HP Forum Archive 06

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HP-19C Message #1 Posted by Randy Dickinson on 6 Sept 2001, 12:03 p.m. I want to rebuild the battery on my hp19C. I assume that I just use tab nicads and solder them the way the old ones were? Also, as I remember, the batteries must be in place and working for the calculator to work. Just AC alone did not do it. I get a movintg display as it is now with a bad battery pack. Is this correct? Thank you. Randy Dickinson

Re: HP-19C Message #2 Posted by Dave on 6 Sept 2001, 7:31 p.m., in response to message #1 by Randy Dickinson Your right, never operate this calc w/o batteries, the printer will be destroyed. Batterie Plus rebuilds pacs fairly inexpensively.

Re: HP-19C Message #3 Posted by Tony Duell (UK) on 6 Sept 2001, 8:45 p.m., in response to message #1 by Randy Dickinson Yes, to rebuild a battery pack just use new NiCd cells and the original terminal contacts, plastic housing and so on (as appropriate). HP machines with 2 pin chargers (Woodstocks, Spice, Sting, Topcat, etc) generally need the battery pack to be installed and working before you can run them off the charger. In fact with the continuous memory Woodstocks and Spices (HP2xC, HP3xC), connecting the charger without a good battery will damage some of the chips. I hope the 19C is not like that, but I've never been lucky enough to investigate one. The exceptions to that are the 71B (where the AC adapter is not used as a charger anyway, just as a PSU) and the 75 (which is documented as working without a battery in place). For the models with 3 pin chargers (35, 45, 55, 65, 67, 70,80), if there is no card reader then it's safe to run them without a battery, and the machine will work fine (the battery is disconnected when the charger is plugged in). If there is a card reader (65, 67) then you can burn out the card reader sense amplifier chip if you connect a charger without a good battery pack in place. I must admit I normally test machines using an adjustable bench PSU set to the appropriate voltage (1.3V per NICd cell or thereabouts) connected to the battery contacts without the pack in place. That's safe for the machine and it means I can test machines even if I don't have a good battery pack to hand. But it also means getting a bench PSU and they are not that cheap.

Re: HP-19C Message #4 Posted by Ellis Easley on 8 Sept 2001, 12:59 p.m., in response to message #3 by Tony Duell (UK) Tony, I've wanted to ask this question for a while and I think you might have the answer: is the memory chip in the 25 (not 25C) especially prone to failure? Possibly due to high voltage caused by missing or corroded-to-open-circuit battery? I collected several 25's before I got one with working memory. Both storage registers and program memory were out on the others. I didn't get a proper AC adapter till late in the process but ran the calcs with nicads that I charged externally. The one that finally worked was a little flaky storing programs at first, till I learned it had been sped up. After I changed the cap to the original value the memory worked fine. This was all a couple of years ago and I haven't opened them up lately, but I'm beginning to wonder if the other calcs had been sped up - I better take a scope to them. But my main question is, does the memory chip in the 25 fail frequently?

Re: HP-19C Message #5 Posted by David Smith on 8 Sept 2001, 4:00 p.m., in response to message #4 by Ellis Easley I have found that the most likely chip to die in plain HP25s (and for that matter the 25C's) is the 22 pin ACT chip. I have had about a dozen machines with bad ACT chips, one '25 with a bad ROM, and one 25C with a bad RAM and a bad ACT.

Re: HP-19C Message #6 Posted by Tony Duell (UK) on 9 Sept 2001, 2:00 p.m., in response to message #4 by Ellis Easley Well, logically, the battery-backed RAM in the 25C/29C would be very susceptible to damage by overvoltage. It's connected to the unswitched battery supply voltage, so if the charger is plugged in and the battery packl is open-circuit, then a high voltage will appear across the RAM supply lines. You're actually better off if the machine is turned on, since the PSU converter takes enough current to pull the (high internal resistance) charger output voltage down. However, others have said that the ROM or ACT fails first (I have no idea why, unless high voltages appear on the bus lines). I don't have much experience of Woodstocks, actually, although I have worked on Spice (HP3xE/3xC) machines which are electically similar.

Re: HP-19C Message #7 Posted by Ellis Easley on 13 Sept 2001, 7:29 a.m., in response to message #6 by Tony Duell (UK) I guess I must attract the ones with bad registers! The first one was described by the seller as having bad program memory but OK storage registers, but in fact both were bad. Then I learned here that both functions are in the same chip. Another one, I bough as non-working, for parts. It was badly corroded, and after cleaning and replacing some PC traces, it worked - except for the registers! Finally, I bought one that was guaranteed all working.

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