Re: DM44 - Droop in Volts reading or eventually lock down to reading 0033 (regardless of range)).

Fabio Trevisan
 

Hello All,
@Roger, thanks for the heads up... I solved the problem yesterday as per
update below.

Yesterday, after I posted the question to the group, I went back to the
schematics, resolute to find some additional component or connection to
that node of the input of the A/D processor.
It happened that the answer was right there on Diagram <1>, the low pass
filter comprised of C3023 (22n), C3024 (33n), R3023 (2M2) and U3023D (1/4
LM324).

This low pass filter is engaged in all functions and in all ranges, EXCEPT
in OHMS *and* 20Meg range (which perfectly explains why, only when I set
that mode, I could see the right voltage).

I was blind to that part of the diagram because, in recollection to when I
got the scope, when the function TIME and 1/TIME was non functional
whatsoever, and when I first studied the schematics of the DM44, soon
enough I learned that this low-pass filter didn't have an *active* role in
any of the DM44's functions and simply started ignoring it from that point
on.

Yesterday, when I systematically started searching for something else that
could still be connected to that node (that could explain the leak), I
finally paid attention back to this circuit and realized that it could be
the culprit.

Upon returning home, I just lifted, one at a time, C3023 and C3024... and
VoilĂ ! C3024 (33n, PoliStyrene cap, 200V) was the one causing the droop.

I didn't have a PoliStyrene replacement, but did have a PP of exactly the
same value.

After replacing C3024 and confirming that the meter was stable again for
more than an hour (which never happened), I was able to remove the LF411
buffer from the A/D processor input, and it remained working fine and was
relieved to know that the LD111 was not faulty.

I think that - FINALLY - I got this 464 + DM44 FULLY FUNCTIONAL.

I will work now on documenting the main chapters of this story and posting
it to the Files / Pictures area (will take some time to put together).

BRgrds,

Fabio



2017-07-04 7:25 GMT-03:00 very_fuzzy_logic@... [TekScopes] <
TekScopes@...>:



Can you measure the current into the input to the DMM during these changes
to see if you have leakage to ground or a high resistance in the signal
path?

Roger





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