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[G-ManBart]

So it's an AEA-200LE right? What is the serial number? You're trying to stick weld with it, correct?

What kind of load are you putting on the generator side?

Does the engine increase in RPM when you try to weld?

The more details the better....

[G-ManBart]

I had to buy a couple of other parts online so I decided it made sense to just buy a new display board which put me into the free shipping range. The new board showed up yesterday and it works perfectly. I haven't completely given up trying to fix the bad board, but wanted to get the machine back...

[G-ManBart]

The sections that don't light up give me around 400K Ohms. The sections that are dim give me around 1M Ohm. The sections that light up properly are all 1.4M Ohms. When I reverse the leads I get Open on all of them. I checked the good display board and got 1.4M Ohms on all sections and Open when...

[G-ManBart]

No, only some segments would light up. Some were bright and normal, some were dim, and some were dead.

Good point about checking the capacitor with the multimeter...I'm pretty sure I'll have to unsolder one to check, but that's not a big deal.

I'll do some more digging on...

[G-ManBart]

As best I can tell, the modules don't pop out. I pulled on them and found no sign of movement.

When I measure R2 one way I get 6.76K Ohms and 6.70 the other way. When I measure R3 the same way I get 35.54K and 39.17K Ohms.

D2 and D3 are marked 1N4732A. That appears easy...

[G-ManBart]

Thanks...I've been on the Miller manual page so many times it automatically shows as a favorite in Safari!

Hopefully this link to my Dropbox works. I tend to search for these manuals on line, and read them from there.
https://www.dropbox.com/s/nvzrpmn0d9...c_251.pdf?dl=0

[G-ManBart]

The display board was completely disconnected when I checked.

I'm not in a huge rush to get it fixed, so even if it took replacing D2, C5 and Q3 I wouldn't mind even if there's a chance the board would still need to be replaced. At this point it's more of an experiment than anything.....that...

[G-ManBart]

I tried testing the LED displays....I got some parts to light up normally, some that did nothing and some that were very faint.

R2 and R4 are both marked as 39.2K Ohms. R2 gives me 6.7K Ohms. R4 gives me 35.5K Ohms. On the good machine both R2 and R4 were in the 35K range, and the same...

[G-ManBart]

Hopefully we all keep learning! At least it was a cheap fix.

When I turned on the machine both displays came on, but only some of the lights on each display lit up...sort of fragmented 0's on both. When I turned the knobs, those fragmented 0s didn't change. I've seen digital readouts...

[G-ManBart]

Update:

I replaced the varistor, powered up the machine...no fan. I ran beads with it and the fan kicked on. I stopped running beads, the fan ran for a few minutes and then shut off like you'd expect. I guess there's a reason why the readings on the two boards were so close!
...

[G-ManBart]

I'll check Q7 that way this time..

Unless I'm mistaken, I would have been reading voltage for R96 wrong, because I was using the center pin for VR1 as board common....at least that's what I thought I was using....

[G-ManBart]

I completely disconnected both boards from the machines and checked Q7 again.

On the bad machine I got 3.5M Ohms. On the good machine I got 1.5M Ohms. That's the first time I've found a clear difference between the two machines. No idea what it means, but it's different ...

[G-ManBart]

The manual mentions the fan in the troubleshooting section if the fan doesn't run, and the remedy it lists is to have an authorized service center inspect the fan on-demand circuit, so I think that's pretty conclusive. I've also had much older MM250, and 250X that all had fan on-demand so I can't see...

[G-ManBart]

Okay, on the bad machine I got 9.98K Ohms between A2 pin 8 and micro controller pin 39. I got 10.0K Ohms on the good machine.

I got 9.98K Ohms on R86 on the bad machine and 9.97K Ohms on the good machine.

Since all of these new measurements are coming out the same on both machines...

[G-ManBart]

Okay, armed with that knowledge I just went back and checked again.

On the bad machine I measured A2 pins 8, 9 and 10 to the center pin on VR2 and got 11.04V on all three.

On the good machine I did the same thing and got 10.95V on all three.

I also went back and...

[G-ManBart]

Okay...got it now! I'll go take new readings....

[G-ManBart]

The pictures aren't showing for me, but the fact you said the notch is in the top of LM324 says I probably was measuring the pins wrong. As installed in the machine, the notch is on the bottom and I was counting down with top left being #1, top right #2, etc. I'll reverse that and start with bottom...

[G-ManBart]

Is there anything in particular you'd want to see?...

[G-ManBart]

I was a bit confused about calculating A2 pin 10...maybe I was reading the equation wrong?

Both machines gave a touch over 46K Ohms when I measured the thermistor. I measured resistance on R70 and got 8.24K and 8.35K Ohms on the two machines. I measured resistance on R33 on both machines...