Farnell DM131 Digital Multimeter

[WaveyDipole]

I have a Farnell DM131, an older, DMM, the design of which dates back to 1978. It measures AC and DC volts and current as well as resistance  with two scales, ' ' and '20M', and also temperature. Initially it was not working and the fault was relatively easy to repair once traced. It looks like the jack for the thermocouple had been replaced but was not wired correctly and an earth connection was not being made which caused erratic random readings.

The DMM seems to work fine now. Voltage measurements seem accurate, current and resistance was only very slightly out when compared to my Fluke. The problem I have with it is when taking resistance readings. If I strap a resistor directly to the terminals it will give a pretty stable reading, but if I attach probes to the terminals, the reading fluctuates making it difficult to take a reading. The higher the resistance, the worse the problem and it occurs on both DMM resistance settings. This is even so with different probes and even if the resistor is strapped across the terminals and then probes are just inserted into the jacks without actually being connected to anything.

I did a little checking with an oscilloscope and found that although being mains driven, the power rail is quite stable without any significant ripple, despite my not replacing any capacitors yet. With a resistor connected to the terminals and with the probes plugged in I did pick up some slight mains hum noise, which disappeared when the probes were removed - i.e. it was not present on the terminals themselves.

I am wondering whether the mains hum noise being picked up by the probes is destabilizing the circuit somehow, but I don't have this problem with my other DMMs. Is this a 'feature' of older, mains powered DMMs - or specifically this model - or is this a fault condition?

[Kleinstein]

Most DMMs use an ADC that averages over a multiple of the grid period (20 ms or 16.6 ms). This give a pretty good suppression of mains related signals. In an old meter the frequency could have drifted, making it more sensitive. Mains powered meters could have capacitive coupling - not unusual to find half the mains voltage.