Control board for a welder - hive mind help needed  - Page 1
Forum section: electronics / repair

author: AvroArrow
author_rank: Newbie
timestamp: November 30, 2024, 11:54:47 am
content: 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: PASS2) 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. FAIL4) 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?

author: CapLeaker
author_rank: Regular Contributor
timestamp: December 01, 2024, 04:13:54 pm
content: Resistors do not always fail open. They also can go high in resistance. In a rare instance they also appear to have normal readings but as soon as you put power to them change their resistance again to high. Visually they looked all good! I’d go after the trigger circuit.

author: AvroArrow
author_rank: Newbie
timestamp: December 02, 2024, 02:26:37 am
content: Hey thanks for your reply. Trigger circuit was isolated and tested in the first few steps - RC3-1/2 feed the trigger, how power gets to the octocouplers/isolators seems to pass through the large ATMEGA microcontroller but it’s really hard to tell, and continuity checks to find the path get interrupted by capacitors / resistors.Thanks for trying to help!!!

author: Paul Moir
author_rank: Frequent Contributor
timestamp: December 02, 2024, 03:26:22 am
content: It's extremely unlikely that the 35V comes from the microcontroller. Hunt around with your multimeter on continuity mode and a nice sharp probe to get through the conformal coating. If you've got some cheap ones or a crappy spare set, consider sharpening them. I think you've run down the wrong path. I'd probably be poking around Q1, Q3, Q4 to see if I could pick up the supply.The 35V likely is what's used to power the LEDs in the optocouplers. I see what look like 10k resistors on them? Possibly a high voltage got on the trigger switch somehow and blew out the optocoupler and whatever supplies the 35V.

author: AvroArrow
author_rank: Newbie
timestamp: December 03, 2024, 10:16:07 pm
content: Hey - great idea. I’ll map out that way. Will sharpen probes like you suggest!

author: AvroArrow
author_rank: Newbie
timestamp: December 04, 2024, 02:01:32 am
content: No dice on that part, all seem ok. Measured in-situ.

author: AvroArrow
author_rank: Newbie
timestamp: December 04, 2024, 09:28:31 am
content: Transistor tests below. -

author: AvroArrow
author_rank: Newbie
timestamp: December 28, 2024, 10:17:39 am
content: Resistors closest to pin out were the problem. One was OL the other 10x its rating (odd). No visual damage.