Unusual HP-21 ?

[jch]

Much later...

Replacing every component of the PSU daughter board, the one after the other, without spare parts readily available, seemed rather unworkable. I took apart the whole thing. Simulation with LTspice confirmed that the ripple was due to tantalum caps' ESR worst case, but it reads quite lower with the module rebuild on a breadboard with all new components.

Alas, still unsuccessful...

As Jebem has noticed, some pcb traces were badly damaged by batteries electrolyte.
And yes, desoldering corroded solder joints was tricky!
Cleaning, desoldering ACT and POR/Bat, re-cleaning, checking pcb traces and resoldering didn't help.

Jacques Laporte's pages and Jim Johnson's "HHC 2013 - HP29C Diagnosis and restoration presentation" are great resources and I was able to check some ACT's waveforms.
Clock phases 1 & 2 and ISA seems Ok but POR (ACT Pin 3) exhibits a square signal. So, once again, desoldering POR/Bat then connecting to pin A2 as on Jacques' schema, but still without success.

I feel like putting everything back in the drawer until I can put my hands on another HP-21 or not being able to withstand the call for an LP module...