Weird HP-71B behavior...

[rprosperi]

(08-29-2018 07:44 PM)KF6GPE Wrote:  Thanks to both of you! I think disassembly may be my next step. Joe, I did try shorting the battery contacts at first, did it again this morning, no luck either time. It's unresponsive to on-/. What's kind of goofy is after shorting the battery contacts for a minute or so and putting it back together it won't power on with the batteries, but does on an AC charger --- back to the last result ('33', I kind of wish it was '42') in calc mode and doesn't respond. Very weird. That leads me to believe I'm not fully draining the back-up cap, because I know these things don't have NVRAM or anything. (And oh, yeah, no modules in any port.)

Thanks! Will update here when I get a chance to get it on a bench if I find anything interesting.

R.

On rare occasions (i.e. I've no idea the cause) a 71B can get into a state where [ON]+[/] with options 1, 2, or 3 still cannot clear the state and reset the system, and even battery removal for hours makes no difference; the old screen (at least) still is retained somehow. When this happens, the only cure I've found is to remove batteries for a long time, meaning several days to, in one case, over a week.

Once you've removed the batteries, short the terminals (a few seconds is fine, longer than that makes no difference) and if you can, press [ON] while shorting. In fact all the keys in the bottom row still 'register' even if the unit is turned-off, so pressing them will help to dissipate any stored energy. But even with these extra steps it can still take days to clear. One of my 71's got into this state, and each key-press caused a high-pitch whine, while another one made no noise, but after the long reset, they seem to work fine, months and years later.

So, maybe try that before disassembling, if you're patient enough.