You should buy a can of electronic cleaner/deoxidizer and clean
the whole board and selector switch with a toothbrush.
Zinc whiskers can cause all kinds of leakage on a circuit like
that and your board looks nasty.
On 7/9/2022 7:21 PM, Stephen Nelson
wrote:
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Thank you,
Now I have more reading to do.....
-Stephen N.
Hi Stephen.? You picture indicates it's a Victoreen 720
model 3A. Attached is the shop manual for that variant, as
well as an apdate done to a Vic 715 that addressed some
leakage issues..
Geo
Hello,
I purchased a non working CD V-720, and found the
meter "nuts" were not present which caused the meter to
not show any movement...obviously. After connecting the
meter, it seemed to be ok enough to zero, but..... most
ranges were maxed out which I take as high leakage
current from something dirty. The insides looked pretty
clean compared to others with battery leakage damage.
I removed the chamber and the leakage currents were
reduced to a ~0.5R/hr reading on the 1x scale. I cleaned
the glass insulators on the chamber with 99% IPA and a
Qtip and the chamber leakage went down to about 12 R/Hr
on the 10x range.
Looking at the schematics, I see mine is different
from what was published in 1961. I have a 8.2k resistor
across the meter, and the 470k resistor near the
potentiometer is 100K ohms. I am wondering if they made
changes in 1971 when a faint stamp shows CD calibrated
the meter. It looks like my meter was made in 1962 with
"62" on the meter and a 62 date code on the vacuum tube.
I scraped off the flux residue on the solder side of
the PCB (kept dry), and that sorda helped, bringing the
1x leakage currents to show ~0.25 R/hr on the 1x scale.
I am thinking the glass insulators are still dirty
which is causing the upscale readings, and there is a
leakage path on the PCB or switch somewhere.
Questions:
What was the 470K -> 100K resistor mod supposed to
do?
What is the resistor across the meter for?
Does this meter require a desiccant in the can to
help reduce the upscale readings?
Thanks for any help, I attached the check results
with and without the chamber, name plate and? one of the
component side. I left them full resolution such that
one can zoom in if need be.
--
David71