hp28s need repair ?. Autotest fails
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01-01-2024, 03:22 PM
(This post was last modified: 02-01-2024 05:44 PM by tlfrance.)
Post: #1
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hp28s need repair ?. Autotest fails
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. |
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02-01-2024, 05:42 PM
Post: #2
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RE: hp28s need repair . Autotest fails
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? |
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02-01-2024, 07:41 PM
Post: #3
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RE: hp28s need repair ?. Autotest fails
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. --Bob Prosperi |
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02-01-2024, 09:45 PM
Post: #4
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RE: hp28s need repair ?. Autotest fails
The following post may help. At least you can see what is involved in opening the calculator.
https://www.hpmuseum.org/cgi-sys/cgiwrap...ead=185173 |
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02-01-2024, 09:49 PM
Post: #5
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RE: hp28s need repair ?. Autotest fails
Here is another resource that describes how to open the HP-28C/S models.
https://www.teawea.com/open-a-hp-28s-calculator/ |
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02-02-2024, 09:47 AM
Post: #6
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RE: hp28s need repair ?. Autotest fails
(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. 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 ... |
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02-02-2024, 09:53 AM
(This post was last modified: 02-02-2024 09:56 AM by tlfrance.)
Post: #7
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RE: hp28s need repair ?. Autotest fails
(02-01-2024 09:49 PM)Steve Simpkin Wrote: Here is another resource that describes how to open the HP-28C/S models. 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 .. ![]() |
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02-26-2024, 05:41 PM
(This post was last modified: 02-26-2024 05:48 PM by tlfrance.)
Post: #8
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RE: hp28s need repair ?. Autotest fails
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) . ![]() https://ibb.co/VjkBwkJ Autotest is pass (OK-28S), and all is working well (many lines in stack, program etc..). Very happy.. ![]() |
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02-26-2024, 08:04 PM
Post: #9
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RE: hp28s need repair ?. Autotest fails
Congrats! I am truly in awe…
How did you know what needed to be replaced? How did you disassemble and re-assemble the machine? |
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02-26-2024, 10:10 PM
Post: #10
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RE: hp28s need repair ?. Autotest fails
(02-26-2024 08:04 PM)vaklaff Wrote: Congrats! I am truly in awe… 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...) Daily drivers: 15c, 32sII, 35s, 41cx, 48g, WP 34s/31s. Favorite: 16c. Latest: 15ce, 48s, 50g. Gateway drug: 28s found in yard sale ~2009. |
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03-01-2024, 07:37 PM
Post: #11
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RE: hp28s need repair ?. Autotest fails
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] |
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