If output is always above or always below the set value, have a Factory Authorized Service Agent check control board PC1 and hall device HD1. Don´t do it by yourself, there is a good chance to destroy the board. You should still be in warranty.

Hi,
This is most likely one or more of the high current diodes have failed.

If you look inside the unit - they will be bolted to a large heatsink plate - and look like threded bolts with a heavy wire attatched to the head.

The good news is that you can buy replacement diodes and replace them yourself - and not have to buy a complete rectifier assembly.

Make sure you get the same type !

You can test them with a hand torch, or car bulb /tester arrangement.
They should allow current through one way only, and not in reverse.

If they all test ok (unlikely) - then the next possibility is a short in the coils of the transformer. This will result in effectively fewer turns. Fewer turns = less current. If this is the case - transformer is scrap.

Hope it's a couple of diodes (available through Ebay etc.)

Try replacing the amperage dial. its a switch essentially.

The amperage dial will have to be replaced as that determines the amperage of the output.

Miller is a great company to work with. They have all the parts in stock and are quick about sending them out.

here is the link to Miller welding. http://www.millerwelds.com/service/

It looks like there is no amp control anymore, amp control may be fried.

Output control and Reostate must be tested. If the unit has a pedal control, you can switch to remote pedal control and see if the problem persists, that will tell you if problem is control.

Is welder is less than one year is under warranty.

Here a manual for a 250 series, you will find schematics at the end.

Here factory website, you will find manuals and schematics for all models and contact info.