Serial number- KG234930

Can't get high freq spark- CR5 will not engage when remote is triggered.
I have 2 remotes- both work with 2 identical Syncrowave 250's.

Machine stick welds fine.

2 Miller techs gave me my checklist methods-

• Backfeed 115 volts (wall circuit) to low side of hi freq transformer.
Worked! Beautiful blue sparks (clean, shiny- gapped.008)

• switched PC1 board from identical, working machine- next serial number : )
Working board didn't help CR5- suspect board controls donor machine.

• switched CR5 from same machine-
No joy- CR5 doesn't function in remote.

• switched S3 switch-
Same thing- nothing, nada- no hi freq sparks.

What am I missing?

Thanks, in advance.