Help - broken HP-25


I've got a HP-25 that is broken and I'd like to fix it if possible.

The battery pack had some corrosion and the contact had some too. I've scraped this off and now the contact is nice and shiny. I've also rebuilt the battery pack, however further tests have been done using a small bench top power supply clipped to the battery contacts (polarity has been observed at all times) while the back is off the calculator.

Initially, the calculator was drawing about 100mA when switched on but without any display.

Okay, open it up carefully and reseat the main circuit board. Now the display shows '8' in random positions. Most typically at the right end but sometimes in other places and sometimes several (2 or 3) at once. No keys respond and the program/run switch does nothing either.

Pull it apart again and I don't find anything obviously wrong. I gently spread the between board connections a little to hopefully improve contact. Reassemble and now nothing. No power draw when on or off.

Does anyone have any suggestions as to what I broke and what the original problem was?

I don't have any for parts Woodstocks :-( I've got a working HP-21 which I could extract parts from but would prefer not too since it works.

I'll take this into work tomorrow and check for bad solder joints under the microscope.

So anyone have an idea on any other avenues to follow?

- Pauli


Is it an HP-25 or a 25C?


It is a HP-25. No C.

- Pauli


With no current draw, it sounds like a bad transistor in the SMPS or something as simple as the ON/OFF switch not making contact.

When it was drawing current, it had all the signs of a dead ACT IC.


Smells like toast. HP's lauded design quality from the good ol' days (one-third century ago!) strikes again.

This is not age-related failure. It could happen to a new Woodstock just out of the box. It should have sparked a class action against HP for the blatantly incompetent and destructive design of the Woodstock charging system. The lowest cost Woodstock, the HP-21, cost $125 in 1975 (equivalent to $500 in 2009). The HP-25 subject of this thread cost the 2009 equivalent of $800. These were never intended to be short-lived throw-away items. The customer deserved far better.

It's a real shame, because mechanically (size, weight, rigidity, keyboard layout, battery pack/door) the Woodstocks are my favorite HP LED calculators. I'd love to have the capability of the HP-34C (so flimsily made...more good ol' time HP quality) available in the sturdy and compact Woodstock configuration, and with a charging system designed by someone more competent than a dull grade school student.

Edited: 17 Sept 2009, 9:48 a.m.


Too bad most Woodstocks around are burned. I still don't have any. This HP-22 is selling for about 60 dollars but it doesn't work... I'd rather get an HP-25C though.


Too bad most Woodstocks around are burned. I still don't have any.

I somewhat disagree, and would rather say "many" are bad. I purchased my HP-21 a little over a year ago from a seller who was willing to ship internationally (DaveHal2001) for $76 USD and it is in exceptional functional and cosmetic condition. I also have two HP-22's, a HP-25C and a HP-27, all of which are in excellent condition. Admittedly, they are hard to find and you just have to be patient. You can always find one if you are willing to pay enough, it's just a matter of your personal limit. So far, I've yet to find an HP-29C or HP-70 that meets my cost criteria, but I'll just continue to keep looking. The one thing I've learned about Woodstocks is that it is better not to buy one that is untested, because in all likelihood it will be "toast."


It should have sparked a class action against HP for the blatantly incompetent and destructive design of the Woodstock charging system.

I'm no lawyer, however, I do know that there is something known a "failure to inform liability" where a manufacturer fails to alert a consumer as to improper use of a product, which could result in damage. I have the owner's handbooks for all the Woodstock models (HP-21/22/25/27/29C), and only the HP-21 and HP-25 manuals have a specific warning (caution actually) that AC operation without a battery installed may damage the calculator. The other manuals simply state that the calculator will not operate or operate properly, however, there is no indication that any damage could occur. I wonder if this is the one of the reasons it is harder to find undamaged HP-22/27/29C's than Hp-21/25's.


I suspect that the HP-21 and HP-25/25C sold many times the number of units compared to all of the other Woodstock calculators combined. Therefore, just due to the quantity originally built, more survive.

I'd like to come across a decent HP-29C, but I've never trusted the ebay listings that I've seen with respect to the functionality of the item being sold. When it comes to Woodstocks, I'd gladly pay more for a guarantee of complete functionality, and I'd pay nothing for anything less. I have no need of a $200 paperweight that so many Woodstocks have become.


I'd like a nice 29c too but I'd settle for a working 25 or 25c at the moment.
I really like the Woodstock form factor and feel.

- Pauli



Thanks for the comments. I suspected a bad ACT IC might be the culprit.
I guess that means I won't be able to fix this one unless I decide to break my 21 (assuming the chips are the same in these two models of course).

Oh well,

- Pauli


They are not compatible... as the 21 ACT was unique.

Since the 21 lacked memory external to the ACT, the 1820-1396 omits the DATA signal on pin 11. The 1820-1596 ACT used in the 25 has it... so while you could calculate, you'll have no programmability.


Thanks again Randy, I'll leave my 21 alone :-)
The 25 will just have to stay dead.

- Pauli

Edited: 17 Sept 2009, 10:01 p.m.

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