C32 preamp repair and rebuild questions

MikeRam

Active Member
I recently purchased a C32 preamp at a reasonable price, but it does have a few issues. Surprisingly, there is not much here in Audiokarma about troubleshooting and rebuilding these units (at least compared to other units). I thank you in advance for the help you can give.

Overall I like the sound very much. Even in original condition, I like the sound a little better than my fully rebuilt MAC 1900. I can’t wait to hear it after it is fully rebuilt. There is definitely a lot more hiss than the 1900 though, but I’ve read that a rebuild will mostly take care of this. Now to the issues:
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1. Expander issue. The expander needs to be on for both channels to work. When turned off, only the right channel works. There is no audio in the left channel, only the hiss. I’ve read several posts about the audio connectors between the boards going out and that reflowing the solder takes care of many of these types of issues. I’ll give that a try once I tear into the unit. Let me know if you have any other suggestions.

2. EQ issue. When I first bought it, both channels worked with the EQ on but after about a week the left channel went out. Both channels work if I bypass the EQ though. Same symptoms as the expander issue, no audio in the left channel, only hiss. I suppose it could be an audio channel connection problem but I noticed that the left channel has a different set of OP Amps than the right. The ones on the right look stock and the ones on the left don’t. The Op Amp is sandwiched between a couple of capacitors and I don’t have the right tool to pull it out yet. I’ll order one from Mouser along with the rebuild parts.
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There are a couple of posts about rebuilding C33’s C34v’s that suggest swapping out the EQ OP Amps to a low noise NE5534AN, or an NOPA132P. Would these also work for a C32? If not, what would be the proper Op Amp to replace it with?


As far as the rebuild goes, I also have several questions:

1. A couple of posts talk about replacing the harsh sounding tantalum caps used in the phono section. I looked for them but could not find anything that looked like what I’m used to seeing. I found some caps that look a lot like ceramic. Are these actually tantalums?
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2. There are some non polar electrolytic caps on the AUX, Tape, and Tuner inputs (C256 – C265). Are those worth replacing with WIMA Film MKS2 caps? Would the difference be noticeable?

3. There is a lot of debate about ½ watt resistors being less noisy than ¼ watt ones. Any suggestions on whether to replace the signal path resistors with ½ watt ones?

4. Luckily my ribbon cables look to be in great shape. Should I expect them to disintegrate if I pull them out of the boards several times during the rebuild? Do most of you replace components while the ribbons are still connected?

Thanks so much reading the post.

Mike
 
Regarding tantalum caps , the pictures you provided show brown ceramic caps in the center. The blue cylinders on the edge of the pictures are electrolytic caps.
 
These are pictures of the left input switching board. The phono inputs are the ones on the left.

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Hello

I will recommend you to change all the capacitors on both pcb . Be carefull some are NP capacitors (non polarized) .

See my previous posts , I change all the "flat cables" and connectors

To answer to your question 133043 is a LM356N (bi FET Op Amp) . I personnally will not use other IC (because of slew-rate and power consumption) .

For my point of view changing all flat cables and connectors is a great improvement :thmbsp:
 

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You first need to solve the channel issues. Do you have the correct schematic for your serial number....I believe there are 3 versions.

Have you cleaned the switches?

The C32 uses sealed reed switches, they seldom fail but there is always a first time. They open and close pathways that engage and bypass various functions and serve as input control.

I have chased many a molex pin issue in this vintage of Mac products. I always now reflow all the Molex pins on all the products from these years.
 
I've got the same issue...

I just got one of these amps from a buddy. It also loses a channel when the EQ is engaged. jiggling the switch brings the channel up, which makes me think it's in the switch. I've got the thing taken apart down to the board...is there a fix for these switches? Or do I just have to replace it? Thanks!
 
clinic-audio,

Just so I have it clear, you are saying to replace the op amps with a 133043?

I will definitely reflow the solder on the molex connections.

I haven't torn into it yet in fear of tearing up the ribbon cables but I guess I'm going to have to get to it sooner or later. I'll clean the switches and report back.

Mike
 
clinic-audio,

Just so I have it clear, you are saying to replace the op amps with a 133043?

I will definitely reflow the solder on the molex connections.

I haven't torn into it yet in fear of tearing up the ribbon cables but I guess I'm going to have to get to it sooner or later. I'll clean the switches and report back.

Mike

133043 is a LM356N (bi FET Op Amp)

133043 is the McIntosh part number. You need to use LM356N.
 
clinic-audio

Very nice job on the ribbon cables. Do you have a post where you demonstrate how you did it? That would be great to see.

Also, I looked up the LM356N and my favorite places like Mouser don't carry it. Who do you purchase it from?

Thanks,

Mike
 
clinic-audio

Very nice job on the ribbon cables. Do you have a post where you demonstrate how you did it? That would be great to see.

Also, I looked up the LM356N and my favorite places like Mouser don't carry it. Who do you purchase it from?

Thanks,

Mike

Thank you !

OK I will try
 
First , you need to delete every bad connectors from pcb's and clean the boards

:thmbsp:
 

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Then , solder the new connectors
 

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When a connector do not exist with the right number of pins just take it 1 off like on my third picture !

making long flat new cables with new female plug will not be a problem :thmbsp:
 
Excellent! I guess it's a simple concept but for some reason I can always understand better with pictures.

Do you have a favorite source for the LM356N chip?

Thanks,

Mike
 
Yes , MOUSER

Mouser part nbr 926-LF356N/NOPB

you can also use TL081

Mouser part nbr 595-TL081ACP

:thmbsp:
 
clinic-audio

I'm making my Mouser order and am having a hard time finding the ribbon cable components that your pictures show. Do you have part numbers for them?

Thanks,

Mike
 
Well, I'm halfway through the repair/rebuild and things are going well. The issue with the left side of the EQ not working when it was engaged ended up being Op Amp related. Once I switched out the two IC's with the 926-LF356N/NOPB chip (thanks clinic-audio) I got my left channel back.

The ribbon cables were in perfect shape when I started but as careful as I was, they started to deteriorate once I tried to put them back in (as you can see in the picture). There is no way around it, I'm going to have to replace them with their modern equivalents as clinic-audio suggested.

I was reading that the molex cables carried audio signal and the ribbon cables carried switching signals only. The Tone board only has ribbon cables attached to it but when I look on the schematic, I notice there is audio signal running through them. The darker lines in the schematic represent audio signal flow.
 

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