HT-Instruments HT 79 Clamp Meter - readings low - zero function behaviour

[soldermarc]

I've got a used HT79 clamp meter here.
Some simple dc current measurement tests show values about 10-20% too low.

When I press the "ZERO" button in dc current mode the meter tries to zero but after some seconds the "mA" in the display disappears and pressing the "ZERO" button again nothing happens anymore. The meter seems to be stuck and only gets reset out of this when it is powered off and on again.

If there is some kind of service instructions available or somebody out there who knows what's going on with the device it would be very helpful. The pcb has a bunch of trimmer pots on the back, but I doubt that only adjusting the correct one solves the problem.

[fant]

Pictures?

[soldermarc]

Found someone worked inside before. VR1 and VR3 were put all into the right position. 

[soldermarc]

It seems to be the same as AEMC Model 556.
https://www.aemc.com/products/clampon-meters/clampon-meter-566

I wonder who's the oem for this meter?
Perhaps this would it make easier to find more information or a service manual.

[soldermarc]

Found more names for the device.

KOICO Model 3541
https://www.koico.co.jp/product/acdc-clamp-meter-for-low-current-measurements-2/

KUSAM KM 2006
https://www.kusamelectrical.com/AC-DC-TRMS-Leakage-Clampmeter-KM-2006.htm

Centertek CENTER 262
https://www.centertek.com/product_d.php?lang=en&tb=1&id=205&cid=52

[soldermarc]

According to a sticker on the pcb (262-3) the correct oem should be Centertek and the device is a Center 262.

No service manual found so far.