First, tack some wires on the back of the board. This is the floating DC rail that powers the daughterboard. It's a 68k resistor, simply because it has 2 ends and the resistance between them is pretty high ie I can ignore the effect on the measurement. It's easy to clip the probes onto and if one end comes off, the other may prevent them jumping where they shouldn't be.
The supply on the floating (torch side) rail is around 8Vdc. That sounds reasonable. With a forward voltage on the arc enable opto of around 1.5V and a 2k resistor, we'd see about 3mA on the LED, which sounds reasonable. This is also the current that would need to be sunk by a remote enable circuit.
The output from the daughterboard varies with the torch-mounted control pot - but not by much. The voltage varies from 0.26Vdc at min setting to 1.59Vdc at max setting.
Twiddling the pot on the front panel has no influence, nor does enabling the arc using the torch-mounted microswitch.
So the bad news is that it seems to be functioning in general terms ie not the culprit that is causing the machine not to respond to the remote amperage control input. Bollocks.
Whether it's supposed to be at such a low voltage or not isn't clear. It's plausible that a 0-1.6Vdc signal into the microcontroller's ADC may be as designed. But even if the voltage was down a bit on the ideal level, it's unlikely that the micro would be expecting much more than 3-5V max, yet the pot has zero effect on the arc current, even then.
Tits. Sounds as if the issue isn't a hardware / component malfunction. I can't be 100% sure but it's looking that way from where I'm sitting.
Time to reflect. And most likely to reassemble the machine. I've put in a support request but naturally China is on full holiday mode, as it's currently between Mid Autumn Festival and Golden Week, so I won't see any action until most likely the end of next week. Hmm. Do I have a TIG welder that can't be remotely controlled?
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