I'm struggling with a mystery issue.

[ranatungawk]

Hi friends;

I'm struggling with a mystery issue. Please help me by giving your expert comments on this….

I'm troubleshooting somewhat old "YF-1150A" capacitance meter. When I received it didn't work at all. I found some signaling transistors got burnt. Having replaced them, I was able to fix major issues and now it's reading is almost correct. However, after 3,4 hours of repairing , the meter's idle value is changed to -00.2,-00.2…-0.4 (idle value should be 00.0 when any button is not pressed.).

The mystery I found was this, when the idle value is changed (to -00.4 or -00.6) If I remove some parts (ex: transistor, capacitor..) from the PCB and resold them again as they were on the board, idle value comes to "00.0" again. but after 2,3 hours it go wrong as it was.

So I thought this can be a result of a "bad capacitor" somewhere in the PCB. so I replaced all caps and checked all transistors. But the issue remains same.

This meter circuit is based on The ICL7106 A/D converter. Please see the diagram and pics I have attached herewith. Could you please let me know what should I check…

Thanks

 

[shrtrnd]

You've done everything I would have done (first).
Since it's not your caps, I'd be looking for a 'leaking' transistor. (Though I suppose it could be an IC). Your problem is one of the semiconductors 'leaking' when the electronics 'warms-up'. An easy way to find it, is after the few hour warm-up time, shoot each one of the IC's and transistors with a can of freeze spray (one at a time). You'll see an immediate return to zero, when you hit the right one.
Good Luck

 

[nomi]

nice work !!!
em a newbie so cant give you any kind of suggestions .
best of luck .