hp28s need repair ?. Autotest fails

[tlfrance]

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.

[tlfrance]

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?

[rprosperi]

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.

[Steve Simpkin]

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

[Steve Simpkin]

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

[tlfrance]

(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 ...

[tlfrance]

(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 ..

[tlfrance]

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..

[vaklaff]

Congrats! I am truly in awe…

How did you know what needed to be replaced?

How did you disassemble and re-assemble the machine?

[johnb]

(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...)

[tlfrance]

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]