Hi everybody. I'm having a problem with
the kit I just assembled. It's reading a reasonably large capacitance (hundreds of microfarads) most of the time, with nothing connected, and also behaving erratically some of the time. The firmware version is listed in "show data" as 1.12k.
Once I got it all put together (apart from the ZIF socket, because I think I want to change its pinout first), I powered it up using a bench power supply set to 8.8 V to make sure it worked. It turned on just fine, but showed a capacitor connected. I don't remember for sure now, but I think it was shown as between 1 and 3. I restarted it many times and reran the transistor test and C+ESR@TP1:3 test through the menu many times as well. Every time, it showed a reasonably large capacitor, with a seemingly random value usually around 200–300 ?F, but sometimes as low as 90 ?F or as high as 700 ?F. Sometimes it was between 1 and 3, sometimes between 2 and 3. I don't remember seeing it between 1 and 2. About half the time, it also listed a Vloss value, ranging from 0.7% to 14% IIRC. The included 220 nF calibration capacitor didn't seem to have any effect—it's much smaller than the random variation between the readings. (With the ZIF socket not yet installed, I was putting it into the holes and holding it at an angle so it made contact.)
I ran the calibration procedure ("selftest") and it measured the 220 nF capacitor fine there (as 210 nF), but back in normal testing, it was the same as before.
I tried running it using another bench power supply, suspecting noisy power, and it worked correctly about 40% of the time. I then tried an alkaline 9 V battery that was working fine in another (non-graphical) tester, and it still read erroneously (though I only tried it once, and then considered the noise idea disproven, so put the battery back where it came from). I also tried swapping in the chip from the non-graphical tester, but it uses a different LCD interface (as well as the LCD being non-graphical), so it didn't show anything onscreen.
Around the time I put the correct chip back in, it started behaving strangely. Specifically, it would start up straight into the menu, then exit the menu back to the transistor test, go into the menu on its own, advance menu item on its own, and go back and forth between menu and frequency generator on its own (when I was trying to test the FG). I suspected a faulty switch in the encoder, but I haven't tested for that yet. It seems to have stopped that behavior now. Actually, it seemed kind of like it did that more the more it had been powered on/used recently.
The board is still a bit messy with flux residue, but I don't see how that could cause false readings of large capacitance. Wrong resistance readings, sure, but it doesn't seem to see resistance as dominant unless I short the terminals. I also looked at the circuitry between the terminals and the microcontroller, and it looks like there are only some resistors between them, certainly no large and failing capacitors.
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Topic: $20 LCR ESR Transistor checker project (Read 3452985 times)