On 19/06/2015 02:31, Arfa Daily wrote:
I currently have a Peavey PVX-P12 amp chassis on the bench. There was no
audio at all from either the HF or LF digital output ICs. The IC in
question has a 'go / mute / standby pin and this pin on both ICs is
connected to a little control circuit. That in turn is driven by the "AC
Detect" circuit on the power supply. This bit of circuitry is very
straightforward, and comprises a bridge fed with AC from the line input,
via a 0.47 uF 275 v ~ x-class cap. Across the output of the bridge,
are two caps in parallel, shown on the schematic as being 22uF at 25 v
working. But here's the thing. They are not shown as being polarised,
nor is there any marking that I can see on the caps themselves, that
indicate any polarity. They are surface mount and small - approx 3.6 x
2.6 x 1.8 mm - and look just like a typical sm ceramic cap. Pink-y
glazed body with conventional 'end caps'. One of these two capacitors is
short circuit. With it removed, everything returns to normal, and the
amps both un-mute. The circuit only produces a few volts, and this is
used to drive the LED in an opto via a 47 ohm R, the transistor side of
the opto being the "AC Detect" signal that connects to the mute control
circuit on the amp board.

So what type of caps are these ? They seem awfully small for any kind of
solid dielectric non-polarised cap of that value, and that sort of
voltage rating. I've had a look around at a few component supplier's
offerings, and can't find anything that seems to match. I checked the
capacitance of the one that isn't short, and it came up at 18 uF, so a
bit low, but in the ballpark of what it says on the schematic. Given
that these caps only seem to serve as the filter for the bridge output,
and that there is only a few volts across them in normal operation, can
anyone see any potential problems with replacing them with a pair of
'conventional' 22 uF 16 v polarised tants, obviously taking care to put
them the right way round ?

Arfa

Why would they have to be unpolarised? Just 2 paralleled together to get
the required capacitance. Any polarising mark on the SM carrier strip
perhaps