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Problems with my C31V preamp

A

act1292

New Member
My C31V is having problems with the channel selectors. Some of the buttons on the front panel work (Speaker 1, Speaker 2, Mono, Phono, VCR Copy 1->2, VCR Copy 2->1) while the rest do not. Very similar issues described in these threads:

https://audiokarma.org/forums/index.php?threads/new-to-me-c31v-issues.626867/
https://audiokarma.org/forums/index.php?threads/mcintosh-c31v-issue.241770/

I've been trying to debug the logic board for this but it would be much easier if I had a service manual. The AK archives don't seem to have this one. If anyone out there has a copy of the service manual it would be much appreciated.

Thanks in advance
 
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If you are in N America you can get the correct service manual from factory parts......there are two different builds, I believe I purchased 3 versions of the manuals.

It would seem that the phono selection is locked on, blocking the other inputs.

If you aren't careful and trash the EPROM, you might want to inquire if Mac parts can flash you a replacement before you get too deep, use static protection measures.
 
Thanks for the input. I know I have one of the later versions that has the motorized pot for volume control. I checked the McIntosh web site but I don't see a service manual available.

The problem seems to be intermittent slowly working my way through the debugging process - basically creating my on schematic along the way. The phono light isn't stuck on so I'm not sure that is the issue. The behavior of the lights seems to change when it goes into failure mode.

BTW - I don't see any EPROM in the circuit.
 
My schematics are buried in moving boxes, so going by memory.

You have to order schematics from Mac parts, they charge a nominal fee.

The last stuck input selector I worked on was a C38, caused by cat puke shorting out the circuit. While a cat caused failure I hope is rare it is not uncommon to find "grunge" build up on a board to cause intermittant switching.
 
Unfortunately without a schematic I can't even tell which capacitor is C38. I'm closing in on the problem though even without a schematic. Fortunately I had a good half day without distractions that allowed me to focus on the problem and I think I may be able to diagnose soon. I did contact McIntosh support to see if I can get a service manual. Being Sunday, I won't hear from them until tomorrow. If I solve it by then I won't have to spend the money for the manual. However, I might anyway cause I intend to keep my system for a while.
 
I have a C-35 service manual. The two preamps are similar, and this manual MIGHT be usable as a reference. PM me, if interested.
 
Sorry - I thought the reference for C38 was a capacitor. My mistake. I'm almost got it figured out so I may not need a schematic after all. I ran out of time yesterday and probably won't get back to it until Tuesday. I did get a response from McIntosh and will be calling them to price the service manual.

Thanks for the offer for the C-35 service manual but for schematics they really need to be to the exact model. Otherwise I'd be doing what I currently doing which is following traces, ohming out connections to figure out how it works.
 
I'm at work right now but I'll check when I get home. I think it is a later one as I had read some threads that the earlier ones didn't have the motorized POT for volume control - which mine has.
 
I managed to figure out and repair the problem. The problem was in the pushbutton circuit board. On this model, the LED circuit board that that light up the selected channels is mounted above the pushbutton circuit board and has two screws that hold both boards. Where the two screws hold them, the pushbutton circuit board has a metal plate attached to it via some sort of adhesive. Behind this metal plate runs several circuit traces - a 5V trace along with the Aux1 and Aux2 pushbutton traces. These three traces had some resistive cross talk. This ended up holding the Aux1 line high which meant other channels could not switch. I suspect that over the years the adhesive got to the traces and provided a signal path.

To fix the problem I cut out the Aux1 and Aux2 traces at the pushbutton end and the JST connector end and soldered new wire in. Put it all back together and it works like a champ.
 
Thanks so much act1292. Exactly the same with unit belonging to my sister in law's father in law. Looks like someone had been in there sometime in the past and jumped a trace from a switch around the area. I cleaned it all off with acetone (after alcohol didn't cut it) then put down a piece of tape and some double stick - didn't remove or check for need of the previous jumper. Guessing the repair may outlast us all. Hoping that will be a long time.
Now that I'm signed up, will try to pass it along.
 
Thanks for sharing this. I am having the exact same problem on my recently purchased C31V, with the input stuck on Aux 2. I will try this tomorrow and report back. I am thinking about cutting the Aux 1 and Aux 2 traces and replace with two jumpers.
 
How do you get to that board? I’m having a similiar problem with power button. I’ve taking it all apart but can figure out how to access the board behind the buttons.
 
Sorry but it's been seven years since I did my investigation and I can't remember how I got it out. I believe I had to disassemble quite a bit to gain access to it. Maybe other can chime it on it.
 
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