On my friend's DM-820 multimeter, the AC V and mV settings give incorrect results. When he originally encountered the problem, the value it would show would be too low and drift around. However, when I test it now, it's stable and the measured value changes proportional to the magnitude of the input, but the value is incorrect. For example, a 1V 100Hz sine wave will show 0.1 V instead of 0.7V, 10V shows 1V, etc.
What I've checked thus far:
1. Switching between auto-ranging and manual doesn't correct it.
2. Input protection resistors, spark gaps, and PTCs are OK.
3. Other measurement modes such as DCV and Ohms measure their respective values correctly.
4. I removed the 22uF 16V electrolytic capacitor that controls the time constant for the true RMS converter (AD737). It measures 23uF with 3.4 Ohms ESR, which is OK on most ESR charts.
5. Measuring a 1V, 100Hz sine without the cap installed, which makes the AD737 act like an average responding converter, still shows the same incorrect 0.1V.
6. Connected a capacitor decade to the pads where the elcap was. Different values (2 - 99 uF) make little to no difference to the measured value.
7. I tried comparing the output of the AD737 in my DM-820A (successor to the DM-820) when both meters are fed the same AC signal. The output is different, but my meter also uses different capacitor values. So, I don't know that it's a valid comparison. Same IC, but different configuration, which may also have different scaling.
I'm not sure that the RMS converter is faulty, but it seems that way. Recommendations? Other checks to make?