MC423 Car Amp Power Guard Light On and Halved Right Channel

smartin53

Well-Known Member
So I got an ailing MC423 car amp and am going through troubleshooting it. Basically the power guard light is on and the right channel is roughly halved in its output volume. A general inspection of the unit shows no major issues with components and nothing overheating. I can actually get both channels equal by adjusting the gain on the right channel so the audio path is clean functionality wise. After doing this and running for a while I again find no faults in a basic inspection.

This leads me to think that there may be a fault in the power guard circuit itself which is forcing the right channel into a lowered power mode. The one issue I could find was an IC that had some corrosion on a lead. Unfortunately I cannot find a service manual for the MC423 so I don't know if this chip has anything to do with the circuit or not. I might try to look at my MC431 manual to at least get an idea how the circuit might behave but I suspect they have different designs since the 423 is missing the PG terminal that lets you defeat the circuit if grounded (it just has 2 ON terminals and these are bridged by a lead on the circuit board too so both definitely have the same function).

While I wait to see if I can get a service manual from McIntosh (unless someone here happens to know of an electronic source that is eluding me) does anyone have any thoughts on what else might be causing this issue? Once I get this fixed I plan to repurpose this as an amp that I'll be mounting under the desk for my PC speakers.
 
Still no luck finding a manual but unexpectedly I ended up fixing the unit anyway. I decided to tear it back down and get a good look at the board on the bench to see if I could find anything wrong. While taking a close look at the various components I discovered that every single one of the 10uf 50v caps appeared to be leaking.

fail.jpg IMG_7108.jpg

I pulled them and swapped em with new ones and the unit is perfect again. Both channels are at full volume and the sound quality in general is night and day from where it was before. Must have been a bad batch as I don't think I've ever seen every single cap of one specific type fail all at once like this. All other caps seem to be fine. I went ahead and hooked the unit back up as my PC's audio amp and all is good. I have an old PC power supply that I've repurposed as a supply for the amp, rigged an on/off switch and I can tuck the whole setup out of the way. Its pushing a pair of SpeakerCraft Monitor Three speakers I have set up in my office.

IMG_7116.jpg IMG_7121.jpg
 
What brand of caps are they?

Looks more like a unit built by a Clarion OEM made than a board built in Binghamton.
 
What brand of caps are they?

Looks more like a unit built by a Clarion OEM made than a board built in Binghamton.

So it is one of the ones built by McIntosh stateside. Not a clarion produced one. As for the caps...either Rubycon or Nichicon I'm guessing based on everything else inside. They didn't have a brand on em thanks to their size.
 
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