HP Forum Archive 14

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HP25C died?? Message #1 Posted by Walter B on 3 Apr 2004, 3:59 a.m. Seems I need some help: I'va got an old HP 25 C with an old accu pack. It didn't hold charge, only some minutes, but with the adaptor connected to the mains it did work until 3 months ago. So, i left it standing on my shelf (just the calculator with the accus inserted, no connection to the adaptor). Now I wanted to use it again - and the diodes just run blinking, even after some minutes charging. I tried a new accupack which works in another woodstock - doesn't work in my HP 25 C. Assume something destroyed in the calculator - don't know where and why. Adaptor seems to deliver 10 V AC perfectly. What can I do to save this old calculator? Any advise will be appreciated. Walter P.S.: Just found Eric Smith's message of Mar 31 about bad HP design: <...Using the battery pack to filter and regulate the power from the AC adapter. Some models, most notably the Woodstock series, can be damaged by operation from the AC adapter without a good battery pack installed. Or if the battery contacts get corroded.> Seems that's what happened to me. What kind of damage is caused? Chances to repair? Edited: 3 Apr 2004, 4:35 a.m.

Re: HP25C died?? Message #2 Posted by Jon on 3 Apr 2004, 8:59 a.m., in response to message #1 by Walter B You have probably fried any chip. When that happens the calculator works when you press numbers´ keys but when you try any operation you get a display like 00000000. Is that what happens to yours? I am afraid you need another unit for spare parts Best Regards from Spain Jon

Re: HP25C died?? Message #3 Posted by Walter B on 3 Apr 2004, 2:32 p.m., in response to message #2 by Jon Thanks, but this is bad news. Just for sake of clarity: The display does not show a stable 000000000, but it looks like the zeros running around in the background (meaning they do not get power all at the same time). Sometimes, after switching on, only the least significant digit shows up (the rightmost) as 8. with reduced brightness. Don't know whether this changes the diagnosis.

Re: HP25C died?? Message #4 Posted by David Smith on 3 Apr 2004, 12:51 p.m., in response to message #1 by Walter B Usually the ACT chip is killed. When this happens the most common fault is a display of 000000000. Quite often the RAM chip(s) is destroyed also. The ACT chip can be salvaged from any HP21/25/25C/29C/67/97. The RAM will need to come from another HP25C or possibly a 29C. Some machines have a single RAM chip, others use two chip. To use a single chip in a machine designed for two chips, you need to add a pullup resistor to one of the pins.

Re: HP25C died?? Message #5 Posted by Tony Duell on 3 Apr 2004, 2:11 p.m., in response to message #4 by David Smith No idea why the ACT chip fails (I agree it does!), it's the RAM that gets excessive voltage (around 12V) from the PSU if the battery goes open-circuit with the machine _off_ (it's a lot safer to have the machine turned on, the rest of the electronics will load the PSU somewhat). AFAIK, you can also use an ACT from a 19C, 91, 92, (and even a 95C). But those machines are a lot rarer than a 25C. What you can't use is an ACT from a 27 -- that machine uses NMOS chips (all other ACTs are PMOS) and has the supply rails 'upside down'. I never run -C Woodstocks and Spices off the charger, or charge the battery pack 'in place'. I am lucky in that I have a good regulated bench PSU that I use to run calculators for testing (clipped onto the battery contacts) and the 'Reserve Power Packs' (external battery holder/charger units) for the Classic, Woodstock, Topcat and Spice series. If you don't have those (and the Spice one is really hard to find), then consider using a cheap non-continuous-memory calculator (a 21 or a 32E) to charge battery packs. Doesn't even have to work, all you need is the charger circuitry (one diode, one resistor, connectors)!

Re: HP25C died?? - AM radio diagnostic tip Message #6 Posted by Andrés C. Rodríguez (Argentina) on 4 Apr 2004, 12:19 p.m., in response to message #1 by Walter B It is possible that your calculator have suffered severe damage, as many posts had suggested before; but I encourage you to read (in the MoHPC Articles forum) an article by Katie Wassermann about Woodstock common faults and repair, since there are some repairable problems (clock signal malfunction or noise) which may cause symptoms similar to your description. I have worked with an HP27 (Woodstock) from a friend which seemed to have been damaged, but after some warming-up and some battery discharge (!), it started to work fine. Diagnostic tip, specially useful for LED based calculators: Put your calculator (powered by batteries) near a working AM radio receiver, turn the calculator on, and hear the electrical noise induced in the radio. You may need to play a little with the tuning dial, but there is no need to be precise about tuning a specific frequency. Press different keys and hear the noise changing. If the noise and the changes are there, then it is not a case of "no brain activity" death (flat EEG): At least the power supply is partially working, there are clock pulses and keyboard scanning signals (which also are closely related to display operation). As the keyboard and display scanning is run by firmware, at least some CPU and ROM activity can be confirmed. You may still have noisy clock lines, clock instability, faulty RAM, faulty display drivers, etc.; but this is a simple , cheap, fast and safe test you can do by yourself and after this first screening you can take better decisions about asking for help, validate diagnostics or try repairs by yourself. Good luck!

Re: HP25C died?? - AM radio diagnostic tip Message #7 Posted by John McLeod on 5 Apr 2004, 6:33 p.m., in response to message #6 by Andrés C. Rodríguez (Argentina) Try replacing the batteries in the pack with garden variety AAs. I did this with my 29C, as suggested by Randy at Fix That Calc, and it revived it. Now it needs new batteries now and then, but I only keep it for sentimental reasons anyway.

Re: HP25C died?? - AM radio diagnostic tip Message #8 Posted by Eric Smith on 5 Apr 2004, 10:20 p.m., in response to message #6 by Andrés C. Rodríguez (Argentina) Quote: As the keyboard and display scanning is run by firmware, at least some CPU and ROM activity can be confirmed. Actually not. The ACT processor continuously scans the keyboard and display even if the firmware is completely corrupted or missing. The firmware can control the content of the display (obviously) and turn it on and off, but it is scanned continuously regardless of what the firmware is doing. And since the display scannning also scans the keyboard, that is automatic as well. The scanning works by the cathode driver chip advancing the column drive once every word time (56 phase 1/phase2 clock periods). Once every 12 or 15 word times, the ACT asserts the RCD signal to Reset the Cathode Driver. The ACT powers up in 12 digit mode, but has an instruction to switch to 14 digit mode (reset every 15 word times). So an ACT in a Topcat or HP-19C with missing or bad firmware might only scan a portion of the display. If power is good and the oscillator is running but the RCD signal is not being generated, the ACT is bad.

AM-radio coarse test - Correction Message #9 Posted by Andrés C. Rodríguez (Argentina) on 6 Apr 2004, 6:07 p.m., in response to message #8 by Eric Smith Eric: Thank you for the correction. I previously understood that the scanning needed some initialization from firmware, it seems I was wrong (at least for the ACT processor). Now, it should read : ... at least some CPU (and Cathode driver chip) activity can be confirmed...

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