Help Diagnose Broken TES-3010 Clamp Meter

[alierengam]

I have this broken TES-3010 clamp meter, it measures 42.6 ohms without any input, measures AC voltage incorrectly (220volts input results in 70 volts) and only DC voltage range works. I have tried testing everything I can though I dont know what some of them are (I measured the suspicious ones out of circuit by desoldering them). Where can I look next? Which components can I test and how?

[alierengam]

That component named RTH seems to make a difference on the measurement when I remove it, the measured resistance goes up from 42.6ohm to 67ohm slowly when removed. It has the marking 152H on it.

[MathWizard]

Well the few chips are a common op-amps and logic chips, and I'd find that PCB the perfect size to work on, and the traces should be pretty easy to figure out. Personally not being familiar with such circuits, I'd start making a schematic in a simulator, just for layout, labelling, and sim of any circuits I could.

The RTH is probably a 1.5k thermistor, I don't see it tho. Was it in series right after the input ? I've seen that on a DMM, with a fuseable 1k resistor before the 10Meg

If you are lucky, it's not a MCU problem. If it is tho, I hope it's not some black tar covered MCU that can't be replaced. But if you could find a replacement, it still probably needs the firmware and maybe calibration settings.

[alierengam]

The thermistor is indeed in series with the input, the mcu is luckily not a blob and can be replaced (though I do not have the necessary tools )

[MathWizard]

But you do have a soldering iron ? Ever solder chips like these before ? They aren't the cheapest clamp meter, do you have a 2nd meter to work on the clamp meter ? Could just be some problem in the input circuits. I fried a couple of cheap DMM's years ago. I don't really remember how, but probably touched the wrong thing while in resistance mode or something, and they didn't have protection circuits up to it.


When/how did all this start? it's not just the battery or some bad connection with the dial is it ?

If it is the MCU, some cheaper meters, just have standard off the shelf chip made for DMM's, that should just work as is, without any programing. I haven't soldered any thing in ages, I'd probably be able to desolder/solder a typical small MCU without hotair, but yeah it's pretty easy to lift a pad.

What tools don't you have ? These days I could de-solder/solder lots of chips without hotair. I haven't worked on any of my repair projects, or soldered anything in months. Now you got wondering what project I should dig out of a box somewhere.

[alierengam]

As for tools, I just bought another multimeter, I have a decent-ish chinesium soldering station and all the soldering stuff like wick, pump, liquid flux... Picture of the MCU under the LCD is attached. I have checked all connections, even remade the 9volt battery connector bit because it was beaten up.

[alierengam]

I have soldered a lot SMD components and even chips with smaller pitch but I dont know about desoldering it. I dont get what a thermistor would be doing there and I dont know how to correctly test that thermistor.
-Update: I have tested the thermistor and seems to be working fine. At room temperature the thermistor measures 1.5kohm and when heated, the resistance decreases.

[MathWizard]

Ok then u have plenty of tools. What parts did you desolder and test? If you know some circuit theory, I'd really map some of the schematic. In circuit you should see if some of the op-amp circuits work. I guess. Check any chip's too, make sure they all have Vcc.

I'd map it out 1st, but at some point, apply some safe voltage at the input, like 10-20V, and try to follow the path in and see if something goes weird.

1 of the worst parts about mapping this circuit fully, is the dial. So find the common ground of circuits, most chips will have a common ground, that should be the common ground for most the circuits in the whole thing. Then map out the input signal paths. That's what I've done before on 2-3 meters. 1 I had a schematic for.

Look up some cheap handheld DMM schematics, a lot of them will have similar blocks. If everything seems to get battery power, I'd focus on the input area and op-amps.

[alierengam]

I dont think I have the skills to make a schematic of it, but i have tested each and every IC they all get correct power. I have tested every resistor and capacitor, I desoldered the ones that were off to test without the circuit intervening. I will do more testing later