hello

jlaporte wrote:

Hi,
12V DC directly on the battery prongs possibly fried the ACT (main cpu chip) ; a steady display of '0000000' signs always a bad ACT.
But a blinking display must be investigated.
Get the calculator and feed it with two AA batteries. Switch it on and give me the picture.

pyt22fr wrote:

Is there a reasonable chance to bring this HP25 back to life ?

Yes with spare parts. I did it once with ACT from an old HP-21 (cheaper on eBay).

Success!

Hello Jacques,

Thank you very much for your answer, and sorry not to give news earlier myself... I had not subscribed to the topic, so not informed about your post.

Well, after investigating a little more, there are good news and bad news about this HP25.

The good news :

the logical circuits are good, nothing fried apparently, run mode and program mode seem to work perfectly well.

The bad news :

1) several keys are picky, don't work well, and opening the keyboard seems to be a headache.

2) PCB fairly corroded in different areas, copper track is sometimes converted into dust, but replacement with insulated copper wire makes it work. Some IC pins are slightly corroded also, and it might go up into the IC package. I'll have to review it all in detail.

3) The most annoying point : the fixing stakes of the magnifying plastic over the LEDs was broken, it fell off, and the contact of this plastic (or maybe my fingers  Grrrr... :-(  ) with the wiring of the 7-segment digits damaged a couple of these, actually the 2 right side digits, the ones for EEX, are damaged.

I tried to glue one golden wire with conductive silver varnish, no way.

I soldered a thin copper wire (Weller equipment) to the corresponding PCB track and tried to solder the golden wire to this copper wire extension, but the main result is now a melted and shorter golden wire   :-(  .

Short story :

I have to rework 2 or 3 golden wires (ball bonding of wedge bonding) plus lift some other wires from the display PCB to prevent potential short circuits.

Any return of experience on golden wire bonding (a few tens of microns in diameter) rework ?

Thanks in advance and best regards !

Hello Jacques,

One of these days, I'll pry this keyboard apart to clean it and once I get it glued back I keep you updated.

Regarding the display, thank you very much for your offer, it is very kind of you.

Please find below a photo of the real situation :



A : golden wire is currently on air, shortened, probably melted, due to the fact that I used a soldering iron with a tip too hot for this repair, unfortunately.  Wire "A" should go down to track "B".

B : track where golden wire "A" should be bonded.

C : stains of conductive silver varnish, remaining residues of unsuccessful intent to glue gold wire "A" when it was still long enough.

D : copper wire that I soldered on track "B" to make a bridge over the four linear tracks, to try and join wire "A", unsuccessful so far.

E : one of the broken stakes from the magnifying piece of plastic that comes over the blocks of LEDs of the display. Repair should be easy.

So far, I have abandoned the idea of joining the copper bridge and wire "A", too uncertain.

I'll try to find a bonding machine and lay new gold wires. If this works, I'll send another photo.

F and G : side lighting for this photo shows little to no space between wire "F" that comes from track "B" and the projected shadow for this wire. So, potential short circuit there, between wire "F" and track "G", I'll have to check this area under a microscope. I also suspect, by another snapshot from a different angle, that wire "F" is probably broken just at the edge of track "G", but this is not much visible on the current picture.

In a word, I think that I have reached some limits in manual repair on this display, more sophisticated equipment, such as a microscope and a bonding machine, are now required for further repair.

Kind regards.

pyt22fr

Last edited by pyt22fr (2010-02-09 08:23:13)

Trying to resurrect an HP25C keyboard -completely not responsive- I disassembled the 'heat staked' keyboard. I found lot of verdigris and washed it with a solution of mild vinegar.
The keyboard's removal was easy with a hot hair dryer, preserving the plastic stakes.

I have followed the Geoff Quickfall's protocol.

Here are two photos of the operation scene.

Photo 1, left, the keyboard PCB and its cover with keys ; right, another working keyboard, for reference. Note the '7' key bad strip.



Photo 2, washing the keyboard ; I fixed before the '7' key.



The second part is more risky.
Tomorrow maybe a success.

Good work, Jacques.

Did you follow Geoff's recommendation to circulate narrow strips of harsh paper below each of the metal strips to ensure removal of any dust or dirt below these flat springs ?

As for fixing the the PCB back against the keyboard shell, I would recommend one of the 2 following methods :

1) hot air soldering iron (Weller repair station, for example) to soften the emerging stakes, and then rapidly press with a hard tool, preferably with a plastic tool rather than a metal one, so that you can preserve the heat on the stake for a longer time. To be able to use this method, it is assumed that you have not cut the stake ends away to disassemble the keyboard.

2) small dots of PVC glue on top each of the plastic stakes once the PCB is back in place, then leave it cure the necessary time.

I have myself used hot air this week-end to assemble back the parts of a small keyboard, but they were 4 stakes, not 40, which certainly means better work preparation in your case.

Good luck.