HP25C repair, help needed

[albertofenini]

Dear All,

we will post pictures later, but this is what we accomplished today.
We are dealing with mainly two issues : the motherboard and the keyboard.
As said before we have opened the working HP25C from our collection, and measured Vgg, Vss and Vdisp.
They were obviously corrected.
Looking at Jaques Laporte scheme we tried to find where to measure the Vgg, Vss and Vdisp at the PSU.
We only have found where Vgg can be measured at the PSU.
The differences between HP25 and HP25C have made impossible, to us, to locate the other testing points in the PSU,
so we measured those values at the top of the PCB.
We were about to de-solder all of the PSU components when we have found this old link :
HP-25C Repairing
and we decided to follow that path.
We de-soldered both 1820-1886 (the double density ram) and the 1818-0154 (rom)
Well, the assumption that PSU was working was a good one, without those two ICs our PSU was working indeed.
In fact, now also on this unit Vgg, Vss and Vdisp are correct.
We have now a steady red 0.00 on the display.
However, while the old thread says the basic math could be performed, our unit doesn't.
Because the keyboard under restoration is damaged we used the keyboard from the working unit.
We have been able to enter digits without problems.
But if we try to do anything like pressing the ENTER, pressing CLX, perform operation math (+, -, x, /) the display
shows a 0.000000000 (1 digit 0, 1 digit . and 9 digits 0)
The unit doesn't lock up so that we can continue to digit new numbers.
Also it doesn't handle the decimal .
Switching to PRGM shows the 00 in the left corner and pressing SST will show
01 13 00
02 13 00
which we can understand as ROM and RAM are missing.
At this point we re-soldered the 1818-0154 to see if it was a cold joint but the machine with that IC inserted looks like dead.
We de-soldered again and the unit worked as described above-

What do you think ?
Are we talking about a partially damaged ACT or this behavior is because of the two missing ICs ?

About the keyboard.
Since it had very much oxidation we decided to take it apart, and clean it.
As you will see later in the pictures this particular keyboard is composed by a thin plastic film with some copper traces,
a plastic films that serves as a separator and the PCB.
Unluckily most of the copper traces were broken ad when put together generate random contacts.
We have seen some keys working, after the cleaning, but the majority doesn't.

Beside the difficulty of putting back together a Woodstock keyboard, we did some tests with the glue that can be
made liquid by warming up with a gun, but the results are very poor, so we will probably look for a replacement
from a donor.

Any idea or suggestion is very welcome !!
Take care !