UNI-T UT61E with wrong dial seting

[rodrigopires]

Hello everyone! I have a uni-t UT61E and used it for quite a while now and last time i was going to mesure some resistors it showed capacitance mode and now it doesn't go normal... it doesn't even make a beep when changing from resistor to capacitor wich indicates it's in the same mode but i dont know what could be wrong. It didn't droped and everything functions as normal since last time i used it. Any explanation? Could it be the selector dial contacts?

Thanks! 

Video: https://youtu.be/pr_os8ni2aA

[jxjbsd]

Clean the PCB, or the spring
I have used it for many years, no problem

[rodrigopires]

opened it and clean it and still the same, there was no marks or debri on the contacts. To note that it happened suddenly last time i used it it was ok used resistance mode and today i found it like this...

[odium]

hi,

could it be that you measured some cap without previously discharging it  ?

in capacitance mode, can you check if you have 1 khz signal at the probes ?

cheers

[rodrigopires]

I don't belive so only used capacitance mode once or twice in years and with discharged capacitors (actualy was to measure homemade capacitor value). Tested capacitance mode with a 103 cap and it works ok 10nF in both modes resistance and capacitance 

[PedroDaGr8]

According to the logic table in the schematic, the only difference between resitance and capacitance is that FC4 is high (DGND) for resistance and forced low (V-) for capacitance. Since FC4 is normally high (DGND) via an internal pullup resistor, something is pulling it to V-.

This leaves two options:

1. Diode D4 is shorted to V- via FC1. If this is the case, removing this diode should force it into resistance mode (the capacitance selection would not work) as long as the internal pull-up resistor is undamaged.

2. The internal pull-up resistor is damaged. If after removing D4, the meter still insists on capacitance mode this is the most likely cause. A 2.5MOhm resistor between the FC4 pin (Pin 119) and DGND (Pins 3-5 or left leg of the voltage regulator) would mimic the internal pullup in the ES51922.

3. The internal circuitry for FC4 is damaged, the meter is toast.

[rodrigopires]

Thank you PedroDaGr8 you went right to the problem! Did some mesurents and got everything right except for FC4 in resistance, capacitance and frequency mode. Also found a reprair series on youtube of the same meter with similar problem by DrCassette, in his case he changed D4 and added a 24 ohm resistor to ground. Next i will be removing D4 to se how it behaves.

[rodrigopires]

removed D4: left diode shorted and right diode ok.

Now i have resistance on resistance mode and mA on capacitance mode, everything else works ok. Diode replacement options? 1N400's or 1N4148? that's what i have in stock