Millermatic 300 - Humm, hot weld, no voltage regulation

[Tanks_Alot]

I have a mm300 that just started a loud hum, too hot of weld, and does not respond to voltage control. I can't find much about troubleshooting mm300's at all. No tech or service manuals. Almost nothing on any web forums.

[ccawgc]

Do you have 3 phase power connected to this welder? It is a 3 phase only welder.
using the diagram pasted to the cover, find to voltage feedback wires to the pcb. Make sure they are not open.
Check diode SCR assembly for shorts. If any diodes or SCRs are bad they are not available from Miller.
If you have good line power, Good diode,SCRs and good feed back it might be the control pcb.
usually I see the pcb fail with no welder output.

[Tanks_Alot]

ccawgc,

Three phase to welder is good. Voltage feedback wires are good back to the pcb and I unhooked them at the pcb and measured the whole range of the voltage potentiometer as good. I also measured the capacitors as good (easy task). Now I will move on to the rest of your suggestions.

It looks like the whole diode SCR assembly will have to be removed from the welder to test the components. That is a bit difficult with all the crap in the way and the connections to unhook. I will post results, but it might be a 2 or 3 days until I post again.

[duaneb55]

ccawgc,

Should he also check the weld output shunt secondary leads connections at the shunt itself and PC1 connector RC3 pins 5 and 6 (wires #26 and #25 respectively)? Voltage sensing being on wires #23 and #24 at RC3 pins 12 and 11.