Can somebody verify this with another 8845A/8846A unit?
The thread can be marked as "solved" now,
| 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 |
Thanks TiN, you're absolutely right. ;D
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 :o
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
I know this is a very old thread but did you find anything else wrong with the unit with regard to the flashing symbol you mentioned?First of all, sorry for the late reply. I just want to confirm that even after all these years, the unit is still operating fine. So besides the flash symbol, there is nothing wrong with it. Also, I think I've seen it appearing in brand new units as well. So it does not seem to be a malfunction, but rather a firmware issue (perhaps only with older units like mine)?
So besides the flash symbol, there is nothing wrong with it. Also, I think I've seen it appearing in brand new units as well. So it does not seem to be a malfunction, but rather a firmware issue (perhaps only with older units like mine)?
There is a shared AC/DC converter for both the current and voltage readings. So if zero current results in a non zero reading, I would expect a similar thing with AC voltage - it this happening ?
Testing is usually easier with the voltage ranges.
The frequency reading usually only uses a simple comparator for the input. So low slope signals are always a bit tricky to measure.
To high a reading could be some ringing or interference causing multiple counts in some cases, especially with a low slope, so the input is longer near the sensitive range.
There is a chance the input amplifier may be oscillating for some reason. This could explain trouble with the frequency measurement and the non zero reading. Such an oscillation may be even visible to the input in a lower voltage (e.g. 1 V) AC range.
Another suspect would be hum on the supply voltages. So as usual: check the supplies.
I agree with Kleinstein. First check all voltages, then inspect anything related to the AC path. Check the component list I've made. It could also be a problem with one of the many opamps, probably early in the AC signal path since anything AC related is having issues. Perhaps several parts have issues.
I'd check all signals around these parts for stability (there should be no oscillations), then also check if voltages around the opamps are okay and behave as one would expect.
The AC input takes some time (e.g. 1-5 seconds ?) to settling when switching range - that is normal with the analog RMS-DC converter.
For testing it would be better to use manual ranging - autoranging only add complications not needed, as it may not be clear which range is used and there may be constant up/down jumping.
Different ranges use different amplifcation setting at the front end an can thus be effected different. Especially the higher voltage (e.g. > 1 V) ranges will have some "divider", which often is a different input path.
The voltage and open input case is of limited use, as it can easily pic up some hum.
DCV and DVI have a different input parts. The problem is likely in the AC input amplifier or RMS to DC converter. After the RMS converter the signal usially joins the DC path somewhere.
15 mV with a short is about as bad as the current readings.
The deviation seems to be combination of some offset/background and a scale factor that is a little low. So different range may be effected differently.
Without knowing the circuit configuration it is a bit tricky to localize the error more.
After checking the supplies one could look at the input to the AD637 - ideally with a scope, to see if the problem is more with the amplifier or the converter itself and maybe the connected filter.
With the AC part one can usually pin-point the defect better than just a curde swap all caps and chips. When changing OPs one may need to do a new adjustment of the frequency flatness. The AD637 is a relatively expensive chip. One can usually at least check at the input of the AD637, if the signal there is OK. This would tell if the problem is with the amplifier or the AD637 and maybe behind this.
It is not so much about the cost of the chips, but the indirect changes and possible stress to the PCB, that one should test more to narrow down the defect more to individual OPs / CMOS switches.
Another way for checking would be to apply a test signal AC signal and then look at the output of the OPs in the area in the different ranges. This would help to know which chip is used in which range. This can help to get a crude idea of the circuit and signal paths.
If some OP is oscillating the defect could be as simple as a cold solder joint / broken (litterally mechanical) small capacitor.
One point to check is if the supply is DC or oscillating.
In quite some older circuits they use one supply (e.g. the positive) as a reference for the opposite sign reference.
Most if the OPs would be using something like a +-15 V supply, so this would be the same current for the positive and negative side.
It would however make a difference of the output of an OP is shorted to ground or similar.
Another point to check is the voltage on the input side of the regulator - with a bad fitler cap one could have excessive ripple there and this can make the voltage drop at least on average.
7 V is still not a dead short - a short from a MLCC is nothing so unusual. With extra series resistors this would usually give quite hot resistors, possibly even burnt ones. This would also cause a much lower voltage for the effected part (e.g OP).
None of the manually adjustable ranges of the 400 mA connector work. Thanks for the link, I will check that out!
At the moment I don't really know what to do. What would you check next? Or does anyone know to which part of the circuit the inductor L9 should be connected?
The entire ACV/ACI measurement was gone, but came back after replacing U18 (AD673) and U20 (AD8510).Hi. I got mine with the overload problem. But replacement of U18 and U20 didn't help. What could be the other possible reason?