sentimental METEX M-3850D

[Q8]

tl;dr
multimeter METEX M-3850D
AC rail broken

open shows this:
 20210901_154614.jpg
probing 230v50Hz shows this:
 20210901_154404.jpg
any ideas?

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Hello,
So i was looking around the web for help regarding an old metex i found in my fathers attic.
Im not that deep into electronics, so it is half a learning exercise for me, and half a repair project.
okay, so the problem is this:
I found it already broken, so i dont know how it broke.
Current 4mA, 400mA, 20A measurements: work fine(ish), as far as i can test.
TEMP: i dont know.
fHE: i dont know.
CAP: doesnt work, but i did parasitized it. and i dont really care for it, and it is on a different pcb. I will fix it when/if the rest is functional
DIODE: doesnt work
OHM: ok
LOGIC: ok
FREQ: ok(ish) well it gives me 50Hz when i plug it to the wall socket
VOLT: DC works fine (as far as i can test it, witch is 0-30V)
   and AC doesnt work, but shows values, only very, very small.

As i said, im not really that deep into electronics. Well, i suppose im not totally useless, but this task makes me confused. Im basically sure of nothing. I dont have a proper schematic. the one i found is crap. Blurry. Half the values are unreadable or missing. And its not the right revision date. I found the part list, but its a different revision date too, so there are differences between schematic and the part list, and the pcb. and on top of that, the pcb is old and it was mangled before. there are tht resistors soldered here for testings, apparently im not the first fool to try to fix this thing.

 voltcraft_dmm-3850d.pdf
so... at first i thought that i'll brute force it, meaning, i'll test all the parts brainlessly with the part list and schematic. Few resistors were shot, so i swapped them, and that fixed the DC function, witch wasnt working at the start. Next i tested the op amps. As far as i can tell, they are fine. I dont think the main uCs are bricked, since most functions are working fine(ish). And screen is working fine. So now i started testing the converter on EAO and EAI pins on the MAX 134.
While probing the wall socket i get this from the EAO:
 20210901_154806.jpg
but nothing comes back to the EAI. I dont have much experience with acdc converters, but i assume, something should be coming back?

Well, if someone with some skill could look at it and give me a pointer or two, that would be greatly appreciated

[coromonadalix]

Service manual

https://elektrotanya.com/metex_m3850d.pdf/download.html

User Manual

https://elektrotanya.com/metex_m_3850d_user_manual_eng.pdf/download.html


Normally  you follow  the signal path / function path

Ex:  you put say an 1 or 5 volt dc  and follow it all the way down to the main IC
Ex:  a 20 volts dc source  with a trimpot, to create the 200mv, the 2 vdc, a 20vdc source ....  for the ac  try to find a signal generator at 50 or 60 hertz  with enough voltage output and do the same

Capacitance :  it should be  time based measurements
Temperature:  it will be a voltage based measurement
diode :  voltage measured junction


Check visually the fuse (s)  PTC / MOV'S   the blue and or green discs

[Q8]

Thx for the replay.

Yes, those are the schematics i found when i started. I had them attached under number 3.
Yes, i followed the function path. all four of them, and those paths led me places. The most important one led me to the range-set resistors and leg 24 on MAX134. Those are all fine. I followed them all probing random values from 0 to 30V DC, and the meter shown proper values. AC rail is harder to test. I dont have a signal generator at home, but i have a wall socket with 240V50Hz. I followed it to the MAX134, and i got the proper wave on it:
 20210902_165659.jpg (462.7 kB. 1409x1035 - viewed 62 times.)

Fuses are fine, PTCs are fine(well one is 1.7k instead of 1,5k but that cant be it, can it) MOV's looks fine. But that gets me thinking, that maybe i should build it whole in SPICE and compare one to one? ...ill do that, seem like an adventure.

[shakalnokturn]

Living dangerously using your mains as an AC source to troubleshoot!

For the AC measurements not working look towards IC11, IC15 but first of all check C12.

[Q8]

Living dangerously using your mains as an AC source to troubleshoot!

For the AC measurements not working look towards IC11, IC15 but first of all check C12.

ye, that's me alright. Some people dont wear seat belts, some people dont do backups and i do that.

Yes, i even replaced C12, just to be sure. So im certain its fine. But what goes through... that im not certain in the least.
As i said, i dont do well with op amps. Well i know basics, but those aint basics. I downloaded some papers, am going through them, its going slow.
The problem is, that i have NO IDEA, what i should be getting at any junction in that op amp circuit.
For example, why do i have a flat out zero volts, measured from GND, coming back to EAI of IC2(MAX134) while feeding 'attachment 4' wave from top post to the EAO from IC2. Is that proper?
Well i built those op amps in SPICE, and SPICE is telling me that i should have about 1.7VDC coming into the EAI... But then i cant be sure of that, since i dont have a MAX 134 model for SPICE, so im just probing air...

[shakalnokturn]

AC up to inverting input of IC15.
You should be getting a DC voltage on EAI.
Have you checked the supplies to the OPAMPs?
No shorted diodes?
I'd probably also check the EAI input is working by giving it some external DC (disconnected from your AC source of course) maybe a 1.5V battery.