Early/earliest HP-41CV halfnut

[AndiGer]

These days I received an HP-41CV halfnut with serial 2405S...
The earliest halfnut serial here https://www.hpmuseum.org/collect.htm#earliest
is claimed to 2511 ...
So I asked the seller. He told me he bought it new and it was neither serviced nor did he opened it for repair (what I think is true due to the well attached rubber feet).
He used it until 2019 as a daily driver. Unfortunately he then left the batteries in and they leaked. I opened and cleaned the corrosion but it doesn't work correctly.
After a MEMORY LOST - which comes up after about half an hour of batteries out - and left after about 15 seconds on it suddenly display 1/X then X<>Y followed by NULL - as if you were depressing that key constantly. After this happens the only keys that show a reaction are the keys in the same keyboard row i.e. X<>Y, RDN, SIN, COS and TAN.
When I press back arrow and ON it immediately shows X<>Y followed by NULL. Sometimes I can switch to one of the keys of that row and after back arrow + ON it shows that key followed by NULL.

a) Anybody knows what to check / repair?
b) This is really a very early serial?
Thanx
Andi

[TallKey]

Addressing a)

Recently read an ebook* on repairing a 41.

Short of it is they use a Caig Deoxit product. It is applied to the battery side of the PCB key contact mechanism. The product allegedly can restore/wash the key contact. Each key location has about a 1mm diameter hole where this product is applied. It makes sense if a small amount of debris is the problem.

Never performed & never used the product they recommend.

Per their writeup a proper KB teardown and cleaning can only be performed on a Full-Nut.

*See thecalculatorstore.com, then the HP41 corner link.

[BillBee]

If its a stuck key I follow the method I learned from our other members.

DeOxit Red + 1 strand of stiff wire (from a wire brush). I put a drop over the corresponding pcb hole (there are little holes on the backside of the keys) insert wire strand and gently move around. Then exercise the key. Repeat as needed.

Hope that helps,
Bill

[rprosperi]

As Bill has confirmed, yes this product works quite well, though note that there are several different DeOxit products for various kinds of cleaning or treating and preserving contacts. You should use DeOxit D100L (also often simply called "DeOxit Red") for cleaning battery copntacts, keyboards, etc.

Another type, "DeOxit Gold" is used as a preservative and contact enhancer on contacts that are particularly worn/damaged, as is often the case when battery leaks may have corroded the 'chrome' finish from battery contacts, etc.

Both are widely available on eBay, Amazon, etc.

[AndiGer]

Thank you all for your suggestions. DeOxit red and gold - I read much about it here but never bought.

To clarify ... although there was some battery leakage the calculator has no stuck keys.
I could locate the KR lines on the board and there is no permanent shortage from the keys. Only when I press the appropriate key in rows 1 or 2 the connection is made to that KR line. Today I measured the KR lines to the key in rows 1 and 2 again and all seems to be fine.
The machine sat without batteries since last Sunday.
So I put in batteries, started the machine and after 2 minutes it still behaved normal, showed 0.0000 after MEMORY LOST.
Then I performed the "forensic test": 9 - sin - cos -tan - arc tan - arc cos - arc sin
in FIX-9 mode and let the calculator sit for a while. Still normal.
Switched it off and let the batteries in. Since about 2 hours I switched it on 2 times. Still normal.

The only thing I didn't do this time: CF 28 to get comma instead of point as decimal separator.
I will observe further.

[AndiGer]

Now I let the calculator sit until it switched off itself - after about 7 minutes.
I changed to decimal separator comma (CF28) and it still behaves normal.
I did some calculations, did change SIZE to 000 and 319 and back to 046.
Changed the battery compartment to a rechargeable HP-82120A - all ok.
Successfully installed Math1 module in each of the 4 ports. Works and everything normal.
Astonishing. Faith healing by my hands or self healing by "let it alone" ;-D ?

Hope it will work in future without further problems.

Andi

PS: Somebody can report he/she has a halfnut of the same age or even older?