Weird HP-71B behavior...

[KF6GPE]

So, I have a few HP-71Bs (I never line them all up, so my wife is none the wiser). The one I keep at work has been flaky at best --- when I got it it had bad corrosion in the battery compartment, and I've mostly salvaged it. Once in a while it flakes out and loses its mind, but I pretty much use it only as a desktop calculator and as something to intimidate the young'uns at the office, so that's OK.

After grinding through some math the other day, I put it down without turning it off. The next day I realized it was locked up with the previous day's results and the CALC annunciator still on the display, unresponsive to keypresses. I pulled the batteries for a couple of minutes and held down the ON key for that time, put the batteries back in, and the LCD was showing the same thing, and it's still locked up.

Anybody seen anything like this? I pulled the batteries tonight before going home and will see tomorrow if it's fully self-drained and resets.

[Joe Horn]

Did you try the reset sequence, [ON]+[/]? If that works, press ENTER to accept the 1 which appears. This causes a harmless warmstart. Or press 3 ENTER for a complete Memory Lost.

Did you try shorting the two farthest battery terminals with a paper clip or pair of scissors? IIRC, shorting the two farthest terminals in the card reader slot with very gentle pressure (so as not to scratch them) also drains the capacitor quickly.

[Dave Frederickson]

(08-29-2018 03:52 AM)KF6GPE Wrote:  So, I have a few HP-71Bs (I never line them all up, so my wife is none the wiser). The one I keep at work has been flaky at best --- when I got it it had bad corrosion in the battery compartment, and I've mostly salvaged it.

If you have the later version hardware with the plastic back, severe battery compartment corrosion can also affect the golden zebra connector between the two PCB's. If the 71 is the later version, remove the 7 screws holding the case together and gently separate the case halves. If there are any signs of corrosion, remove it with vinegar, water, and alcohol. If the golden zebra is affected then very gently clean it in a similar manner. Don't brush the zebra contacts as they can be easily damaged.

If the zebra connector is damaged then the only solution is to wire the case halves together with fine gauge wire.

Dave

[KF6GPE]

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.

[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.

[KF6GPE]

Thanks, Bob! I will do that, and let it sit for the week. It's not anything I desperately need to fix, and I have a hunch the less I take apart, the better off I am. I have an increasingly poor do-it-yourself track record.

That's interesting, because what you describe (other than the sounds) is exactly what I'm seeing. Good to know.

[J-F Garnier]

(08-29-2018 07:44 PM)KF6GPE Wrote:  Joe, I did try shorting the battery contacts at first, did it again this morning, no luck either time. It's unresponsive to on-/.

You should, as Joe suggested, try shorting the two farthest terminals in the card reader slot, not the contacts in the battery slot because there is a diode in the battery supply path.
Be sure to remove the batteries first!

J-F