HP-25 - what could be wrong, simple question

[albertofenini]

Hi Andy
We had two hp25 and one Hp25c worth this project
As a start we just ordered two PANAMATIK hp25a replacement

When the two PANAMATIK chips arrived we decided to give up with the board as we had two pcb worth the restoration with no big issues to troubleshoot
Both were working except for the capability to store values and programs
So we didn’t try to repair that unit postponing the troubleshooting as well

However when we started installing the PANAMATIK one of the two good units
was acting weird and we found out the most probably the rom/anode chips was gone

At that point we harvested the rom/anode driver from the unit with Vdisp too high in order to repairs one of these two

At the end we recovered one Hp25 and one Hp25c

Also from that unit we recovered the back of the keyboard
and once the rivets will arrive we will use it to replace a keyboard we repaired some times ago mixing domes from a classic machine on a Woodstock keyboard

About that vdisp our guess is that I t was probably something bad in the cathode driver
I say this because when we tried to repair the one gone bad we harvested both
chips from the one with vdisp too high but we didn’t succeed
When we mixed the rom/anode driver from the donor with the original cathode driver things went well
It may be worth to check now if the PSU is working good on the board
Rainy days are coming so we may as well check it out

As schematics we use those from Tony Dwell

Take care