But .... hold up! What's up with the TIG welder??

The whole aim of this "TIG button" thing is to be able to control the arc current at the torch. This uses the remote control input of the machine, which can take inputs from a torch-mounted button and knob or from a foot operated pedal. 

The inputs for these current and arc controls are typically brought in with a so-called "aviation" connector, although I doubt you'd find (m)any of them on an actual aircraft. These are a 5-pin connector that snaps into place on the frornt panel receptacle. There doesn't seem to be a standard pinout for these, so you have to do some reverse engineering to check how it is supposed to connect up. The torch that came with the machine had both "arc an on/off" switch and current control pot. 

I obtained a foot pedal from AliExpress, as it wasn't possible to get a pukka Arc Captain one here in the UK without a great deal of buggerage, delay and expense. Of course, I've not had any need to use the TIG much since I got it - only a few quick jobs on the Bridgeport plasma conversion and converting some BT40 holders to ISO40 - and I didn't twig that the current control input wasn't doing anything.

Yep, neither the torch nor the pedal seem capable of controlling the arc current remotely. The switches work fine but the only way to control the arc current is via the knob on the front panel. Hmm. What's wrong here?

Last week I removed the cover and had a quick check to see if there was anything obvious amiss. The connections all beeped through to the main PCBA but beyond that I looked no further, as my focus was mainly on determining if the torch controls were actually making it to the internal gubbins.

The pinout of the aviation(?) connector seems fairly clear:

• Pin 1 - Vcontrol
• Pin 2 - +5V
• Pin 3 - Relay (NO)
• Pin 4 - Relay (NO)
• Pin 5 - GND

And there's no galvanic connection between the relay (arc enable) wires and the potentiometer (arc current control) wires. There's no obvious reason why the current control won't respond. 

I may need to look beyond where those 5 wires go onto the control board. The signals go vertically up along the edge of the PCB. There is a small array of Y caps straddling the slot in the PCB near the bottom of the photo. I suspect that 4 pin device is an opto for the arc enable signal. The analogue (pot) signals seem to go into the forest of compts between the 2 vertical slots. Could there be something amiss there?

Of course, the internal wiring colour scheme differs from that of the torch, so I can't tell from here which tracks are which. Time to take a closer look at that board....

Front panel comes off easily enough.

Here's the cross reference for the various wires. The PCB connector is numbered from the top of the connector as it sits in the unit.

Here's that view again:

The inside side of the main board has this daughterboard on it. It sits across a routed slot, so clearly forms an isolation barrier, presumably between the torch side and the digital control circuit.

That transformer is clearly part of a power supply. Looking at the tracks around it and leading away from it, it provides floating rails for the power circuit located on this board. 

But there are 2 unusually large 8 pin DIP devices present too, one each side of the transformer, and both sit astride the isolation barrier. I'm thinking they may be analogue isolation devices of some form. One of them would perhaps bring the control voltage over to the main control micro.

Here's where that daughterboard fits in the larger assembly

Although the daughterboard of interest has the identifier "PS-33-A1", the other daughterboards are either "PK-something" or "PS-something". Here's what looks like the control micro board:

And a snubber board for the main power devices:

So, the "PS" notation doesn't tell us much and certainly doesn't seem to indicate a PSU function. Possibly "PCBA Subassembly"?

The transformer is the yellow box and the 4 pin devices are at the top red (through hole 8 pin DIP) or bottom red (PCBA mounted).

The top device with its through hole pins clearly tracks over to the bottom right of the board ie nothing to do with the torch controls. So that leaves the bottom red box as the area of interest.

There seems to be one pair of pins on the bottom RHS of the transformer, that presumably powers our torch circuit.

That is indeed a linear opto - HCNR200 from Broadcom / Avago (formerly HP at one time in the past):

It has one emitting diode and 2 receiving diodes, one on each side of the isolation barrier. It can be used in sort-of closed loop on the driver side and this gives a fairly accurate transfer function over to the other side of the barrier. This one is actually the one that is associated with the power circuit but they look almost identical, so I assume the one on the daughterboard is the same device - I can just about make out the part number but it doesn't photograph well.

So, it looks to me as if the daughterboard drives the pot on the foot pedal / torch handle and drives the signal across the opto to the control micro. It's powered from the little transformer, so there's little else to the circuit beyond the 2 wires that come out on the control board side. These are most likely something like 0-3.3V or possibly 0-5V. 

I should be able to power up that daughterboard from an external PSU and check the output, which should follow the torch analogue voltage. Of course, if it does, yet the machine still won't vary the arc current, there's not a great deal more I'll be able to do unless I magically spot a dry joint on the way over to the micro board. One step at a time.....