I got a few different replies, so I will try to answer them all in one go. I too could not understand how 5V pulse could possibly be at a steady voltage given the input. But today I found a capacitor on the physical board that is missing from Tony's schematic. I haven't yet worked out how it is connected but I am trying to re-draw the schematic in KiCad so I can add corrections. I am not sure I have the patience to completely re-draw the whole thing but I will try to get at least this area correct.
I measured AC OK by adding a pull up resistor, but if I am honest, I forgot about this in my latest measurements. I have also looked at Power OK, knowing that AC OK needs a pull up resistor, and it is logically false, so it is definitely saying the power is not OK. However, I am thinking that I shouldn't be too concerned about the AC OK signal because it is clear that the 5V output is out of spec being at around 4.3V, so I think I probably need to fix the 5V output and then perhaps the AC OK will sort itself out, or at least I can fix AC OK once the outputs are actually valid. As far as I can tell the Power OK and AC OK signals don't influence the operation of the PSU. Given that I do get an output of 4.3V, I would have thought that the thyristor was not being switched on. I have measured Raw 5V and it too is around the same voltage as the 5V output. In fact both now measure about 4.4V. I checked the resistance between the junction of the +5V rectifiers and the +5V output and it is about 0.1R, so I don't think there is a break in continuity. I hadn't spotted the capacitor on the 12V Pulse output. The rectifier output for 12V looks like this: https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-12v-secondar y-side-rectifier-output.png and 12V Pulse looks like this: https://rjarratt.wordpress.com/wp-content/uploads/2025/10/h7842-12v-pulse.pn g. Vstart is 11.3V and Vz is 6.4V. I have not been able to find where Vz comes from on the schematic either, if I elaborate the KiCad schematics further I will try to find its source. The supply to E1, E2 and E3 is 11.3V, referenced to the GND output, the GND pin on E1 is -12.9V and the GND pins on E2 and E3 are -0.7V Let me know if I have missed anything. Thanks Rob
