HP Forums
Restoring HP25 S/N 1702S13138 - Very strange take a look - Printable Version

+- HP Forums (https://www.hpmuseum.org/forum)
+-- Forum: HP Calculators (and very old HP Computers) (/forum-3.html)
+--- Forum: General Forum (/forum-4.html)
+--- Thread: Restoring HP25 S/N 1702S13138 - Very strange take a look (/thread-13040.html)



Restoring HP25 S/N 1702S13138 - Very strange take a look - albertofenini - 05-29-2019 09:00 PM

Hi Everyone,
while looking for a good RAM chip we stumbled across a "bargain" and we purchased this unit.
We knew it was not working, so we are not complaining.
When we opened it we found this situation :
[attachment=7330]
and after several steps of water/vinegar and soda we cleaned to this point.
[attachment=7331]
Still it is not working.
We decided that to test few voltages, and we found out that Vgg and Vss are good, while Vdisp
is higher then expected.
[attachment=7333]
When we compared the unit with a working one, we found however some strange differences.
Both units have an identical PCB version, and all of the components are identical
except for the ROM, the working one (the unit pictured on the left) has a ROM 1818-0153,
the faulty one (pictured on the right) has a ROM 1818-0168
[attachment=7332]
However if you look at the yellow circled areas you will certainly note some differences
The working one has three diodes and a capacitor more where the faulty one has
two less diodes in the PSU area and just below the ROM has a resistor instead of an expected diode.

Any explanation for this ?
The faulty units looked absolutely untampered ...
What do you suggest ?
Thanks everyone for reading and posting a reply


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - teenix - 05-30-2019 05:44 AM

(05-29-2019 09:00 PM)albertofenini Wrote:  Both units have an identical PCB version, and all of the components are identical
except for the ROM, the working one (the unit pictured on the left) has a ROM 1818-0153,
the faulty one (pictured on the right) has a ROM 1818-0168

However if you look at the yellow circled areas you will certainly note some differences
The working one has three diodes and a capacitor more where the faulty one has
two less diodes in the PSU area and just below the ROM has a resistor instead of an expected diode.

The 0168 is a replacement for the 0153. Maybe it was a code change.

Maybe the power supply had some modifications also. The PCB holes look filled with solder from the original assembly process.


cheers

Tony


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - isanchez - 05-30-2019 06:58 AM

HI,

may I kindly recommend you to implement the PANAMATIK LC board...I have done it in an HP25 and I have to say that I am enjoying every minute with that tiny Woodstock....impressive work, I have to say!

Kind regards


Ignacio


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - albertofenini - 05-31-2019 12:09 PM

ciao Ignacio, thanks for the idea !
We are trying to rebuild two units, one is an HP-25 and one is an HP-25C.
Both units have a good PSU, display and keyboard working, ACT and Rom working, bad RAM
We have purchased this last unit with the hope to recover a working RAM for the HP25 project
and we were thinking of putting a Panamatik kit in the HP25C, since finding a donor for
an HP25C is more difficult and more expansive.
However, when this came in, we thought that cleaning the corrosion would have been enough
to bring it back to life and us it as a whole logic for the HP25 project.
Honestly, we could just de-solder the RAM and see if it is a good donor for the HP25 project,
but we are curious people and we want to learn why in a unit apparently in good shape the Vdisp is wrong.
Will keep testing on it and will see, however we also want to try the Panamatik !

BTW, do you have any idea why Vdisp could be wrong ?
Thanks for help


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - albertofenini - 05-31-2019 12:16 PM

(05-30-2019 05:44 AM)teenix Wrote:  The 0168 is a replacement for the 0153. Maybe it was a code change.

Maybe the power supply had some modifications also. The PCB holes look filled with solder from the original assembly process.

cheers

Tony

Ciao Tony, we knew about the 0168 as a replacement for the 0158, so this doesn't surprise
However, the PSU modifications are exactly about where the Vdisp is generated and in this unit Vdisp is too high
We'll try to see if Ø1 and Ø2 are good, it's a test we have not done yet
The board looks intact, no previous attempts to be repaired so the modification looks original
What's your guess for a wrong Vdisp, where and what could we investigate ?
Take care !!


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - teenix - 05-31-2019 11:48 PM

(05-31-2019 12:16 PM)albertofenini Wrote:  What's your guess for a wrong Vdisp, where and what could we investigate ?
Take care !!

Hi Alberto,

It looks like here were 2 power supplies for the HP25.

One is shown at Jacques Larporte's site and another at the HPCC site.

https://archived.hpcalc.org/laporte/Woodstock/ws_HW_main.htm

http://www.hpcc.org/cdroms/schematics5.0/index.html

One design has less components but a more complex coil arrangement.

If the other 2 voltages are ok, and the display works ok, then the Vdisp voltage only has a few components to check, coli, 2K2, 10uF and a diode coming off the coil as shown at the HPCC site.

Other usual problems as generally discussed, such as high resistance (or shorts) due to corroded pcb tracks, "gunk" under chips, old components, something wrong with the power path to the display.

Only guessing as usual, but hope it helps.

cheers

Tony


RE: Restoring HP25 S/N 1702S13138 - Very strange take a look - albertofenini - 06-02-2019 09:00 PM

ciao Tony,
we couldn't spend much time on this today, however looks like our is definitely not like Jacques Larporte one, neither like the other two ...
These are the three ... Jacques Larporte is the last one, the first two are from the HPCC
Besides that in one of the HPCC there is a coil with eight pins ....
We will try to see which one is our and return with some comments ...
If in the meanwhile you have any idea ....
Thanks again !!
[attachment=7349]
[attachment=7350]
[attachment=7351]