Adjusting HP 10 and HP 25C



I have a HP25C and a HP10 (the one with the build in printer) under restoration and cleaning this moment. When you remove the display (the circuit board with the leds) there is one potentiometer for the HP25C and even 4 potentiometers on the HP10 on the circuit board below.
Sofar I have not tried what happens when changing their adjustment, but I am very curious.
Anyone an idea what they adjust and if so how to adjust them optimal or what the original settings should be ?
Pictures are available if requested to identify what I mean offcourse.

Any help would be really appreciated.



On the 25C, the preset pot sets the low-battery dectecion level. IIRC there are _5_ presets in the 10. They set the clock rate, display intensity, low-battery level, printer motor speed and printer intensity (I will have to check the diagrams to see what the order is).

Please don't fiddle with them...



This is very interesting, I thought it was allin silicon defined, but in the old days more like the real HP measurment systems, where a lot could be adjusted by adjustable coils, pot's, variable C's.


Hi Tony,

I had some troubles to get the HP-10 printer running again, but a total cleaning and lubrication of the printer got this one up again.
Now I was stuck with all dots in the display the moment the printer was accessed and also that the characters were stretched out horizontally. Some carefull fiddling with the potmeters got that up again. By comparing the speed of the printhead mobing from left to right from to a HP-19 and adjusting potmeter 4 got that back on track. Potmeter 5 is the density of the printer. Potmeter 3 is the brihghtness of the display, potmeter 2 is for adjusting the low voltage warning (I set it to trigger at 4.8 volts for now).
Potmeter 1 is still not clear to me, it does something on the horizontal speed of the printhead, but potmeter 4 also has that effect.

I counted the potmeters from 1 to 5 looking from the top from left to right.

Another observation is that the battery has to be really oké. I was during the repair working with a stabilised PSU with a current limit of 500 ma. This made the printing giberish. When switching to a more powerfull power supply, one who can deliver up to 3A, the print out was clear dark and 100% oké.

So it looks like the HP-10 will have a second working live now. Your info on the potmeters were very usefull.

Thank you for that.


Pot #1 sets the CPU clock speed (the oscillator in this machine uses an RC network, not an LC network). I think my notes include the freqeuncy that I measured in a working HP10 (I connected a LogicDart to some point in the oscillator circuit).
Again, I can look this up if you need it, but if the machine is working, I'd leave it alone now.
Incidentally, the 10 uses NMOS chips (like the 27, and unlike just about all other HP calculators of the period which use PMOS devices) so the PSU is somewhat strange!


Hi Tony,

If you are able to find the frequency it would be very nice.

I marked the original position of the first potmeter, but if you twist it to one side, the printer will only print horizontal lines, which seems consistent with you remark on the CPU freqency. I will take a reading myself also to see what frequncy he is running. I assume a lower frequency will mean a somewhat lower powerconsumption, but realistically this will problaby minimal in respect to the drain to the Led's and offcourse a printing printer.


OK, I've found the notes. It appears you connect a freqeuncy counter/scope to pin 39 of the main IC (ground == -ve side of the battery pack), then adjust the leftmost
preset for 1.26MHz


Tony and Ronald,

This is a very useful and interesting thread. Could one of you please write this up as an article and post it in the articles section here? Other than Michael Meyer's article on the HP-19 overhaul, there's very little information on this site concerning HP-10/HP-19 repairs.




The HP19C printer system is very much like the HP97... it uses the same hybrid multivibrator module, etc. The HP97 service manual has a lot of useful info.


There is one important difference between the Topcat and Sting printers. The motor in the former is reversed to bring the printhead back to the home position, in the latter the motor always turns in the same direction (and the printhead travel is reversed mechanically).

Otherwise they are very similar. The 19C uses the same printer control chip as the Topcats (it's in the big 40 pin chip on the 10). They use a quad comparator chip (looks like an LM339) and the same hybrid circuit to set the motor speed and printhead drive time (intensity).

Incidentally, the 19C has very similar hardware to the 95C... I've had both in bits.


I had both the HP10 and HP19 open next to its other. The printers mechanisme is the same.
I have variuous pictures from the inside if interested.

One observation:
What I found out is a that is very important to have the horizontal speed of the printer precisely adjusted. To slow, and you have smaller digits printed (width).

If however to fast and you get larger (wider) digets printed, but funny enough, due to the fact that the printerhead keeps on moving the effect is for numbers with 8 or 9 digits the last digits gets printed on the way back of the printhead and are mirrored.
At first glance you think the printer is printing garbage, defective driverchips etc. but later when really looking what is printed this becomes very obvious that it were only mirrored digits printed over the first charecters. The paper feed is only done when the printhead is at 50 - 60% of its travel back to the homeposition.


Hi Katie,

No problem, I can make my story including some internal pictures, and if Tony would be able to add his remarks and measurements to it, it could become a nice story.
Let's see if Tony is also willing to add some of his experience.


My home box is a text-only machine so I can't help with diagrams, etc, but I'll be happy to share any information that I can...


Hi Katie,

I have sent a draft last week to Tony for review and his additions.



Thanks! I'm looking it'll be great to have this information readily available to all.




If you want to publish my part already with pictures etc. send me your email adress please and I will forward it to you.



I don't think I ever received it.

If you sent something other than plain text, then it's likely I deleted it without attempting to read it (I _cannot_ handle proprietry word processor files like M$ Word !).



Any way I can get it to you exept by fysical sending you a CD or Floppy ?


If you send me you mail adress you want to have it I can send you already my part.



You can send me e-mail (my address above is un-munged), but only plain text. That applies to sending me physcial media too. There is _no way_ I can make sense of proprietry word processor files, and graphical images give me severe problems.

Possibly Related Threads...
Thread Author Replies Views Last Post
  Shutdown with the Apps key and more than 10 variables in a program. Davi Ribeiro de Oliveira 10 2,717 11-05-2013, 01:26 PM
Last Post: Han
  Updated PPC DVD Version 2.10: HP-41 Searchable Program Files and Scannable Barcode Jake Schwartz 3 1,289 09-27-2013, 09:51 PM
Last Post: Olivier (Wa)
  Rounding of 10^x Olivier De Smet 8 1,812 08-28-2013, 06:33 AM
Last Post: Dieter
  HP-25/25C Potential Matt Agajanian 14 3,102 07-17-2013, 02:57 AM
Last Post: Matt Agajanian
  Later HP-25C Keyboard Repair aj04062 2 924 04-23-2013, 07:25 PM
Last Post: aj04062
  For sale: HP 25C and Stat Pac 2 for HP 65 Michae Altmann 1 766 04-21-2013, 01:24 PM
Last Post: Raymond Del Tondo
  HHC 2012 RPN Programming Contest - 10-Step Way-After-Nashville Solution Jeff O. 32 4,696 10-12-2012, 01:41 AM
Last Post: Paul Dale
  Library 4 update on 10/2/2012 ? Gene Wright 10 2,240 10-11-2012, 02:31 AM
Last Post: Ángel Martin
  e^pi - pi + 9/10^4 + 1/(10^4*ln(2) + sqrt(10)/6)^2 Gerson W. Barbosa 47 8,275 08-08-2012, 10:58 PM
Last Post: Les Koller
  [WP-34S] Upgrade in 10 mins. now v 3.1 2988 Chris Tvergard 12 2,579 06-01-2012, 02:39 AM
Last Post: Walter B

Forum Jump: