The first thing we would succeed to do is to let this power supply work correctly with the 3 known good working boards ....Later, we will worry with the failure of the other boards.
One thing we don't know is if these boards are exactly the same or not and if the components have the same values....so long I know, those boards had a lot of modifications, that's possible that output limit to 28V would be only a problem of values of components on the control board of the pass transistors.
D23 is only a protection of pass transistors against reverse polarities as it could happen if you connect a battery to the power supply switched off.
I would like to know if below 28V, the power supply is working properly with the boards known as good.
Does the current limitation work?
Is the voltage well regulated?
Does the voltage drop on the pass transistors vary greatly when the voltage is set at 5, 10, 15V and with different loads?
In short, is the 28V max output voltage the only fault?
The control amplifier board that seems to be the one that is faulty is the same part number as the one I replaced it with from the smaller 25 Amp supply. The schematic shows no alternative component values. Similarly the "Supplies Board" is identical. The SCR control board is borrowed from another 50 Amp supply and I cannot see any different component values on it, but the SCR and the Control Amplifiers boards do have some preset adjustment pots. The one on the Control Amplifiers board that should raise the max voltage level is already nearly on full, but that board in the 25 A supply it's from allows over 60V. I agree this is a bit hit and miss. The other thing is if I turn the supply on with the 3 good boards in it, and the volts on max, there's a pause of three or more seconds where a very low voltage shows on the V meter, then it suddenly jumps up to its max of about 25V.
I have just checked the supply with two off 1 Ohm 100W metal clad resistors in series as the load. Things are *NOT* normal. This is what occurs:
V and A knobs fully counter clockwise, no V or A show on the meters. I advance the V coarse know clockwise, leaving the A knobs at zero. No volts show. Leaving the V knob or knobs advanced a little, I rotate the coarse A knob clockwise. Current shows and so does V on the meters, however current follows the V knob, the A knob has no effect. If I rotate the A knob fully counter clockwise the supply should not output and V, but it still does and the current showing on the meter does not reduce.
Volts across the terminals 6 and 10 on the Control Amp Board change with the V control knob in so far as they reduce as V is increased. At the max attainable of about 25V I see about 6.5V, and as I reduce the V knob back to zero the V across 6 and 10 rises to about 40V and then decays, fairly fast at first, but slowing unti after about 30 seconds or so I see a resting V across pins 6 and 10 of about 17.2V That seems very odd... I am hoping it means more to you than I
Thank you!