DMM Metex M-3850 damage by high voltage

[Michaelt]

I wanted to step up voltage using transformer I took from AC adapter, so I connected secondary windings to 12VAC adapter, which would in theory produce 300-400V on primary. Then I wanted to measure voltage on primary using my multimeter(which is rated for 750VAC), but the problem is that I probably accidentally disconected transformer from power supply which resulted in high voltage being produces on primary? I'm not really sure but then I just heard some spark, like that from "high voltage generators", multimeter instantly turned off.
After turning it on I found out that all functions except "TEMP (Room)" are dead. More specifically they show same UI as temperature function so the units are always degrees celsius.
But millivolts work a bit, they can distinguish polarities and also show some values according to increasing decreasing voltage, but as I said it's in "TEMP (Room)" mode so the values aren't accurate. Resistor meassurements shows 0L when disconnected which is correct, but once I meassure something and then disconnect it the value on display doesn't change to 0L, it just starts rising slowly. Milliamps function has "LOGIC" UI instead of "TEMP", it shows "rdy" value which corresponds to LOGIC, but on bottom of screen there is another "display" that shows values as if it was on analog ammeter, suprisingly it shows values absolutely correctly in 0-40mA range, when I go over the range nothing happens, normally it would switch to 40-400mA range or something like that.
Also I can't switch between AC and DC measuring mode but it doesn't surprise me much because I'm stuck in Temperature function.
I appreciate any tips although I think it is permamently damaged. Tried looking into service manual but found nothing about my problem.

After reading my text again, seeing how badly damaged it is, I guess that I'll use my multimeter as room thermometer

[Emo]

Hi,

Had this myself long time ago
If you are lucky the chip under the display, probably a maxim ICL7106 or ICL7107, is in a socket and easy replacable.

Eric

[Kleinstein]

AS the meter seems to be 4000 count, the main chips is not a 2000 count chip like ICL7106.

It sounds a bit like the main DMM chip does not get the info which range is selected. If lucky this is some burnt resistor of switch contact that can be fixed. If no so lucky the main DMM chip is damaged and chances are than small to fix it.

A first point would be opening the meter and look if there is visible damage (chared PCB, burn-marks on a resistor, maybe a bad MOV or PTC ).
I have not much hope for a successful repair, but one could still have a look.