Going mad in repairing Metex MS-9150 Ohmeter

[Fred_TV]

Hi
I own an old Metex MS-9150 instrument with a strange behavior in its DMM ohmeter:
The displayed resistance is exactly HALF the value of resistor under test in all the (automatic) resistance ranges !
DMM is based on MAX134  together with Metex KS57C2016 custom uP
I got (bad resolution) schematics from electrotanya https://elektrotanya.com/metex_ms-9150_sm.pdf/download.html ,  I measured a lot of voltages/references and components values they are ok
MAX134 datasheet clearly talks about half resistance measured value that must be multiplied by two by MAX134 itself or externally by microprocessor, so my idea is this operation doesn't take place.
But the X2 (or called /2) operation is a bit set by logic dialog between MAX134 and uP , it seems strange to me something in this has broken...
I cannot find the failure, I'm trying to repair it by more than a month and I'm almost giving it up.
Any suggestion or already similar issue ?
Thank you   

[MathWizard]

Here's a better copy bottom) of the schematic, from
https://www.eevblog.com/forum/reviews/metex-universal-combined-test-instrument-systemsstations-a-look-inside/

Nice machine tho, with the combined DMM, PSU, counter, function gen

IDK about the uP or the other chips having problems. And I'm having a hard time seeing where the inputs and switches are are going. BUt I'd say on page 3, is where the reference current comes from. Maybe it's even from the max134, across those range selection resistors, just making a divider. Or some of those op-amps there near that chip, making a constant current, then then goes through some known range resistors ?

If something really was wrong and you can't get that 2x mode to work. Maybe the analog circuitry would only need a little change, like doubling the reference current, or swapping a few range resistors, or changing the reference level on some op-amp, or changing it's gain. If it was some dual use circuit, you might need to add a switch as a by-pass, to a little PCB with your different resistors on it.