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+--- Thread: HP-41C Shows 12 Commas, Otherwise Unresponsive (/thread-12850.html)
HP-41C Shows 12 Commas, Otherwise Unresponsive - quoner - 04-23-2019 09:18 AM
I have an HP-41C that I haven't used for a while. With the old batteries, it wouldn't turn on. I installed new batteries, and now it displays a string of 12 commas like this:
,,,,,,,,,,,,
The calculator is otherwise unresponsive to all key presses. I can't even turn it off. Is this unit completely dead? Is it repairable? Thanks.
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - Diego Diaz - 04-23-2019 03:09 PM
Hi, Since it's a 41C chances are that it's a "coconut" model (AKA fullnut) therefore the CPU board is connected to the keyboard/display assembly by some elastic connector.
The plastic posts holding the screws are well known to cause these kind of symptoms (and many others) when the plastic cracks and the connection between both boards gets loose.
There are several methods to fix such problems depending on the severity of the damage.
If you open your calculator and share some pictures it will be easier to address the problem in order to find a suitable repair action.
Hope this helps.
Best.
Diego.
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - burkhard - 04-23-2019 08:00 PM
(04-23-2019 03:09 PM)Diego Diaz Wrote: Hi, Since it's a 41C chances are that it's a "coconut" model (AKA fullnut) therefore the CPU board is connected to the keyboard/display assembly by some elastic connector.
The plastic posts holding the screws are well known to cause these kind of symptoms (and many others) when the plastic cracks and the connection between both boards gets loose.
I was thinking the same thing. If you squeeze the top and bottom of the calculator together with your fingers (try a few places) while attempting to power/de-power, does it help?
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - quoner - 04-24-2019 10:18 AM
Thanks for you replies Diego and Burkhard. Here is a picture. It doesn't show much. Do I need to disassemble further?
https://www.dropbox.com/s/784o6u7csy1ix96/2019-04-24%2002.30.55.jpg?dl=0
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - burkhard - 04-24-2019 02:57 PM
Good picture!
This must be a quite early 41C because it shows two nuts around the lower black posts, holding the daughter board onto the main board. The later versions of the 41 series don't have those nuts and only squeeze the daughterboard onto the main board with the skinny screws going into those two plastic posts. With age and use/abuse, the plastic posts crack and the "squeeze" is no longer so good, which causes all kinds are weird behavior because of the poor contact between the boards.
For yours, though, those two nuts *might* be holding things tight... I think they can be removed the get the daughterboard off at which point you could clean the contacts between the two and apply some DeOxit Gold. But.... I haven't taken apart one this old with those nuts myself and don't know if there are special cautions. You obviously do not want to break either of those two plastic posts off...
So, let's let somebody reply who has disassembled one of these early ones with the daughterboard nuts.
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - Diego Diaz - 04-25-2019 02:17 PM
Hi, Good Hi-res picture indeed!
As Burkhard pointed out, those nuts are far less prone to cause the "cracked post" syndrome. So this may not be the cause of your display problem.
First I'd short the capacitor (the "SPRAGUE +" labelled cylinder) terminals for a few seconds, just to make sure that there's no remaining charge that can keep the processor into an "undefined status".
If this doesn't help. Nuts can be removed using a 1/4" (or 6.5mm) socket wrench.
Cleaning the contacts with a Q-tip and isopropyl alcohol is always a good idea and won't hurt anything. Though I'm not very confident it can make a difference in this case.
Next step in diagnosing the fault would be CPU board replacement. Do you have any other HP-41 (fullnut type) available? If so, you can swap the CPU boards (C, CV or CX) and see whether the problem persists or not.
These are great machines but somehow fragile. Extreme handling care is highly advisable.
Keep us posted.
Diego.
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - g8241CV - 04-25-2019 03:04 PM
Suspicious of your comment that batteries went dead.
Rightmost battery contact in the photo looks like it may have the blue discoloration of battery leak residue. May well be just an artifact of the photo, but worth cleaning this and all contacts for a power on retry. Any discoloration under black protective coversheet on left photo?
++ on IPA cleaning + Deoxit treatment of contacts.
RE: HP-41C Shows 12 Commas, Otherwise Unresponsive - AndiGer - 04-25-2019 06:20 PM
Even if looks clean try cleaning all the contacts at the IO block (both - the IO block itself and the counterpart on the PCB) with IPA.
Removing the nuts can cause problems. Try to hold the posts when turning the nuts otherwise those can be shearded off - making your problem bigger = broken posts.