4/6/2023 0 Comments Tip31 pinoutPulling the null detector output high with a resistor to +5V does switch off the overload condition and displays the last measured input, so it's definitely something in the null detector signal chain but after the ADC integrator. Looking at Q1 as the likely culprit because it triggers display blanking during the measurement cycle, I stuck in a replacement with no effect. The schematic fragment shows essentially the dual-slope ADC reference current comes in from the top and unknown voltage from the left. Unfortunately, I suspect that cycling the power a few times during the checkout has resulted in something else failing, because now the display shows overload on all modes/ranges (it blinks 0.0000). Replaced both of the transistors just to be sure (used TIP31/32, though the pinout is different) and voltages look fine now. One of them didn't measure quite right so I took it out and in the process saw that the leads had separated from the package. New filter caps didn't help, so I looked at the series pass transistors (MPS-U05/U55). I started by looking at the PSU voltages, which were on the low side. Unfortunately, when compared to my voltage/resistance references and my other DMMs it's definitely out of cal now. It's pretty nice for a manual ranging DMM and has current measurement too. I've had this unit longer than any other piece of gear I must have picked it up in the late 80s at a hamfest.
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