Tenma 72-8155 (UT 603) LCR meter - bin or repair?

[mianchen]

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.

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)

[SeanB]

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.

[PA0PBZ]

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.

[sombat3960]

Hi,

I know this topic very old but it will help if C/L not function.

follow  above previous advice to repair my ut603, 
the issue found is IC6(TL062C) under the LCD display in mother board is dead.
replace it, then the reading value of measuring a known C value is correct.
but the display shows with negative sign ( Don't know why).

for the negative rail -6v,
     before replace about -2.xx to -3v.
     and after -5.69v.

please advice to do next if there another issue.