i removed the mcu last night, cleaned underneath and resolder. put the batteries, still no go, but sometime, few unit symbols lit, intermittent and then hanged, try remove,reinstall batteries again, its dead again, so i almost lose hope, in the end, thinking that its already dead, i just put it aside with batteries installed. i think maybe i can see some burnt element later or flat batteries so i will know something is shorted. but after few minutes sit on the bench, when i hit the power button, it beeps and turned ON! i tried again several times from last night, now its consistently to be working again and ON everytime. so just now, resolder back the input connections, metal shields and everything, put back together and test some measurement. right! calibration is unknown, so now i have one more meter just to confuse me even further. attached showing reading from 2 china meters beside the xtra. the china meters read very closely together, so i'm not sure which one is correct. reference is AD584JN (the best i got here, how poor) 2.5V, 5V and 10V. and the xtra showing 0.6mV when probes shorted together. the other 2 china meters are correctly showing 0V. so maybe this gossen is out of calibration.
one thing i made stupid mistake (besides broken connection to input plug) is as i left the DMM on the side of the bench, i marked the protecting plastic just to remind me later. but it turned out the plastic "absorb" the permanent marker ink and cant be erased with anything without destroying the plastic, even with the venerable thinner. so it toasted (5th picture).
so as a conclusion, the biggest trick is, after cleaning from battery leakage, try reinstalling new batteries and let it sits for sometime. not sure what happened, maybe this meter doesnt like to be turned OFF for long period of time, something needs to charged up slowly? or maybe some deterioration components inside i dont know. i just got lucky i think, i didnt fix anything except cleaning it and look out for disconnected traces, which i didnt find, only damaged to solder masking.
fwiw, ymmv, cheers.