Millermatic 200 pcb problem.

[ccawgc]

I have a strange one here. In six months this mm200 has killed 3 motor pcbs.
When we installed the first one it was DOA and Miller supplied a new one. That one lasted about 3 months and it died. We put in another new pcb and also replaced CR1. And now 3 months later it's back again. I have checked the parts all around the pcb and not found any bad. Welder works just fine with our test pcb.
Have talked to Miller tech support and they are puzzled. I have narrowed it down to either a bad batch of unijuction transistors (Miller is checking on this) or something at the owner's shop is killing the UJTs.
May question is has anybody seen this before. I am looking for external cause to these pcb failures.
One thing, problems with the drive motor, brush noise, current over loads will kill the drive SCR Q1. And do not cause problems with the UJT.
This question is for other techs and repairman out there.

[duaneb55]

Mmmm.

2-wire or 4-wire motor?

If 2-wire, how is resistor R22?
Finite life varistor VR1?

I'm sure you've checked but is it jumpered for200VAC and operating on 230? If owners shop power is right at 230V, jumpered for 200V would put secondary voltage from T1 just above a dangerous 10% overage. If even higher (such as 245-248V I see at my shop) then even worse. PC1 supply power should be 102VDC +/- between pins 8 & 10.

[ccawgc]

2 wire motor. I think R22 is ok. I tested most of the large resistors in that area, but will double check when I get back into this machine in a few days. I have ordered the parts I need to repair the motor pcb, then I will look back at this machine and check the different ideas I get from people.
Right now possible causes are, motor over current, This can damage the SCR and sometimes take out the UJT. Drive current should be less that 1 amp.
On my line power I did check the board power and it was 22 volts. Which is good.
Just looking for off the wall ideas that might make sense as to the cause of these failures. Soon as the parts for the pcb arrive I am going to look at it again.
Most of the time with this pcb. IT dies you change it and never see it again.

[duaneb55]

Might want to change VR1 (or at least test it) seeing as it is there to protect against surges that could take out Q2.

Agreed. The 057314 board used in the MM200 and a number of wire feeders is pretty bullet proof.

[DougAustinTX]

I always suspect bad grounding, either internally, or on the line when I see weird behavior.

[Silicon-based]

Right now possible causes are, motor over current, This can damage the SCR and sometimes take out the UJT.

Could it possibly be a bad commutator in the motor or a shorted armature winding such that the driver sees a dead short if the motor is in a certain position? I realize this is more likely to kill the SCR but you never know... Just an idea that popped into my head.

John

[ccawgc]

I got some parts coming in. Soon as I get the SCR off the pcb going to test it.
At this point going to repair the pcb ( about 20.00 in parts ) Replace the MOV and then do some current and voltage tests. Try to see if we have a motor problem or maybe the owner does have an odd problem in his shop. WE will see.

[duaneb55]

Any solution to this matter ccawgc?

[ccawgc]

Parts came in. Finally was able to get to it. Just replaced the UJT on the pcb.
Checked current draw on the motor and it was very low. 100ma to 200ma stopping the wire. Miller told me anything less than 1 amp is good. I did change the MOV. The new one has a different value. New one is 130 volts and the old one was 200 volts. same joule rating. Plus I found the ground wire and one hot wire were loose in the line plug.
Now we send it home and see if it lives.
Duane your mail box is full.

[duaneb55]

Parts came in. Finally was able to get to it. Just replaced the UJT on the pcb.
Checked current draw on the motor and it was very low. 100ma to 200ma stopping the wire. Miller told me anything less than 1 amp is good. I did change the MOV. The new one has a different value. New one is 130 volts and the old one was 200 volts. same joule rating. Plus I found the ground wire and one hot wire were loose in the line plug.
Now we send it home and see if it lives.

What was it Doug was saying about bad grounds?

Here's hoping you got it. If so, would be nice if it was one factor or multiple but not likely one could determine.

Duane your mail box is full.

Still?! I thought I emptied it out today. ???

[ccawgc]

I found it full the day I started the thread looking for a S-22 feeder cover.

[duaneb55]

OK. Well there's room now.

[ccawgc]

Don't happen to know where a Junk S-22 with a good cover is hiding?

[duaneb55]

No I don't and if I figured anyone would my first guess would be Steve (7A749).