Substituting capacitor in MA-5100

[mjarve]

I have attached a link to the circuit schematic for my McIntosh MA-5100. One of the capacitors in the power amp intermittently shorts (becoming a 2- 20 –ohm resistor) and causes the DC offset to go berserk. The original capacitor (C208 in the diagram) is a Sprague “Trans-lytic” cap, rated at 200uF and 3V.

I have an Elna cap rated at 220uF and 6.3V. Would this be a suitable replacement, or too far out of tolerance?

Click image for larger view…(approx. 512KB)

[udip]

There should be no problem, and it will work just fine.

[mjarve]

Thank you very much. I expected that it would work, but this McIntosh has re-written a lot of what I thought I new about amps.

Just curious: Judging by the circuit design, could the failure of this cap cause severe DC offset? That was the initial problem, and as I was comparing the “good” channel to the bad one, I noticed that the same cap in the good channel responded to a resistance test how it should: starting a low value, and quickly rising in impedance. The one in the bad channel first measured at 2-ohms. I removed it and tested it, and it was at 20-ohms. A third time I tested it, it did as it should.

This seemed to be consistent with the occasional and intermittent DC surges at the speaker terminals: some times as little as 0.1mV of DC offset, other times as high as +27VDC.

Whatever the case, the DC offset is consistently low after replacing the cap. I replaced the same one in the other channel, just to be safe, as it is of the same brand and age as the faulty one.

[Elso Kwak]

Hi mjarve,
I would replace the cap by a bipolar.

[anatech]

Hi Mike,
Failure of that cap will increase the gain at DC from 1 to whatever the AC gain is. A small DC offset will become much larger. A bipolar cap might be nice, but only if it isn't too large. Otherwise, replace it with whatever was there. I find 16V caps to be the lowest reliable voltage rating.
-Chris

[Tube_Dude]

Originally posted by mjarve
The original capacitor (C208 in the diagram) is a Sprague “Trans-lytic” cap, rated at 200uF and 3V.

I have an Elna cap rated at 220uF and 6.3V. Would this be a suitable replacement, or too far out of tolerance?

Don't worry about the tolerance...usually electrolytic capacitors are made with tolerances of 20% and sometimes more. And 220 are only 10% of 200.

But use in both channels the some type of capacitor...

PS: Your Mac don't have the output transformer?

[anatech]

Actually, you could use 330uF or 470uF and it really wouldn't make much of a difference in that circuit. Be at peace with your caps.
-Chris

[toddbailey]

thanks for the info, I had the same question about a 100 @ 12 in the feedback loop of a ma-230
I replaced it with a high temp 105 c 100 @ 25 with a leloa name.
Not that the pair failed but 50 yr old electrolytic caps ... scary...

I'm in the process of replacing all the caps, that I skipped 3 yr.s ago.