2020-08-10 LC-100A – New L C Meter Refurb

I bought an LC-100A from an eBay seller and upon receipt, it would give a very low reading for capacitors and gave zero reading for any and all inductors. I contacted the seller that I had a defective item; they offered to refund me some money but I wasn’t satisfied with that so I gave them a negative feedback. They kept messaging me with the same offer begging to change the feedback to positive. I finally replied that I wasn’t going to change it, I had been receiving too many defective eBay items and I was sick and tired of the junk (of 8 items I bought in Jan 2020, 7 have failed).

I’m aware that often the problems are caused by solder bridging between traces so I removed some switches and checked for solder but I didn’t find any, so I gave up.

I decided that I would get another LC-100A to allow me to compare the defective one to it. I received it a week ago and it measured both C and L, but the values were more than 10% low. This is way too far out of tolerance for any piece of test equipment — this is sold as a new item. I searched online for a user manual and found one and I also found a schematic. They don’t send any documents with them; if they did the document would be in Chinese (useless) or in Chinglish (nearly useless). Also while I was searching I found that someone had this same problem with measurements that were too low. They had found that the inductor in the schematic was labeled as 100 uH but the one on the PC board was only 56 uH. So the person replaced it with a 100 uH inductor and it was much closer – the capacitors measured only 1 or 2 percent low. That’s a lot better than the ones from the factory. The manual said that the accuracy on some ranges should be 1%.

So I unsoldered the inductor labeled L1 on the schematic. I measured it with my very accurate LC meter IIb and instead of 100 uH, it measured only 48 uH! That is way too low! I removed the wire from the toroid core and found that it uses a yellow powdered iron core, T50-6. I rewound the core with thinner 26 AWG magnet wire. I put much more wire than the original, I covered the core with a single layer of turns but it still measured less than 100 uH, so I put more turns on it until it measured 101 uH.

I soldered the toroid back into the circuit and put it back together. The manual tells how to save the zero value in memory. It now measures the capacitors only 1 or 2 percent low – much better than the original from the factory. There is no reason why the makers of this meter should be using an inductor that is so far off from the specified part value that the meter can’t make an accurate measurement.

Now I have to do something similar with the inductor measuring ranges. It also measures too low. There are two capacitors that have to be the correct values to make the meter measure accurately. I’ll have to trace out the wiring of the four switches. They’re the L/C and range switches and they’re making it much harder to understand the way the circuit works on each range.

Update Aug 15 – I unsoldered the small capacitor labeled C14 on the schematic. The yellow capacitor had no markings at all. I measured it as 967 pF so it was supposed to be a 1000 pF or 1 nF capacitor. I tried various combinations of values of capacitors until the meter gave correct readings for the 4.7 nF test capacitor. I ended up with an 820 pF and a 100 pF in parallel for a total of 920 pF for the capacitor labeled C14.

The bigger 0.1 uF capacitor is for the Hi C range. I don’t need a Hi C range because I have three DMMs that have a C measuring range and I’ll use those instead. I got these LC-100A meters to measure low values of C, below 10 nF, and inductors. Years ago I bought two C meters which the seller claimed measured ESR, but they didn’t. I got a refund. These meters were unable to measure below 100 pF, which are values commonly used for RF projects. So these meters were not useful. The only thing nice about them is the case looks decent.

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