Thanks TiN, you're absolutely right.
My bad. I guess I was just blown away after several weeks of troubleshooting when I finally started making progress.
Talking about progress: I have just solved the last problem and (apparently) fixed frequency and period measurements. I was wrong with the suspicion that U16 would have something to do with the frequency measurement. Rather, it seems to be used for autoranging when in ACV mode. The IC I was looking for was U32 (also an LM393). I found it after close inspection of the PCB. I figured that for frequency/period measurements one would want the AC signal normalized to a certain amplitude range first. So the signal should be tapped off somewhere, where this is the case. And I knew from the ACV repair part that the signal has its largest amplitude, and is also within limits at all times, at the output of U20. And in fact, the output of U20 would feed U18, the True RMS converter, and *something else* (the trace splits into two on the TOP side of the board, and one of them mysteriously vanishes into a via). It took me quite a while though, to find U32 sitting next to the power connector of the analog PCB part. I thought it would rather have something to do with power rail regulation. I could not be more wrong
Now, the multimeter also has frequency and period measurements back, BUT: When measuring period or frequency, the flash symbol appears in the display, which usually means "potentially harmful voltage at the input terminals". I guess it may be that there is still something wrong somewhere. Can somebody with a working unit confirm that under normal conditions the flash symbol does not show (when measuring frequency)? Apart from that, all basic measurement functions are back. But I'll perform a complete test, testing every single function right through the user manual to be sure.
I have made a list of ICs and their function, in case somebody wants to repair this beast. Note that the entire AC circuitry is under a metal can and cannot be seen easily as long as it is not removed.
Designator | IC Code | Function |
U5 | LF356 | AC input buffer, only used for higher ranges (at least 750V range, maybe 100V as well-- not checked) |
U12 | LF356 | AC input buffer, input and output are at the same level (no amplification), only used for lower ranges |
U14 | AD825 | AC input buffer, input and output are at the same level (no amplification) |
U16 | LM393 | AC amplitude comparator, 99% positive that it is used for ACV autoranging. Compares against +0.5V and -0.5V. |
U18 | AD637 | True RMS AC -> DC converter, used for True RMS ACV/ACI measurements |
U20 | AD8510 | Input amplifier/ buffer for AC, frequency and period measurements. |
U36 | LM393 | Comparator for frequency and period measurements |
U37 | DG444 | Analog multiplexer used for DCV range switching |
U49 | AD8510 | Main integrator opamp of the ADC |