HP-29C Low-battery warning, help needed please!

[AndyWalter]

Here in UK, I bought my beloved 29C in 1978, and have been using it regularly since. I swapped the nicads for Nimh ages ago, but even with new ones I'm seeing the warning flashing DP appear within about 5 minutes of removing the mains charger. The battery pack is still fully charged at this stage, and the internal power-supply board seems to have decent voltages (very many thanks to Jim Johnson's for his HP 29C Calculator Diagnosis and Restoration PDF). I suspect the 1820-1983 POR chip used in 22, 25, 25C & 29C may be failing, but I don't know what signal/voltages etc it feeds into the ACT (F1 signal line?) to indicate this. If I know this, it might be straightforward to bypass that chip, but at the same time I'd really like to check the other voltages around to make sure they're within tolerance.

Here are the voltages I have, compared to Jim's doc page 14:
V batt-in Jim: 2.4V Mine: 2.441V
V ss 6.0V Mine: 6.163V
V disp 4.37V Mine: 4.369V
V ram 6.5V Mine: 6.535V
V gg -12.0V Mine: -11.62

These look okay to me, but what's the tolerance especially on the -11.62 ?
Does anyone here know what the problem is please, or does anyone have a Service Manual for this superb machine?

Also, Does anyone know how to contact Jim Johnson? I think there's an error in the circuit diagram page 12 he's published in his PDF. Resistor 33Ohms connected to POR pin 1 is in fact 3.9 Ohms, it's on page 8, Orange White Gold Gold just below the bright orange tantalum capacitor.
Andy