C32 Bad Phono Channel - Can't Figure Out Edit: Figured it out

[Johnny_Law]

My C32's phono is dead in the right channel, and I've isolated the issue to the input switching board. Here are the measurements (this is the Right channel phono section of the input switching board 045-023). I'm stumped and looking forward to figuring this out.

-All transistors were pulled and test OK (green boxes)
-All diodes test OK in circuit (green boxes)
-All resistors were measured in circuit and are identical to measurements of good channel
-All electrolytic capacitors are new and have good solder joints, and are installed in the correct orientation (including C220)

The 17V drop across R238 is huge ... but R238 measures fine. I also checked whether the R238/C220 solder joint was open or shorted to ground ... neither: it's a good joint and isn't shorted to ground.

Ideas?

 

[TSmith8605]

. . .Ideas?. . . View attachment 2440171

Assuming your purple numbers are DC voltage relative to ground, it looks like a short to ground at the R238/C220 node - might be that C220 is failing under load or it might be a short elsewhere. I would pull C220 and measure the DC resistance from the R238/C220 node to ground with the unit off to see if it is short. If so, find the short (wire or trace touching somewhere or something). If not, test by installing the old or a different C220.

 

[Johnny_Law]

Figured it out. This is a good one - the C32's right input board sits on top of a coated chassis shelf, and it has two spacers underneath: one in the middle, one on the right side. The spacers keep the underside of the PCB from touching the chassis. Apparently, the middle spacer was JUST squished enough to cause the LEFT side of the input board (where the phono circuit is) to BARELY make contact.

Here's a pic (see the two rubber spacers on the shelf towards the top of the photo ... this is where the input board sits ... note the indents in the left spacer which sits under the middle of the board):



I had the board out of the chassis and was measuring each component individually ... with none of them bad. I also had checked for a short to ground between the R238/C220 node when it was in the chassis, and there was no short.

HOWEVER after re-installing the board this morning, all of a sudden the voltage values were all correct at first, but they would go back to the bad values I posted above. (Is this a quantum pre-amp???)

But then I realized that it was the SLIGHT pressure of my test leads, pressing down slightly to get a voltage reading, that was causing the board to bend down and the leads would make contact below.

After trimming the excess leads under the PCB and putting in another spacer, we're back in business and phono is sounding great.

(@TSmith8605 you nailed it about the short, even though I had tested for a short and not found one. Good lesson here - the numbers don't lie, and Occam's razor comes in handy.)

Could be a good reason for C32 owners to make sure the spacers under their right channel input switching boards are in good shape!

 

[Hofvox]

How did those Flexible Flat Cables survive all those inserts and pull outs without crumbling? This is what happened to mine (see my avatar or the left).

 

[Johnny_Law]

I've got them fancy cables.

 

[MRL_Audio]

^^^ is that the kit cables from eBay?

 

[Johnny_Law]

^^^ is that the kit cables from eBay?

I think so. It's been a while but I kept the instructions. Has the name Col. Frank Trevino (us army ret) of Roanoke VA.

 

[Hofvox]

Sweet job on those ribbon cables.

 

[perryinva]

Wow, the C32 layout is completely different than the C33 layout. I’ve got a local C32 coming in for repair next month or so. Nice heads up!

 

[Johnny_Law]

Instead of a new thread - maybe I can ask here.

Anyone heard from McIntosh about ETA on new C32 glass? I heard it was in their production queue, but no recent updates.

Edit: Got the new glass in later in 2022.