HoldPeak HP-770D issue

[valera]

Hi everybody!

I got this guy back in 2017 for 44$, and it had been working pretty well until the last week.

Now it seems that its analog part freezes after a few seconds. I made a short video of that:


1. If I let it rest for a while and then turn it on, then for a few seconds, it works normally, shows voltage and other values ​​correctly.

2. Then, the readings freeze and stop changing, even if I remove the probes or connect something else (s.a. the video above).

3. If I then switch the mode, it shows exactly 00000, and readings do not change anymore.

4. If I turn it off / on right after, it shows exactly 00000, and readings do not change.

5. But if I keep it turned off for an hour, then it works, but only a few seconds (see item 1).

I would appreciate any advice on that.

lygte's review for this DMM: https://lygte-info.dk/review/DMMHoldPeak%20HP-770D%20UK.html

Since I wasn't able to find the circuit here I'll attach PCB photos from lygte article (back side if flipped for convenience).

[valera]

This DMM has two chips inside: HY3131 as analog front-end and HY11P14 as main controller.
I suppose the issue somewhere between HY3131 and it's analog parts. I think main controller works well as high-level functions like hold and min/max seems to be working.

Here is what I've already checked and tried:
1. Voltage regulator IC4 (7530) correctly outputs 3.0V (+1.5 and -1.5 at C13).
2. Voltage reference IC5 (LM185-1.2) looks like it was badly soldered, but on the backside it was soldered okay. Anyway, it correctly outputs 1.235 REF0 to the IC2 pins 4 and 7. I also resolder it just to make sure.


3. DMM's current load is 1.5..1.6mA and there's no visible difference between working and freezing.
4. It's not a HOLD button issue. The button seems to work well.

This is how I decided to power on the device while debugging (with main rotary swtich removed):


Correct position of rotary contacts (just for future reference):