Re: Random noise on LV power rails

Harvey White

On Sat, 20 Oct 2018 19:00:20 -0700, you wrote:

On Sat, Oct 20, 2018 at 05:56 PM, Harvey White wrote:

On Sat, 20 Oct 2018 17:08:56 -0700, you wrote:

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

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

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.

Sounds like something is not regulating. What it sounds like is that
you're getting coupling from the AC line into the power supply output.

First thing I'd do is to (if you have a variac available) run the
supply voltage up and down by a bit and see what the power supply
voltages are doing.

Secondly, I'd look at the noise on the AC line, and see if it
correlates to the jitter on the scope. Definitely want to see what
the noise is on the power supplies. It suggests to me that a number
of the bypass capacitors could be bad (as a thought, that's just noise

Now, if the regulator can't regulate fast enough (and they're supposed
to be pretty good about some of that), then it's possible that the
noise on the AC line is coupled through the regulator to the load. It
is possible, but it requires something to be not working.

If the regulator doesn't have enough headroom (what's the voltage
setting on the scope and the line?), then it can drop out of
regulation. That lets lots of noise get through.

What does the line do when that AC is on? How much noise and what is
the voltage?


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?

No variac :(
I have not looked at the mains with a scope (because I'm scared to do it)
Reasonable enough. As was suggested, a filament transformer or other
transformer designed for AC line use would work well. Even one of
those little AC supply wall warts would be reasonable. Not the best,
but reasonable.

The low/high switch his set to high and the voltage on the line before the air conditioner kicks on is 120-122V and drops to about 119V when its on.
Fairly reasonable....

I plugged the scope into a APC 1500 UPS and that SEEMS to almost totally correct the issue. The issue is so random one time the air will kick on and the scope will display no issues the next time it will. There is no way I have found to reliably induce the symptoms. I think I need a variac.
It could help, but the absolute line voltage may not be the problem.
Noise may be.

You know that UPS supplies can have filtering in them?

You may want to just see if you can find a filtered outlet strip,
borrow it, and see what it does for you.

I have spent 10 hours a day for about a week trying to figure this out.
not surprising, you want a good hard repeatable failure every time.

You don't get them.


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