Re: Random noise on LV power rails


On Wed, Oct 17, 2018 at 07:54 PM, lop pol wrote:

On Wed, Oct 17, 2018 at 07:16 PM, <skv1958@...> wrote:

While trying to figure out what is going on with my 468 I decided to monitor
the -8V and 15V supply DC coupled. When the trace starts to jitter there is
noise on both of them. Not ripple, it almost looks like a digital signal. What
would cause that? This has been a pretty cool trouble shooting experience.

Just loud thinking...when jitter starts in some circuit, sudden current draw
changes may cause sudden voltage changes, which may appear as noise. What
the amplitudes of these noises on power rails? You may also want to check
ripple specifications and bypass capacitors. My two cents on this problem,
apart from the mention of some failing pots earlier...

I ordered new filter caps today. I'm unable to get any capacitance reading
from C926 15V unregulated line. I'm hoping that will cure the issues. The caps
will be here Friday. I already have the old caps out, while I was in there I
replaced all the bridge rectifiers also. I replaced all the small caps in the
storage power supply today and will do the big caps when they come. Figured I
may as well give it the full treatment.
Ok this is what I have figured out so far. I replaced all the filter caps on both power supplies. Replaced the bridge rectifiers on the main power supply. F4009 was blown have no idea how or why. Still has horizontal jitter. Now here is the kicker. I live in Arizona its always hot so my central air conditioner is almost always on. The last few days it has been cool enough to have it off for long cycles. Long story short the jitter happens mostly when the air conditioning kicks on I measured the voltage drop when the air kicks on its only about a volt. I have many other scopes that are not affected in the same way. When the air kicks on I can even hear the fan pitch change. I'm now at a loss in what to do. Could this have anything to do with the EMI line filter?

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