HP25C repair, help needed

[albertofenini]

Dear All,
we worked on the unit, but we are sorry to say we made little to no progress ...
I can confirm our unit has a double density RAM 1820-1886, which means no chips are missing.

First we measured the Vss, Vgg and Vdisp values, with the following results :

Vss (should be 6V) was 2.33V
Vgg (should be -12V) was -0,37 V
Vdisp (should be 4V) was 2.32

It must be said that differently from Jaques Laporte documentation being this a HP25C unit
the Anode Driver is not a 1818-0153 but is a 1818-0186, so for Vdisp we have assumed the
same pinout for both Anode Drivers and we have measured Vdisp at PIN 5 of our Anode Driver.

Given that the values were totally wrong we worked on the PSU.
So far we have substituted : both the transistors, most of the diodes and the little
transformer with components from an HP25 donor machine.

After that, we checked, and the values for Vss, Vgg and Vdisp were exactly the same as before.

Then we took apart the keyboard and we cleaned all the oxidation.
We did this because we suspected it was somehow impacting in the process.

Nothing changed.

Then we removed the ACT, we wanted to see if without the ACT the PSU was generating
the correct values for Vss, Vgg and Vdisp.

Which it wasn't because without the ACT the three values dropped close to 0 V (all of the three).

Then we re soldered the ACT and the values for Vss, Vgg and Vdisp remained close to 0.

What can we check ?
What can we do to isolate PSU and try make it working ?
Can we use components from a Classic Calculator PSU ?
Also, does anyone has more detailed information specifically about the HP25C ?

As always, thank you very much for you precious support !!!
Take care !