[Solved] Extech EX530 multimeter repair

[shakalnokturn]

I'm trying to repair an Extech EX530 clone that has been abused on the mA range.

By the looks of the mess around the 10A fuse (R52, R53) it seems someone has already attempted something.

I've found input protection diodes DH4 and Z1 shorted.
FTTB Z1 has been replaced by 2 series silicon diodes as seen in this place in some of the Fortune semiconductor datasheets for input protection.

Current measurements on mA range are displayed at about 6x reality.

Could anyone with one of these measure the resistance between COM and μA / mA socket with switch on mA and μA ranges please?

[goaty]

The resistor left of the PTC also looks blown.

[t1d]

The resistor left of the PTC also looks blown.

A blown resistor could explain the x6 reading.

[shakalnokturn]

The resistor left of the PTC also looks blown.

They all look in a sad state, they've been bent or squashed by the rear cover or whatever storm this DMM has been through, the middle 1k resistor had a weak solder on the short leg. They all measure good and AFAIK are only related to the V/ input.

Not seeing anything obviously wrong after the diodes, considering I was measuring 60 at the socket for the 400mA range (that's too much power for any of the Melf resistors in there, in fact too much power for all of them added...), I turned towards the suspected previous butchery near the 10A fuse.

A 1/4W 💯 T.H. resistor seemed to have been added over R51. Removing resistors to check them I found that the R51 Melf was open, original value is 1 .
Replacing that with the right value (I have no 1 precision resistors at hand so I selected value using my Fluke 1520 on low Ohms) is giving me readings quite close to normal. Unfortunately there doesn't seem to be a trimmer for Amps calibration so if I want it closer I'll have to parallel some kOhms...

Resistance measured at the sockets is now 10 , probably right, still I find Extech have been a little optimistic considering that over 2W of power are put into the 9  Melf resistor R53 before the 500mA fuse breaks.

Now I still seem to have a bad contact problem to solve somewhere, let's try cleaning the switch and reflowing the ES51966 IC.

[retiredcaps]

They all look in a sad state, they've been bent or squashed by the rear cover or whatever storm this DMM has been through, the middle 1k resistor had a weak solder on the short leg.

The sad looking state is likely how they came from the CEM Instruments.  Many threads on the poor build quality here on the forum.

[shakalnokturn]

Yes I also discovered it was a CEM rebadge shortly after starting the topic, no idea what the corresponding CEM multimeter is though.

I must say considering the retail price of these they're one of the worst built meters I have ever put my nose in.

The best Taiwanese DMM's so far are from CIE.

[shakalnokturn]

No luck cleaning the switch or reflowing the main IC, after some extensive poking-around my bad contacts and jumpy readings were narrowing-in on the precision resistor network.

I removed it to measure values out of circuit, starting to dread an internal failure on this potted component. It tested good and was reassembled flush with the PCB rather than lifted and askew per factory assembly.

Readjusted VR1 (Vref) to get DCV spot-on readings are stable for now. (Multiturn trimmer is another crap design IMHO: Multiturn is good but not the first thing to do when you have to turn it less than a ° to get the LSD right.)

After that I got the explanation for the bad contacts (cracked solders) at the resistor network, I couldn't reassemble the case. The aligned resistor network was now in the way of one of the screw barrel reinforcements. (Black marker on picture.)

At this point it was time to cut the crap.

[rsjsouza]

The best Taiwanese DMM's so far are from CIE.

BK Precision is one of their OEMs and their quality is quite good. However, Brymen is also from Taiwan and I would say they seem to have an edge on quality.