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Old DMM display problem

Hi to all, 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? Thanks.
Reply to
Eremita Analogico
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Of course, go ahead. :-)

I would check the quality of the internal powersupply in the next step.

Olaf

Reply to
olaf

Ok, thanks. I see that the 9V battery is directly connected to V+ and V- pins of

7106 (through power switch)

There are another 2 chips, a JRC 062D (dual op-amp, probably for Vref) and a CD4070 that I think is for decimal point drive.

I will check these two chips...

Reply to
Eremita Analogico

How is the display connected to the PCB below it? Is it a wide flat flexible cable or a pair of edge conductive strips? If strips is the display firmly seated on them? If you gently press on these edges when displaying the incorrect values do the display then show correct values? If so, rather than taking it apart, can you insert some small bits of foam rubber to gently press the display down when it is installed back in the case?

John :-#)#

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Reply to
John Robertson

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 waveform: If there's no oscillation, and it's not one of the nearby components, the likely culprit is a blown 7016. Replacements are available:

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Reply to
Jeff Liebermann

Il 26/05/2019 17:40, Jeff Liebermann ha scritto:

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? Thanks.

Reply to
Eremita Analogico

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