A (safer) way to look at the mains.....

Dale Chayes

Noise on the “mains” are often an issue on modern “all electric” ships where I have spent way too much time and with the advent of so many poorly designed switching power supplies and drive systems they can appear anywhere.

The THD from those switchers is a continuing to grow issue. It can cause all kinds of obscure behaviors.

When I have to do this, I used to start with a 10:1 transformer - to get down to 12 (or 22, or 44 or 60) volts AC in front of my scope probe, even though I usually carry a high voltage probe for troubleshooting sonar system power supplies.

Depending on the transformer it will/may have some impact, on the signal but it’s a decent place to start.

I eventually graduated to a Fluke 43b

As always, be thoughtful and careful when poking around any high voltage stuff.


On Oct 20, 2018, at 18:00 , lop pol via Groups.Io <the_infinite_penguin=yahoo.com@groups.io> 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)
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. 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. I have spent 10 hours a day for about a week trying to figure this out.

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