HP Forums

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.

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?

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.

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

Here is another resource that describes how to open the HP-28C/S models.
https://www.teawea.com/open-a-hp-28s-calculator/

(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 ...

(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 ..

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..

Congrats! I am truly in awe…

How did you know what needed to be replaced?

How did you disassemble and re-assemble the machine?

(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...)

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]