Restoring HP25 S/N 1702S13138 - Very strange take a look

[albertofenini]

Hi Everyone,
while looking for a good RAM chip we stumbled across a "bargain" and we purchased this unit.
We knew it was not working, so we are not complaining.
When we opened it we found this situation :
   
and after several steps of water/vinegar and soda we cleaned to this point.
   
Still it is not working.
We decided that to test few voltages, and we found out that Vgg and Vss are good, while Vdisp
is higher then expected.
   
When we compared the unit with a working one, we found however some strange differences.
Both units have an identical PCB version, and all of the components are identical
except for the ROM, the working one (the unit pictured on the left) has a ROM 1818-0153,
the faulty one (pictured on the right) has a ROM 1818-0168
   
However if you look at the yellow circled areas you will certainly note some differences
The working one has three diodes and a capacitor more where the faulty one has
two less diodes in the PSU area and just below the ROM has a resistor instead of an expected diode.

Any explanation for this ?
The faulty units looked absolutely untampered ...
What do you suggest ?
Thanks everyone for reading and posting a reply