HP25C repair, help needed
01-19-2019, 06:09 PM (This post was last modified: 03-05-2019 06:50 PM by AndiGer.)
Post: #21
 AndiGer Member Posts: 231 Joined: Oct 2015
RE: HP25C repair, help needed
The parts on my HP-25C board look just as yours but single density chips. I only have the board. Got it together with a HP-21 board which looks like your HP-25 board as I look at the parts.
Didn't get it to work yet but didn't try very hard. Didn't measure voltages.
Thought maybe there are differences as it is an older version as I guess due to 1820-1630 chips.
So not of help for me and you.
01-19-2019, 07:58 PM
Post: #22
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Dear All,

we will post pictures later, but this is what we accomplished today.
We are dealing with mainly two issues : the motherboard and the keyboard.
As said before we have opened the working HP25C from our collection, and measured Vgg, Vss and Vdisp.
They were obviously corrected.
Looking at Jaques Laporte scheme we tried to find where to measure the Vgg, Vss and Vdisp at the PSU.
We only have found where Vgg can be measured at the PSU.
The differences between HP25 and HP25C have made impossible, to us, to locate the other testing points in the PSU,
so we measured those values at the top of the PCB.
We were about to de-solder all of the PSU components when we have found this old link :
HP-25C Repairing
and we decided to follow that path.
We de-soldered both 1820-1886 (the double density ram) and the 1818-0154 (rom)
Well, the assumption that PSU was working was a good one, without those two ICs our PSU was working indeed.
In fact, now also on this unit Vgg, Vss and Vdisp are correct.
We have now a steady red 0.00 on the display.
However, while the old thread says the basic math could be performed, our unit doesn't.
Because the keyboard under restoration is damaged we used the keyboard from the working unit.
We have been able to enter digits without problems.
But if we try to do anything like pressing the ENTER, pressing CLX, perform operation math (+, -, x, /) the display
shows a 0.000000000 (1 digit 0, 1 digit . and 9 digits 0)
The unit doesn't lock up so that we can continue to digit new numbers.
Also it doesn't handle the decimal .
Switching to PRGM shows the 00 in the left corner and pressing SST will show
01 13 00
02 13 00
which we can understand as ROM and RAM are missing.
At this point we re-soldered the 1818-0154 to see if it was a cold joint but the machine with that IC inserted looks like dead.
We de-soldered again and the unit worked as described above-

What do you think ?
Are we talking about a partially damaged ACT or this behavior is because of the two missing ICs ?

Since it had very much oxidation we decided to take it apart, and clean it.
As you will see later in the pictures this particular keyboard is composed by a thin plastic film with some copper traces,
a plastic films that serves as a separator and the PCB.
Unluckily most of the copper traces were broken ad when put together generate random contacts.
We have seen some keys working, after the cleaning, but the majority doesn't.

Beside the difficulty of putting back together a Woodstock keyboard, we did some tests with the glue that can be
made liquid by warming up with a gun, but the results are very poor, so we will probably look for a replacement
from a donor.

Any idea or suggestion is very welcome !!
Take care !

Edoardo & Alberto
01-19-2019, 08:43 PM
Post: #23
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
The pictures : PSU

Edoardo & Alberto
01-19-2019, 08:44 PM
Post: #24
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
The pictures : KEYBOARD

Edoardo & Alberto
01-20-2019, 05:00 PM
Post: #25
 AndyWalter Junior Member Posts: 11 Joined: Mar 2018
RE: HP25C repair, help needed
I don't know if the 25 & 25C psu is the same as that in the 29C, but you can get the full 29C Service manual with schematics & scope waveforms etc here:
https://www.dropbox.com/s/7yh1e0dqtgg2k1...l.pdf?dl=0

Whereabouts are you located? I'm in UK. I have just restored a 25, which was totally dead, nothing on the display when battery connected; I stripped every component except keyboard & the display panel, and used Panamatik's brilliant PIC-chip replacement; this has it's own ram, display drivers, uses less power than the originals, faster, can emulate quite a few of the handhelds, has a beeper + timer, can have GPS, ... ! It just might be worth ditching your components & using one of these!

I have all the old components I took out - no idea if any of them work or not, but if it helps I can post them to you! I also have some photos of the board before I removed the chips, will try to post those here in a few minutes...
Andy
01-20-2019, 05:37 PM
Post: #26
 AndyWalter Junior Member Posts: 11 Joined: Mar 2018
RE: HP25C repair, help needed
The HP25 I restored came from Jim Johnson here, and looking at my photos, I think he'd already salvaged the ACT chip! Would explain why there was nothing at all on the display!!! FWIW here are the photos I took of this, before removing the remaining chips:

Attached File(s) Thumbnail(s)

01-20-2019, 05:40 PM
Post: #27
 AndyWalter Junior Member Posts: 11 Joined: Mar 2018
RE: HP25C repair, help needed
I think there's a 5 image limit on posts, here are the last few pics of HP25 ...
Andy

Attached File(s) Thumbnail(s)

01-21-2019, 10:32 AM
Post: #28
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Thanks Andy, that would be nice to have,
how can we pay you back besides paying the shipping costs ?
Do you also have the keyboard or just the PCB ?
Thanks again !!

Edoardo & Alberto
01-24-2019, 07:50 PM
Post: #29
 mpark Junior Member Posts: 8 Joined: Sep 2018
RE: HP25C repair, help needed
(01-19-2019 08:44 PM)albertofenini Wrote:  The pictures : KEYBOARD

My HP25C keyboard is similar and a few keys were not making good contact. I applied little squares of copper tape to the pads on the printed circuit board and now all the keys work.
01-26-2019, 10:23 AM (This post was last modified: 01-26-2019 11:13 AM by albertofenini.)
Post: #30
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Thank you very much for the excellent idea !
We looked on Amazon but looks like the adhesive side is not conductive.
Can you suggest us the model of copper tape you have used ?
Take care anche thanks again Alberto

Edoardo & Alberto
01-26-2019, 04:02 PM
Post: #31
 AndiGer Member Posts: 231 Joined: Oct 2015
RE: HP25C repair, help needed
OT:
Hi Alberto I clicked to "last post" of your thread. Congratulations for a lucky number of that post:
Wish you good luck while repairing your calculators!
01-26-2019, 06:14 PM
Post: #32
 Geoff Quickfall Senior Member Posts: 689 Joined: Dec 2013
RE: HP25C repair, help needed
If you find that you have a dead ACT chip, as stated Bernhard at Panamatik has a replacement PIC version of the ACT which is incredible.

I posted a video for an HHC conference for replaceing the ACT:

Replacing the ACT
01-27-2019, 07:28 AM
Post: #33
 mpark Junior Member Posts: 8 Joined: Sep 2018
RE: HP25C repair, help needed
(01-26-2019 10:23 AM)albertofenini Wrote:  Thank you very much for the excellent idea !
We looked on Amazon but looks like the adhesive side is not conductive.
Can you suggest us the model of copper tape you have used ?
Take care anche thanks again Alberto

I used this tape from Adafruit. The adhesive is claimed to be conductive, but it wasn't conductive enough to satisfy my continuity tester, so what I ended up doing was to cut the tape into squares with a tab (see crude diagram below), then remove the backing to expose the adhesive, fold the tab under the square, then press the square onto the PCB, so the copper surface of the tab makes direct contact with the pad on the board.

Hope that made sense. Good luck!

Code:
+-----+ |     | +-+ +-+   | |   +-+
01-28-2019, 09:01 PM
Post: #34
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Thank you very much,
we have ordered the tape today, will keep updated on the keyboard,
how did you closed it back anyway ?
Take care Alberto

Edoardo & Alberto
01-28-2019, 11:35 PM (This post was last modified: 01-28-2019 11:38 PM by Geoff Quickfall.)
Post: #35
 Geoff Quickfall Senior Member Posts: 689 Joined: Dec 2013
RE: HP25C repair, help needed
Just for the heck of it, I don’t know how far into the restoration you are but I have some thoughts on restoration.

Firstly do as little harm as possible. Ripping apart the heat staked keyboard is a pretty major undertaking. The problem is reattaching the keyboard to the keyboard PCA.

this can be accomplished by removing as little of the heatstake as possible prior to the disassembly, then reheating / melting the heatsake back into a mushroom.

Others have tried mini screw heads.

My caveat is to isolate the keyboard and PCA then clean the board first in hot soapy water. If copper sulphate (battery corrosion on the contact domes) is suspect then a soak in a white vinegar solution followed by a rinse then drying thouroghly. The inside of the dome then is scraped by inserting a fine stiff wire in the pore on the keyboard PCA board and twirling it around. Introducing into the pore DeOxit red and work the key.

If these steps don’t result in a functional key then consider the major surgery.

See this link: Pores in various HP calc Keyboard PCAs

ALSO, the keyboard silk screening on the Woodstock’s is notorious for being faded and being wiped off. Dab the keyboard dry, do not rub it dry or you risk removing the silk screened labels.

Geoff
01-28-2019, 11:57 PM
Post: #36
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
(01-28-2019 11:35 PM)Geoff Quickfall Wrote:  Just for the heck of it, I don’t know how far into the restoration you are but I have some thoughts on restoration.

Firstly do as little harm as possible. Ripping apart the heat staked keyboard is a pretty major undertaking. The problem is reattaching the keyboard to the keyboard PCA.

this can be accomplished by removing as little of the heatstake as possible prior to the disassembly, then reheating / melting the heatsake back into a mushroom.

Others have tried mini screw heads.

My caveat is to isolate the keyboard and PCA then clean the board first in hot soapy water. If copper sulphate (battery corrosion on the contact domes) is suspect then a soak in a white vinegar solution followed by a rinse then drying thouroghly. The inside of the dome then is scraped by inserting a fine stiff wire in the pore on the keyboard PCA board and twirling it around. Introducing into the pore DeOxit red and work the key.

If these steps don’t result in a functional key then consider the major surgery.

See this link: Pores in various HP calc Keyboard PCAs

ALSO, the keyboard silk screening on the Woodstock’s is notorious for being faded and being wiped off. Dab the keyboard dry, do not rub it dry or you risk removing the silk screened labels.

Geoff

Thanks Geoff
I'm afraid we are already beyond that point ...
If you look at message #24 of this post you will see that we had to open the keyboard apart,
although we know it will be very hard to close it again.
However most of the copper traces on the plastic film were broken,
we will try to replace them with thin strips of copper tape.
If we will be successful in that we will think on how to put the pieces back together
Thanks for your help, always very welcome !

Edoardo & Alberto
01-30-2019, 01:30 PM
Post: #37
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Copper tape arrived !
Will posting very soon
take care Alberto

Edoardo & Alberto
02-16-2019, 10:35 PM
Post: #38
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
First of all, thank you very much Andy !
We received the package, and we added a working ROM to the unit.
The RAM was not working, and even if it was, it should be a double density RAM, or we should add
two single density ROMs.
However, it was a big step forward, thank you very much !!

We also repaired the plastic foil with broken traces with the copper tape.
The copper tape was guaranteed to have conductive adhesive, which is somehow true,
we applied a thin strip to all the traces, and we recovered some of the keys.
Please keep in mind that before it was not working at all ...

Also, we practiced on how securely close a Wookdstock keyboard.
In our experience this was a very good result,
keys have a normal profile and click well.
We used Vinavil, while keeping the layers of the keyboard pressed together with some clips.

It takes a night to dry, and it must not be touched before,
but when it dries is quite strong and keep in place things, is not invasive and very clean.

Unluckily this has to be opened again, since not all of the keys are working,
we'll keep posting results, and if someone has a spare double density RAM we'll be more than happy

Edoardo & Alberto
02-18-2019, 07:04 PM
Post: #39
 burkhard Senior Member Posts: 345 Joined: Nov 2017
RE: HP25C repair, help needed
(02-16-2019 10:35 PM)albertofenini Wrote:  Also, we practiced on how securely close a Wookdstock keyboard.
In our experience this was a very good result,
keys have a normal profile and click well.
We used Vinavil, while keeping the layers of the keyboard pressed together with some clips.

It takes a night to dry, and it must not be touched before,
but when it dries is quite strong and keep in place things, is not invasive and very clean.

Living in the USA, I'm not familiar with Vinavil, so looked it up and see that in the broader sense it's an Italian adhesives company. I assume one of their big / original products is known just as "Vinavil" in Italy, and that is what you meant. I'm not 100% sure which one that would be, though. I'd guess that there might be an alternative from another company possible as well, but it's hard to guess at what is equivalent without precisely which adhesive you meant. Do you have a link to a part number for the adhesive?

thanks!
02-19-2019, 01:48 PM
Post: #40
 albertofenini Member Posts: 293 Joined: Mar 2014
RE: HP25C repair, help needed
Here you go ..
Amazon Vinavil
It's a very common glue, also used in schools, and it's quite cheap.

Edoardo & Alberto
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