David, thank you for your answers to my questions. These are interesting problems; I'm not sure if they are due to faults on your machines, or a general behaviour that no-one has noticed before. Increasing the wirespeed is not expected to change the OCV, but perhaps this has never previously been monitored.
I asked about the OCV with the wirefeed motor disconnected because I thought that maybe the speed controller is feeding the motor with pulses, and inductive kickback from the motor armature is generating high voltage pulses. The speed controller component TIC 116D is a thyristor ( SCR ) rated at 400v and 8 Amps. It will be sending variable-width pulses at 100Hz from each half-cycle of the rectified DC welding output to the motor. If a power transistor had been used in this role, it would be providing a variable linear current source. A digital meter may show inductive pulses as a higher voltage. Using an oscilloscope would confirm this, or using an analogue meter may draw enough current to absorb the spike.
Your photo of the wiring from the transformer secondary to the rectifier shows that a full-wave bridge rectifier, with four diodes, is used. I had though that perhaps a centre-tapped secondary, requiring only two diodes and having one of them shorted would explain the high voltage readings.
Generally, these Italian-made MIGs show high values for OCV ( "Uo" ) which then drops considerably under load.
The Special Car/Cebora Mighty Mig does seem to have a permanently live torch, as a111r suggested, so voltage is present before the trigger is pressed. The trigger enables the wirefeed motor, which takes it's supply from the main welding current, so the voltage at the torch tip decreases slightly.
Thanks very much for taking the time to reply and for all the detail. Much appreciated.
I Think they are OK then as they are both usable, the Special Car / Cebora Micro Mig does as you say have the live torch, I am guessing this is due to it being so old.
I have been using it today and it works well. The Snap On 2135 is also working but I am finding it harder to get the settings right.
Out of interest I asked a friend to test the OCV on his SIP Migrate 150 and see if it was affected by wire speed and it is on all settings.
An example would be:
Setting Min / Low goes from 18.9 OCV on min wire speed to 24.7 OCV on max wire speed.
I have also been advised that the welding voltage readings are most likely unstable because there is no load and with no load multi meters can read wrong. I have been told to place a resistor across the output and measure the welding voltage again to get better readings. It was suggested a resistor around 4K7 3 watt would be suitable. I have ordered the resistor so will try this.
Anyway they both work so thats the main thing I was just interested why I was seeing the unexpected increase in OCV with wire speed. As you say this may just be how these work.