HP 21 repair - Printable Version

HP 21 repair - Printable Version

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HP 21 repair - surgdog - 06-15-2015 01:28 AM

Just found my college calculator from 1975 which has been in the attic for over 10 years fortunately with the battery pack out of it.

I followed the instructions here to remove old batteries and replaced first with alkaline then with rechargeable batteries. With the alkaline I did get the LEDs to light up after awhile but no function of the cal. Nothing happened with the Niad batteries.

I took the device apart and found that a piece of foam that was between the two PCB had corroded and when I removed it found several traces damaged on the PCB's.

Is there anything to be done or do I just write it off? Is this repairable?

I really like this calculator and have an attachment to it especially now that I have found it again.

Any advice is appreciated.

Thanks,

mk

RE: HP 21 repair - Dave Frederickson - 06-15-2015 01:46 AM

I've been experimenting with conductive ink pens to repair damaged traces.

RE: HP 21 repair - surgdog - 06-15-2015 01:55 AM

I saw them when I did a search for "circuit board repair". More negatives than positives.

If they work that would help me a lot.

How does it work for you?

RE: HP 21 repair - Dave Frederickson - 06-15-2015 02:14 AM

(06-15-2015 01:55 AM)surgdog Wrote: How does it work for you?

I've been trying to use it to repair a Thinkjet flex cable.

So far it works.

Dave

RE: HP 21 repair - PANAMATIK - 06-15-2015 02:56 AM

(06-15-2015 01:28 AM)surgdog Wrote: Is there anything to be done or do I just write it off? Is this repairable?

If only some traces on the main board are damaged, they can be repaired with isolated copper wire (german Kupferlackdraht). The HP-21 main board is easily accessible for repairs. If the ACT chip is damaged, you can replace it by the "new ACT". Your HP-21 is most likely repairable. Don't write it off!

Bernhard

RE: HP 21 repair - isanchez - 06-15-2015 11:10 AM

Hi,

I have been using "conducting silver"( http://www.amazon.co.uk/Conducting-Silver-conductive-repairing-heaters/dp/B000YJ2P2I )
to repair the keyboard circuit of an HP41C, successfully. It must be applied with a very small and thin brush. It gets dry in half an hour and is very effective and reliable..

Kind regards

Igancio

RE: HP 21 repair - surgdog - 06-15-2015 03:16 PM

(06-15-2015 02:56 AM)PANAMATIK Wrote:
(06-15-2015 01:28 AM)surgdog Wrote: Is there anything to be done or do I just write it off? Is this repairable?

If only some traces on the main board are damaged, they can be repaired with isolated copper wire (german Kupferlackdraht). The HP-21 main board is easily accessible for repairs. If the ACT chip is damaged, you can replace it by the "new ACT". Your HP-21 is most likely repairable. Don't write it off!

Bernhard

Forgive my ignorance but what is the "new ACT"?

RE: HP 21 repair - Marcus von Cube - 06-15-2015 05:30 PM

(06-15-2015 03:16 PM)surgdog Wrote: Forgive my ignorance but what is the "new ACT"?

Bernhard has created a replacement circuit for the processor (ACT) in some older HP calculators. The chip is prone to fail under not so rare circumstances. This is the thread: ACT available