Hameg 8011-3 4.5 digit DMM (not so broken) repair

[anotherlin]

Hi everyone,

I'm a little puzzled. The problem is a 8000 series Hameg 8011-3 4.5 digit DMM module.
I have included links to the service manuals for both the 8011-2 and 8011-3. The 8011-2 has more readable schematics in pdf.
The 8011-3 is a later model that's basically the same circuit as the 8011-2, components even share  same numbering, but with some additional features.

This DMM can measure voltages up to 0.2V to 2000V, both DC and AC.
It can measure resistances from 0.2kOhm to 20MOhm.
And resistance from 0.2mA to 2A, both DC and AC. And it also has a 20A measurement connector.

It's not so broken because when it does measure, it actually measures accurately in all described mode.
I have both a handheld Fluke 179 and a 5.5 Agilent U3606 to double check that.

I have two problems, the first may actually be a "standard behaviour" :

1) It may take some time (1 or 2 seconds) for the result to "ramp up or down" to the correct result. That is, if I measure a 9V battery, I may first read 5.23V then 7.90V, and eventually 8.96V. That's the case in all "modes"

That may actually be standard behaviour. The DMM uses an ICL7135 integrating ADC. It's just doing its measurement and it need some time, with intermediate results in between. Can somebody with experience with that kind of IC confirm that ?

2) Sometimes, it is stuck with a wrong result. For instance, I'm measuring mA (I have linear DC supply for testing), then I switch to DC voltage measurement. The reading stays stuck at let's say 2.20V and my probes are not pointed to a voltage source. It should zero ? When I'm measuring a 9V battery, it stays stuck. I just press the various mode selection buttons, turn the range selector button, and I'm back on track. It measure the correct around 9V value.

In particular, if I stay in one mode (current, voltage, or resistance measurement), but change range, etc. I always get correct result. But when I switch, that's where I have to press and depress button.

May be a coincidence, I get the problem in voltage measurement mode.

I have checked the schematic. And from the current behaviour :

1) I believe the ICL7135 along with its charging cap, etc. is working properly. Results are accurate when there are results.
2) Because the current measurement input is so simple (precision resistors), I'm pretty much sure it's ok.
3) The voltage input should be correct. Results are accurate, precision resistors are ok. On the schematics, there are some caps in parallel with the resistors for "frequency compensation", can this be the erratic behaviour ?
4) The resistance measurement also uses the voltage input resistance network, and it is accurate, so I guess that would confirm 3) ?
5) The AC measurement is accurate. So I guess the AD536 AC to DC RMS converter works.

Any idea ?


Sorry for this long post. But I don't know where to look at. Especially because I don't know how to reproduce the problem.
 

Thank you.

Best regards,

Here are links to the service manuals :

https://drive.google.com/file/d/0Bxd9xq3tnxpiN01yb2RNV3lQNUE/view?usp=sharing
https://drive.google.com/file/d/0Bxd9xq3tnxpicDBMQVQteUxkMGc/view?usp=sharing

[anotherlin]

Ok, I've found the problem and fixed it.
It's one of the mode selection switch which had a bad contact.
I traced the entire circuit !