Advice for a recalcitrant 29c

[[kby]]

(06-06-2019 11:01 PM)PANAMATIK Wrote:  

(06-06-2019 04:40 PM)[kby] Wrote:  This would make sense for any “C” machine as x has to be part of the Continuous Memory and therefore must be in a CMOS RAM area as the ACT would presumably not be powered when the machine is off.

Except for the first "C" machine HP-25C, where X is not part of the Continuous Memory.

This allowed HP to use the same firmware for HP-25 and HP25C for faster time to market. The CMOS RAM chip of the HP-25C just ignored the Clear Memory instruction which is issued at initialization, while the PMOS RAM chip executes this instruction. This is the only runtime difference between the two machines.

Bernhard

Ah…should have done my research better before spouting off generalities.

Update (but no progress):

Checked connections for continutity (with a meter) the ACT all of the RAM and ROM chips for Phi 1, Phi 2, ground, power supply connections, SYNC, DATA, and IS/IA lines as well as the PORO line and also did a closer visual check. Found some suspect ones, but fixing them doesn't seem to have changed anything.

A couple of anomalies (both probably useless/irrelevant):

1. Display sometimes shows an extra "0." about three positions to the right of the correct one (as well as the correct one on the far left) if you hold down the "GTO" key squeeze the keyboard and battery pack in just the right way at the right point while trying to ensure that the batteries are making good contact. However, that was somewhat reproducible last night but doesn't seem to be now, so I'm attributing it to making the contact actually worse. Similarly, sometimes the "Error" display will show one brighter character with similar extra pressure on the batteries. [ignore this—see note below]
2. arc cos function behaves slightly different than any other functions—returns "0.0" rather than "0." But the display change doesn't persist if you do anything else.