"Robert Morein" wrote in message
...

"Richard Crowley" wrote in message
...
"Robert Morein" wrote ...
I just fixed my Parasound HCA-2200ii, a John Curl design, 225 wpc
Class A up to 6 watts.

A power rail bypass smoked, and took afew others with it. The rail
bypass is etched for a bunch of three caps in parallel, of
indefinite size and shape. It was populated with three tubulars,
piggybacked with three low ESR "block" film caps, three of each,
varying between .01 and .2 ufd. One of the blocks caught on fire,
damaging two neighbors. While I had the amp apart, I found a similar
arrangement, deeper in circuitry; the tubular was smoked.

I called up Parasound. They didn't have anything like the originals,
offering instead to sell me .22ufd caps to make a wholesale
replacement. Anxious to see if the amp stil worked, I salvaged one
undamaged cap off the board, and reached into my junkbox for some
old Cornell "CDR" yellow tubular film caps. One of the low ESR caps
on the bank survived. The amp now works fine, and I really don't
know if I'm missing anything or not.

What are opinions about the choice of caps? How do very old film
caps, say from the 70's, compare to modern low ESR caps? Should I
take the amp apart again and replace the bank with something
"better" ?

If it is power rail bypass capacitance, you can add as many
microfarads of whatever type you wish with virtual impugnity.
The ideal would be to have rock-solid rails that never move,
and the more capacitance you add, the closer you approach
the ideal.

I think that Curl's concern was that the speed of the capacitors is
also important. It's not simply a matter of capacity. This is why
electrolytics are frequently bypassed by small caps with low ESR.

Go for it. Find some nice low ESR microwave-quality
chip caps and heat up your soldering iron.