EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: mianchen on November 24, 2013, 07:00:43 pm
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Bought 2 x cheapo Tenma LCR meters off Toploser recently. One of them working the other does not measure L/C as described.
These meters are exactly same as a UT603 shown in this thread (https://www.eevblog.com/forum/testgear/review-of-uni-t-ut603-lcr-meter-is-it-worth-the-40-usd/msg189226/#msg189226).
After opened and poked around the faulty one, couldn't find anything obviously wrong with it. The only difference is that it draws 20+mA in L or C modes whereas the working one only takes <10mA. The 6V regulated rail reads 5.8V, 200mV lower than the 6.0V in the working one.
I ended up wasting a lot of time measuring most of the components in circuit and compared that with the working unit - no difference. Swapping the 'daughter board' confirmed that the fault is on the 'motherboard'...
Well it's still a good deal for 1 working unit. I don't want to waste more time on the faulty one and I'm gonna bin it (well maybe 'shelf' it and re-purpose it for something else). However before I do so, I'd like to ask here to see if anyone knows that this is a common fault and maybe an easy fix.
(OT: If I want to buy an LCR meter I will definitely avoid these cheapo, doesn't do much, and give no confidence what-so-ever in the results you get from them. lowest range is 2mH, terrible for any uH inductor measurement.)
Attached is a schematic I found on a Chinese forum. The L/C section of the UT603/Teman 72-8155 is identical to this. Almost all of the components and their values are identical. (sorry for the picture quality, couldn't find any better ones)
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Have you checked the voltage rails. there is a -6V rail, and if it is missing you will have very unhappy chips and no working. Likely the 7660 is either not generating -6V or the TLO62 on the input side is fried. Otherwise check the oscillator is working on the 7106 and the 555 timer is oscillating at the right frequency.
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Since it doesn't do L or C, is the R measuring correct?
If so maybe there's something wrong with the oscillator, did you measure/compare the output with a scope?
It looks like the oscillator is the OP62 TL062 somewhere in the middle of the diagram.