HP 41CV Repair

Everything about 41C 41CV and 41CX Hardware

HP 41CV Repair

Postby MikeHshm » Wed May 06, 2009 7:54 am

Help !

I have an HP 41CV that has been my trusted servant for many years but has now died.
Problems started about 6 months ago when it started eating batteries, previously a set would last about a year but were now down to a few weeks.

There were a number of postings on the web which suggested that this problem was due to a break-down of the capacitor C1 which holds the memory during a battery change.

I opened the 41 up and changing C1 looked straightforward so I replaced it with a new 470uF capacitor, the only difference being that the new one was a 16 volt as opposed to the original that was 6.3 volt but I don't think that should matter.

After re-assembly I inserted the battery pack and immediately (without touching the on button) the display came up with a string of 'hieroglyphic' characters and would not respond to any keystrokes.

I have since cleaned all the contacts on the back of the CPU board and on the silicone rubber strip below. I have also tried putting some thin rubber washers over the mounting posts to increase the pressure on the contacts.

Finally in desparation I put the original C1 back but all to no avail !

The guy who replaced the capacitor for me spends most of his time repairing circuit boards so I am reasonably confident he won't have damaged anything on the board in the process, especially as the capacitor is right at the edge of the board.

I guess I must have disturbed something when I opened the case but I can't see any dry joints etc.

Can anybody help ?

Does anybody know of someone in the UK who can fix HP 41s ? - I am aware of FixThatCalc in USA but don't really want to ship it out there.
MikeHshm
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Re: HP 41CV Repair

Postby lunasix » Sat May 09, 2009 10:29 pm

Hi Mike,

could it be that the lower display clamp spring (a profiled strip made from thin steel sheet with black finish, running all the way across the edge of the display P.C. board) is contacting circuit traces on the top edge of the keyboard P.C.? I realized that this spring is very (VERY!) close to the gold plated traces on the underside of the P.C. There it eventually shorts out signal traces or even the supply traces; the result can be a temporary short of the battery, leaving the battery drained down, and producing strange effects & memory losses.

My HP-41CV once went completely crazy with hieroglyphic characters and producing wheezing sounds out of a sudden, and it turned out to suffer exactly from such short-circuit. A piece of insulating tape on the display clamp perfectly recovered my companion.

Inserting the tape where it's needed is not quite straightforward because the culprit lies underneath the top edge of the keyboard P.C., access is barred by the display's leadwires.

Method 1 (best but labor intensive): remove the keyboard P.C. together with the display, allows to clean the keyboard at the same time. Keyboard P.C. can be remounted with solvent based contact glue once the plastic rivets are clipped off; pressure from the silicone strips helps keeping the board in place. Make sure the glue is only on the rivet positions and does not contaminate the contact strip areas.

Method 2 (easier but beware of static electricity): desolder the leadwires to the display P.C. board, take out the display and insulate the spring clamp.

Method 3 (quick & dirty): insert some plastic pieces (thin wire insulation, e.g.) to force the display P.C. board away from the circuit traces on the keyboard P.C.

For restarting the fixed calculator, short first the supply contacts to let the unit start from scratch in MEMORY LOST condition. I used method 1 and my HP-41CV is now like new. Hope this helps.

Luc

P.S. everything in technology has its definite cause, even in HP41s :-)
lunasix
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Re: HP 41CV Repair

Postby MikeHshm » Thu May 28, 2009 2:15 pm

Luc,

Thanks for the posting.

I have just had a look at my 41 with a magnifying glass and it looks as if there is a factory fitted strip of clear plastic between the black clip and the display circuit board.

Did your 41 have this and it was still shorting out or is this a later modification to the design ?
MikeHshm
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Posts: 2
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Re: HP 41CV Repair

Postby lunasix » Wed Jun 03, 2009 8:46 am

Hi Mike,

Yes there are in fact clear plastic strips clamped together with the display board. Actually what is clamped there is a sandwich consisting of the display board, the display glass, and two interconnecting zebra strips: NEVER NEVER try to disassemble this sandwich!! The display sandwich forms a compact unit and should stay intact forever - and it cannot be the cause of eaten batteries!

What I am referring to is a short of the lower of the two clips to the KEYBOARD P.C. assembly (the keyboard P.C. measures some 6cm by 10cm and is the one that is riveted to the housing) - so the short occurs on the KEYBOARD P.C., not on the display.

Interestingly, HP DID fit an insulating plastic piece (its a black angled profile) over the display - but on the UPPER of the two clips. Its nice there but
I'd have expected it rather on the LOWER clip because the risk of short arises down there. I found it on both my HP41CV and a HP41C which I own too. In both units, I found the black plastic on the UPPER clip and I never figured out why it was there - maybe for mechanical or cosmetic purposes.

Anyways, look out for proper clearance BETWEEN the clip at the lower edge of the display sandwich, and the KEYBOARD P.C., and insulate if needed. Both of my HP41s went through the 'Method 1' procedure, and both are perfectly recovered, no sticking keys anymore, no strange characters, no MEMORY LOST, no eaten batteries - just like new.

good luck!
lunasix
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