Humans! Lend me your ears (eyes?)
I have a Millermatic 252, 2018 vintage, out of warranty, that has stopped seeing the gun. I have generated a comprehensive testing plan to trace back the possible faults and I keep ending up that it must be a microcontroller. But before I go buying a new board / hunting out the microcontroller that could be causing the problems, here are the present steps - who knows, I will make a YouTube video of this whole process to share the knowledge, and eventually when I’m done I’ll upload the link here.
https://youtube.com/@stormbringermm?si=dhfDayLojqmwy-EsHere is the basic problem: One day, the mig gun stopped responding (not a spool gun). No help messages were displayed on the user interface board (the one with the knobs). So after much swearing, I brushed off my brain and got into it. Here is the procedure I followed, and perhaps if you’re having the same problem this may help you too.
1) Tested continuity across the mig gun with trigger closed and open: PASS
2) Tested continuity by hauling off the plug (RC3 on the main control board), shorting the input pins for the gun on the front of the machine and testing continuity across pins 1 and 2: PASS (this means the wires are good when the trigger is pulled on the gun) I did this again with the gun installed and the trigger pulled, again: PASS
= this means it’s not the gun=
On to the hard part:
The input pin block where the mig gun connects to is labeled RC3. This block connects up to two octocouplers labeled on the board as “OC2 and OC1”.
3) there is supposed to be +35V (dc) from pins 1 and 2 on the RC3 board. Nothing. Zero volts when powered on. Input board voltage is fine. FAIL
4) tested OC1 and OC2 using a diode check on the multimeter across their input pins. OC2 failed, I replaced it. Now both OC1 and OC2 are testing fine. You can test these by shorting the collector and receiver pins (4 and 5) with the power on. OC2 controls the feed motor it will turn it on, OC1 controls the rate of the motor.
5) power to the board is fine, used my thermal camera to check the board for obvious shorts. Only found pants. Eyooooo. Ok but seriously no shorts on the board generating wild heat.
6) still no power to the gun. No +35V across RC3-1 / 2 No visible signs of death on the board.
7) I have back lit the board to try and follow the rails to see if there is anything else, the next steps. I have tested a single upstream resistor (though I assumed this would have been a useless test as resistors normally fail in a let the smoke out kind of way) and the immediate upstream resistor R62 passed fine.

where would you guys go next with this?