Re: 2236 DMM issue - anyone able to help please?


Kevin Wood G7BCS
 

check cr1976....and look at anything on the schematics that can be a low z
to the 7.5v feed.
if the cd4051 (u1905) configures 14/15 as an output and cr1976 is
shorted......
jim
I think I might be getting somewhere.

Wind up the voltage slowly (so the Hi-Z buffer output only reaches 0.5v or
a little more) and on some of the ranges the +7.5v is getting pulled down
by a volt or so but this doesn't appear to cause a huge problem.

Wind up the voltage quickly (so the Hi-Z buffer output rails at +6.5v) and
the +7.5v rail drops to 2.5v and, significantly, I've now found that the
-7.5v rail also disappears completely (this one isn't affected by the
range switching at all).

I have a theory - some of the darlington emitter followers driving the
relays are working but low on gain, so drawing more base current from the
latch IC which is on the +7.5v rail, causing it to sag on some ranges.

The CMOS switches handling the Hi-Z buffer output are across the +7.5 /
-7.5v rails. If the Hi-z buffer output is only 0.5v the low +7.5v rail is
of no consequence. If the Hi-Z buffer rails at 6.5v and the +7.5v rail is
low, it could be that we're causing the CMOS switch to latch-up, which
takes out both the +7.5 and -7.5 rails and kills the supply to all of the
relays since the latch has no power.

Once the input filter discharges, the latch-up obviously clears and we're
in business again.

I think I'm going to replace the darlingtons and see if that helps.

Kevin

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