Roberts Model 3001 Insulation Tester

[WaveyDipole]

I discovered that my insulation tester has failed recently. I haven't used it in a while but it was working when last used. The readings made no sense and when I checked I found that there seems to be almost no output.

The oscillator for the high voltage generator is a TL494 PWM chip which has two outputs. These are coupled together and fed into the TX via a 100Ω resistor. There is an output at about 6kHz form the IC. I checked the oscillator transistor which is fine. The TX primary and secondary both have continuity. The secondary, which is ground referenced at one end, has an output of approximately 256VAC. The output end feeds into a stage with two 0.047 HV caps and diodes. I checked the output of this stage and I get approx 620VDC and 1200VDC so the HV generator itself appears to be working. There is a round thick black disc with no markings further up the board but without a diagram I can't tell where in the circuit it is positioned, but I presume that it is there for protection. I lifted one end and tested with it disconnected, but still no output.

Next comes the tricky part. The DC output feeds into a 100kΩ resistor. One side has the full voltage, the other only about 180VDC. The resistor reads at just under 100kΩ on the DMM so it looks OK. I could really do with a circuit diagram to understand what happens next but I just can't find one anywhere. Does anyone have a diagram for the Robin model 3001 please? Failing that, I guess I will need to spend some time reverse engineering the circuit before I can proceed, but I thought I would ask first. Also, has anyone repaired one of these that could provide some pointers perhaps?

[WaveyDipole]

Ah well! I guess it was a long shot!