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+--- Thread: hp28s need repair ?. Autotest fails (/thread-21084.html)
hp28s need repair ?. Autotest fails - tlfrance - 01-01-2024 03:22 PM
Hello guys,
I encounter some issues on my HP28S calc. Batteries are new *
1/ On autotest mode, result is : "2 FAIL"
2/ The screen do reset (clear) after i enter more than 2 or 3 lines in the stack
3/ when trying to plot sin(x) for ex, just 2 first dots begin plotting then cacl seems to be freezed. Must interrupt process whith "ON + UP"
Maybe issue can be the RAM chip , but not sure. What do you think about ?
Thanks for help.
RE: hp28s need repair . Autotest fails - tlfrance - 02-01-2024 05:42 PM
I've found in owner's manual about repeating test: "A message such as 1 FAIL indicates that the calculator failed the test. The number indicates the nature of the failure."
Where can i find a service document relating to error codes of HP28?
RE: hp28s need repair ?. Autotest fails - rprosperi - 02-01-2024 07:41 PM
No service manual for the 28s (or any of the "Champion" folding-case models) has even been captured, or even seen, in the wild. I strongly suspect that none was ever created as the machines are effectively not serviceable, from a manufacturer's perspective; the plastic welds holding it together cannot systematically or cost-effectively be rejoined sufficient to return to a consumer for use. It was cheaper for them to simply replace the very few of these that died within the warranty period.
Sorry for the bad news.
RE: hp28s need repair ?. Autotest fails - Steve Simpkin - 02-01-2024 09:45 PM
The following post may help. At least you can see what is involved in opening the calculator.
https://www.hpmuseum.org/cgi-sys/cgiwrap/hpmuseum/archv020.cgi?read=185173
RE: hp28s need repair ?. Autotest fails - Steve Simpkin - 02-01-2024 09:49 PM
Here is another resource that describes how to open the HP-28C/S models.
https://www.teawea.com/open-a-hp-28s-calculator/
RE: hp28s need repair ?. Autotest fails - tlfrance - 02-02-2024 09:47 AM
(02-01-2024 07:41 PM)rprosperi Wrote: No service manual for the 28s (or any of the "Champion" folding-case models) has even been captured, or even seen, in the wild. I strongly suspect that none was ever created as the machines are effectively not serviceable, from a manufacturer's perspective; the plastic welds holding it together cannot systematically or cost-effectively be rejoined sufficient to return to a consumer for use. It was cheaper for them to simply replace the very few of these that died within the warranty period.
Sorry for the bad news.
Thanks rprosperi . I can try to repair myself the unit, and i see on pcb only few compononent (2 ROM + RAM chip ) and other discret component such as resistor, capacitors..
The difficulty is to find what is faulty. I 've searched over web for similar issue, but with no chance. But my idea is the ram chip faulty (50%). I can find it on ebay with different brand (nec or mitsubishi ) , i guess it's same ...
RE: hp28s need repair ?. Autotest fails - tlfrance - 02-02-2024 09:53 AM
(02-01-2024 09:49 PM)Steve Simpkin Wrote: Here is another resource that describes how to open the HP-28C/S models.
https://www.teawea.com/open-a-hp-28s-calculator/
Thanks Steeve . I'll try to open the unit as best i can do (i'm confident for that) ;-) There are full of different way to do that, and i think best way is described from https://www.teawea.com/open-a-hp-28s-calculator/
Next difficlty is to find the faulty component , and i didn't see over the web same issue as mine ..
RE: hp28s need repair ?. Autotest fails - tlfrance - 02-26-2024 05:41 PM
Hello,
I've repaired my 28S . I've changed the RAM 32KO chip for a new one on ebay for 7.20€
The original chip is M5M5256BFP 15L (150ns) from Mitsubishi. I've changed for a Nec D43256BGU-70LL (70ns but compatible 150ns) .
![[Image: pcb.jpg]](https://i.ibb.co/VjkBwkJ/pcb.jpg)
https://ibb.co/VjkBwkJ
Autotest is pass (OK-28S), and all is working well (many lines in stack, program etc..).
Very happy..
RE: hp28s need repair ?. Autotest fails - vaklaff - 02-26-2024 08:04 PM
Congrats! I am truly in awe…
How did you know what needed to be replaced?
How did you disassemble and re-assemble the machine?
RE: hp28s need repair ?. Autotest fails - johnb - 02-26-2024 10:10 PM
(02-26-2024 08:04 PM)vaklaff Wrote: Congrats! I am truly in awe…
How did you know what needed to be replaced?
How did you disassemble and re-assemble the machine?
Same here!
Post photos and description of your thinking / actions, please! (I have a 28s in very good condition, but the sands of time wait for no man...)
RE: hp28s need repair ?. Autotest fails - tlfrance - 03-01-2024 07:37 PM
For opening, I’ve followed this article https://www.teawea.com/open-a-hp-28s-calculator/. Using a dremel it’s easy.
For the memory malfunction, i didn.t have any doubt, I could not store more than 3 rows in stack, and had some screen clear when attempting to store some var into the calculator.
For replacing the chip , i ‘ve used hot air desoldering tool. Be carrefull to protect with tape the 2 Saturn chip as they are very fragil. Solder new chip was not difficult at all.[/align]