Miller spoolgun 100 only feeds in reverse. how to fix

Run the first layer of wire of the spool you will see a color change. The wire gets a fine layer of oxidation and this causes a bad contact.

Check for binding int he inner sleeve first, then increase tension as long as feed motor spins. If feed motor does not spin check wiring to make sure it is connected correctly.

The control box for the spool gun it's just an adapter to feed the right voltage to the wire feed motor, the heat (voltage) must be adjusted from the three knobs:
1) Process Contactor knob must be dial on wire feeder CV using remote.
2) Polarity Must be set on DC
3) Voltage, depends on thickness of your work piece and wire gage, but 20 volts must be enough for up to 1/4" thickness and 035 4043 aluminum.

I never heard of a 50A spool gun, probably you meant 30A.

If your voltage it's erratic check the following:

Clean and tighten connections both inside and outside unit.

Check and secure lead connections to remote control.

Be sure connection to work piece is clean and tight.
Remove excessive coils from weld cables.

If continues to be too high (hot) and can't control it, then you should send your machine to be checked that's probably a problem with the PC1 or PC2.

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Your Millermatic 175 has syncronize wire speed-voltage feature in order to reduce the wire speed first reduce your heat (voltage). Try again.

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Machine will run only on 220V, check your braker nust be at lest 20Amp.

Did you install the switch that it comes with the spool gun inside the 180? and did you turn on the spool gun side switch on the side of the unit?

First determine if the wire drive motor responds to the trigger.

If not, then there is a fuse for the motor I believe.

If the motor tries to drive the wire, the wire MAY have jammed into the side of the transfer tube or the end fitting preventing it from moving.

Check the Clutch on the wire reel, usualy a bolt centred on the hub this is possibley too tight, slack this off and try again, Tack the tension off the WFR check the speeds are viriable through the range,

check for an inline/or internal safety fuse could be pop noise if no fuse burnt up circuit board

If your welder is less than 3 years old your local Miller authorized service center will repair it for FREE. They can also check the serial number of your welder over the phone to see if it is still in warranty. For goodness sake DON'T tell them you've been mucking around inside the welder or they won't be able to help you.
That said, if the welder spins the feed wheels without any wire between them it probably isn't the board. There is the switch in the gun, the board, and the motor and brake assembly all needed to feed the wire.