I'm struggling with a mystery issue.

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

ranatungawk said:

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

Click to expand...

Maybe a long shot, but did you thoroughly clean the flux from the PCB after soldering?

Yes! many flux is there .. but do u think that flux may leads short on PCBs?

regards!

Flux isn't particularly conductive. So from a DC point of view its generally not a problem. But with AC? varying frequency and sensitive electronics, it can play havoc. It has been known to make RF circuits simply refuse to work correctly. Perhaps its causing capacitor leakage?

Also, since its an apparent 'drift' in the zero ofset, you were correct in replacing the caps, but perhaps the crystal should be replaced too?

You have done 90% of rescue works.The ICL7106 A/D converter is available here in SL.I'd say after checking all doubty components then go for that 7106