytic caps are past their accepted design age and a preemptive recap is warranted. As easy as it is to recap, though still time consuming, it’s not that expensive compared to much equipment.

There is nothing to deoxit under the phono shield. FWIW, I had a noisy phono on both channels in one of the C33s that turned out to be a set of factory installed electrolytic caps under the shield, IIRC, installed backwards. They were backwards per the manual and compared to my other C33. Over the years those caps finally leaked (very low voltage there, so it’s no surprise they worked fine for 30+ years.) replacing the caps and opamps fixed everything.

 

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Here is the link to what FETs I used to replace the originals. That thread was before I had done my 2nd C33 and it turns out it was a muting transistor. It’s a possibility yours is just permanently in Mute mode. The other was in the tape monitor section. Both have been superb since those recaps; the best preamp I have ever owned.

Hey thanks, I don't see the link but feel free to DM it to me if that's easier.
Great to hear from another C33 lover — Thanks for your insight!

 

[jlovda]

First, there are two FETs in series for each input. This was done I assume to increase the attenuation when the channel is off. If you have to replace one you might as well replace both. I had a problem with "leakage" of a signal even when the channel is off. You really need to change both FETs in that case.

Second, the FETs controlling the phono preamp are on the separate small phono preamp board inside the metal box at the rear, left hand corner of the chassis.

 

[perryinva]

DOH!! Forgot the link! Jeez!

C33 132-170 substitute interchange?

There are 6 132-170 jfet switch transistors in the Tape Monitor switch boards. They are listed as the NLA MPF970. The sub comes up as a P1087 online. D & S are swapped relative to the G, but P1087 appears symmetrical. I will bend the leads to match, but wondered if anyone has used it as is...

audiokarma.org

 

[perryinva]

Thanks for the clarification @jlovda ! It’s been a few years since! I was going completely from memory. I didn’t remember the ones under the shield at all. Yes, 2 in series makes sense.