[volt-nuts] Fluke 731B battery & charge circuit

[Orin Eman]

On Sun, Jun 23, 2013 at 2:27 PM, Christopher Brown <cbrown at woods.net> wrote:

> Do have have a shorting bar in place between guard and gnd?
>


No, but it didn't make any difference if I shorted guard and either
terminal.



>
> One of the issues I tracked down on my worst unit was a leek from output
> + to guard.  Some of the tin had flaked off of the traces and formed a 6
> ohm connection from + PS out to guard.  Besides guard being hot I saw
> intermittent output.
>
> Simply probing with a scope would return voltage to normal.
>
> Simply placing a _dry_ fingertip across the traces/guard in front of the
> guard would do the same.
>
> I asked about the shorting bar, because connecting guard to gnd in my
> case would pull the ps output _way down_.
>
> Anyway, worth checking for voltage on guard.
>
>

Yes, there is some leakage to guard and a couple of V is readable, but the
leakage in the order of M ohms.  I did some checks with the DMM on ohms,
one lead on guard and ran the other probe around the power supply board and
there are definitely areas with measurable resistance.  I scrubbed some
with alcohol and electronics 'q-tips' and got the meter to read overload.
It looks like I'll have to take the bridge rectifier out to scrub under
it.  I was hoping I wouldn't have to do it, but I guess contamination from
a previous battery leak wasn't cleaned up well enough and I'll have to take
the board out.  Oh well, I can fix the broken pins at the same time.

The output voltage is behaving itself now though and holding steady at two
counts above 10V on the 3456A with 6-digit display selected.

Another thing I noticed is that the zener voltage has a lot of broadband
noise on it.  0.1uF or so across the zener makes it invisible on my 'scope.

Orin.