Repair chinese multimeter - Holdpeak HP890CN

[palazziandrea@yahoo.it]

Hi,

I've brought this cheap chinese multimeter - an Holdpeak HP890CN - but soon after it stopped working properly: basically it will only measures volts, while when measuring millivolts it starts from zero and then slowly rises. On all the other modes it measures zero.
On a visual inspection of the board I couldn't see any component that have obviously failed, everything seems ok to my newbie eye.

The multimeter is based on the DTM0660 (translated datasheet here https://www.improwis.com/projects/reveng_multimeters/chips/dtm0660-brochure.pdf

Does anyone have some ideas or some troubleshooting suggestions to try and fix this?

Thanks
Andrea

[Kleinstein]

Assuming the DMM uses the typical application circuit shown in the data-sheet, the first parts to check are the 2 transistors used for protection (may be short) and the PTC and possible extra resistor in series with it (may be open).  A bad contact at the range switch is also possible - though usually more like a problem after longer use.

The fuses may be a problem for the current ranges.

[palazziandrea@yahoo.it]

Hi,

Thank for your answer, if I correctly identified the transistors it seems in fact that they are shorted: I have a short circuit when I test the pins highlighted in the picture below.
The PTC seems ok.

So, next question: should I trust the typical schematics and get two 8050 or should I look for something else? There's a Y1 mark on the component, but I couldn't find anything specific when googling it...

Bye
Andrea

[picburner]

If you look carefully at the schematic in the pdf you posted, the two transistors T3 and T2 (Q1 and Q2 in the schematic) have the base and the collector short-circuited. So it is normal that you have measured a short circuit where you have highlighted, they are just the base and the collector.
The Y1 smd code corresponds to a SS8050 transistor.