HP-25 Isopropylalkohol

[Tom Flatterhand]

I received an HP-25 today, which did not work at first. Apparently the accumulators or batteries leaked once and attacked the battery contacts. After a test with a lab power supply and test clamps on the rivets of the contacts it could be brought to life. With a nail file I have then filed the battery contacts something and now he runs.

Unfortunately it is a bit dirty and I cleaned the feet with and the outer housing with gasoline. This has worked well. Then I cleaned the plate around the power button with some isoprophyl alcohol, one spot got a little lighter. Is it known that isopropyl alcohol attacks the housing or especially this plate?

[Dave Britten]

DON'T clean the front plate with alcohol! It will rub off the painted labels for the shifted functions. And who knows what it might do to the old plastics. I would play it safe and use distilled water.

[Tom Flatterhand]

(04-06-2022 05:09 PM)Dave Britten Wrote:  DON'T clean the front plate with alcohol! It will rub off the painted labels for the shifted functions. And who knows what it might do to the old plastics. I would play it safe and use distilled water.

Thank you for the warning. In the meantime, I had cleaned a bit more with alcohol and it seems that the lighter spot comes from a grease that is on the plate. I also ran a cotton swab dipped in alcohol on the edge once and it was quite brown on that! But then I think I will do the rest with water and a drop of dishwashing liquid.

[Frido Bohn]

(04-06-2022 04:56 PM)Tom Flatterhand Wrote:  After a test with a lab power supply and test clamps on the rivets of the contacts it could be brought to life. With a nail file I have then filed the battery contacts something and now he runs.

Just as a reminder in the case that you don't know that the Achilles tendon of the HP25 is the use of external power supply without batteries being in place. The use of power through the battery contacts is fine but beware of applying power on the contacts at the bottom of the chassis without batteries that serve as a kind of overvoltage/overcurrent buffer.

[Tom Flatterhand]

(04-06-2022 06:32 PM)Frido Bohn Wrote:  Just as a reminder in the case that you don't know that the Achilles tendon of the HP25 is the use of external power supply without batteries being in place. The use of power through the battery contacts is fine but beware of applying power on the contacts at the bottom of the chassis without batteries that serve as a kind of overvoltage/overcurrent buffer.

Thanks for the tip! When I bought an HP-29C on an auction platform the other day, the seller also pointed this out to me. That's why I used a lab power supply, set the voltage to 2.5 volts and the current to a maximum of 200 mA.

It is quite possible that I have destroyed a Privileg PR 56 D-NC, which I bought a few years ago, in this way. Also then I had not inserted batteries (3*AA) and connected the power supply. But only through the references to this weakness with the HP calculators I have now recognized the connection.