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Eremita Analogico Eremita Analogico is offline
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Default Old DMM display problem

Il 26/05/2019 17:40, Jeff Liebermann ha scritto:
On Thu, 23 May 2019 20:45:09 +0200, Eremita Analogico

I have collected an old (80s) HT 1100A multimeter that seem a clone of a
Fluke of that era.
It mount a classic 7106 A/D converter that drive driectly a LCD display.
When turned off, the display appear in good conditions, no black marks,
brownings or burning signs.
When I turn on the DMM, the display show almost all segments, except
when the display is not 00.0 (that appear like 88.8)
If I measure something that implies display to show fewer segments
(like, i.e., 11.1) the reading is ok without ghost segments.
If I reduce supply from 9V to around 4-5V, the display start to show
00.0 correctly, but with still some light ghost segments.
Replacing the 7106 with another don't eliminate the problem.
Do you think that I can try to solve?

The clock in the 7106 is not running. I would look at the 100pf or so
capacitor on pin 38 of the 7106 for some kind of clock oscillator
If there's no oscillation, and it's not one of the nearby components,
the likely culprit is a blown 7016. Replacements are available:

Ok, thanks to all.
I have checked the dual op-amp and the CD4070 and both is ok.
I have checked the capacitor and resistor at pins 38, 39, 40 of 7106
(clock) and the values are correct, BUT if I power on the DMM without
capacitor at pin 38 the display is perfect 00.0 without ghost segments
(but obviously without any A/D conversion).
This is a significant symptom?
Replacing capacitor with another of same value (100p) make always
display corrupted.
I have 3 7106 and I checked with all, same problem.

The display is mounted in a small PCB connected to main PCB through a
flat plastic flexible "foil" with argent traces, pressed to pcbs with
the plastic support of small display pcb. The resistence between every
pin of 7106 and pads in small pcb is around 3 ohm.
From small pcb to display there are classical siliconic conductive
strips that I have carefully cleaned, together with plastic flexible foil.

Where I can try to found the fault?