MR78 Backwards Capacitor?

[Corktowner]

Hello All. I just started work on an MR78 which I got in a multi-piece purchase last year, and when I hooked it up there was no sound, or at least none I could pick up. I read some of the posts and decided to open the unit up, check for burned components, replace the tantalum and electrolytic caps and then drop it off to Punker X who lives maybe an hour up the road from me. However, when working on the output amp section I have run into a problem - capacitor C410 on the board did not match the polarity of the service manual which matches my board.

Some quick numbers - Serial AD4967.
Output Amp PC Board number 044-794

Now according to the SMs in the AK Database, I should have the older 044-349 board, but from reading through the forum posts, this type of mismatch is common with serial numbers close to the official cut off of AD5165.

The problem I have is that C410 on my board was installed reverse of what the SM circuit diagram says it should be. So now I have the age old problem of following the manual or following the previously built board. It looks original and not modified in any way.

Suggestions? The mark-up on the photo shows how my original C410 was oriented.

 

[skippy_ps]

The MR78 schematic that I have, starting with sn AD5165, is the same as yours with the + side of C410 connecting to R409. Board 044-794.

Murray

 

[Corktowner]

The MR78 schematic that I have, starting with sn AD5165, is the same as yours with the + side of C410 connecting to R409. Board 044-794.

Murray

I’m starting to think the schematic is wrong. Every photo I could find of this board on a google search showed that cap the same orientation as mine was on the board.

I did also find a photo of a 2004 Modaferri mod, which had what looked like a Panasonic film cap as a replacement for C410. So that would be bi-polar, I believe.

 

[skippy_ps]

I’m starting to think the schematic is wrong. Every photo I could find of this board on a google search showed that cap the same orientation as mine was on the board.

I did also find a photo of a 2004 Modaferri mod, which had what looked like a Panasonic film cap as a replacement for C410. So that would be bi-polar, I believe.

It seemed unlikely that McIntosh would repeat the same mistake on subsequent schematics but maybe they did. If it was me, I'd follow what you have on your old board and then tell Punker about it.

Murray

 

[SaturationPt]

.. and post to the McIntosh "hints and kinks" thread if you are able to verify.

 

[whoaru99]

Yeah, presuming the unit was otherwise working and or no signs of that part being previously messed with, general protocol is as installed on the board takes precedent over schematic.

 

[SaturationPt]

A better look at the schematic is IMO worth the time. If it's not a DC application it might be oriented with the foil on the low-impedance side for reduced noise, if there is DC then certainly orient it per the DC voltage. Might be a production change that the schematic didn't cover.

 

[perryinva]

I always replace to follow the original. The PCB diagrams are unfortunately often wrong.