Need some help from an Opamp expert!

[rjmusto]

Hi all,
I am trying to fix a Siemens Multizet analogue multimeter and am a bit stuck ....

The unit looks like it has never been used, not a mark on it, but the meter winding has been zapped.  I have been trying it out with another movement (seems to need something around 20uA meter) hooked up to it and for the most pasrt it all works.  However, on the resistance ranges, it works by running the meter over to full scale for open circuit and working back from there as you measure resistance.

But I am concerned that it fair whacks the meter over on open circuit - which might explain why the original was burnt out. Full scale needs aroung 80mV across the meter connection, but it applies 190mV to it on these resistance open settings, so way too much. It does though make correct resistance measurements, so it seems like it is only when open circuit that things go awry.

The circuit uses a bunch of opamps to handle all this and I have been unable to get my head round it all and work out why this is happening. So could somebody smarter than me help with the challenge?

Attached are two extracts from the schematic; one shows the amp for the resistance readings and the other the amp chain driving the meter. I've checked all the diodes and zeners and not found a fault.

[Kleinstein]

The circuit is relatively straiight forward:
IS2 works as a non inverting amplifier with 2 possible gain settings depending on the range.

IS3 works as a kind of voltage to current converter an precision rectifier. depending on the polarity the current flows either through D11 or D10 and than a bit like 40% through the meter movement and the rest though just a resistor (R49, R50).

IS4a seems to indicate the polarity. IS4b works as a differential amplifier for the voltage over the meter movement part. What is done with this signal is ouside the picture.


The usualy meter movements are OK with a somewhat high than nominal current. So 2 or 5 times the norminal value will usually no to harm to the movement.

[rjmusto]

That’s very helpful. Thanks.

So I guess I’m being over cautious and haven’t found a fault because there isn’t one!

This meter seems to auto-sense polarity and has an LCD window to indicate it, so I think the rest of the circuit out of shot is that bit. It also auto-shifts the scale between x1 and x3.

It’s a nice meter so will be good to get it back into use…..