20 Jul
2006
20 Jul
'06
9:02 p.m.
On Thursday 20 July 2006 07:45 pm, Scott Quinn wrote:
O.K. - now on to the power supply.
The +5v channel 1 supply is good, 69 mV peak to peak AC noise (measured on
a scope with AC coupling) the +-12v channels are ~292 mV of noise
the -5v channel has 500 mV of noise, even with an additional 1000 ?F
capacitor across the output (to hopefully filter this junk out) (-5V also
has periodic fluctuations at a lower frequency)
All of this is high-frequency, excluding the -5v hum.
A 1000uF cap isn't going to help you at all with high frequency noise, at
which point it probably looks more like an inductor than a capacitor. When I
want a really quiet power supply I generally put a 1 - 10 uF tantalum cap
across the big electrolytics, _and_ a smaller ceramic disc cap of somewhere
say up to 100nF but no higher, which tends to give you better coverage.
In short, it doesn't look like adding an external
filter cap will help
much, unless I go all the way and throw in a LC filter.
If what I suggest in the above paragraph doesn't help then maybe a pi-filter
is perhaps the next logical step.
Pulled the P/S apart, and the board with +-12V and -5V
has peculiar
capacitors mounted on it - 3 lead Mallory Aerovox. Are these really the
multipart electrolytics that went out of style (I thought) with solid state?
Probably not.
The wiring is 2 leads connected to one side of the
power rail, one lead to
the other. Could twin-lead caps be substituted?
Maybe.
I haven't seen these but I'd guess that they're something special with that
configuration. I remember years back (1974!) Zenith used a special 4-lead
cap in their TVs on the output side of the flyback transformer. And some cap
maker messed up, changed what the materials were or something, and ended up
hurting bigtime because they failed left and right and you had CRTs getting
ruptured from 40KV+ voltages and similar nonsense. I heard that Zenith
covered all of that and that they got it all covered by the cap maker...
Have you done any searching for those parts?
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