HP-41C/LCD Display Problems



#14

Hi All,
I bought an old HP-41C calculator recently from the auction site that was listed as "for parts" since its LCD does not display a few segments. The serial number is 2046A03570. Not 100% positive but I think it is the old "fullnet" kind of HP41. It seems to have boxier keys and the square LCD style.

I bought this because it was cheap and I wanted the Time Module and a couple of other modules that came with it. I don't actually need the calculator. However, other than the few missing LCD segments (just parts of the first couple of numbers) it seems to be in remarkably good shape. All the keys feel good.

Is there any way to repair the LCD? I don't want to put a lot of money into it - so it would have to be a do it yourself repair...

Thanks.


#15

Hi,

I have no information on repairs, but I can tell you that the earlier 41C models had "boxier" more upright keys. This was changed in later 41C's and all CV's and CX's. So you have one of the earlier calcs. I had one of these in college and then traded it in for a CV. Back then I could not afford more than one calculator. Years later I went on a special hunt for that particular model and was lucky enough to get one in excellent working condition.

Good luck getting yours fixed.

Cheers!

#16

Hi,

sometimes it helps checking and resoldering the joints between the LCD tray and the kbd pcb,

but in most cases it's easier and less time consuming just to replace the whole LCD unit.

Special care has to be taken about the solder lands on the LCD tray side, since these are VERY sensible against mechanical and thermal stress,

and tend to lift off, sometimes also pulling the trace to the controller, which renders the LCD to an unrepairable state.


For this reason I strongly suggest to desolder the kbd pcb side only.


Most of the non-halfnut LCD trays are interchangeable, but on the early HP-41C units there's a chance to find a tray which back side looks somewhat different (square controller covers instead of round blobs).

AFAIK those should be replaced by the same type.

HTH

Ray

#17

as a general repair of LCD issues (not HP specific) where a flexible 'printed' cable is bonded to glass or a PCB: if you suspect the cable has lifted from the glass/PCB, try for a strategically places SMALL piece of foam rubber to lightly press the cable in contact when the casing is assembled. the current flow is next to zero, so only a light pressure is necessary to restore function. i've fixed quite a few issues with LCDs in the past using this method - calculators, pagers, etc, and it can last surprisingly well... certainly a number of years.


cheers,
rob :-)


#18

Quote:
as a general repair of LCD issues (not HP specific) where a flexible 'printed' cable is bonded to glass or a PCB: if you suspect the cable has lifted from the glass/PCB, try for a strategically places SMALL piece of foam rubber to lightly press the cable in contact when the casing is assembled. the current flow is next to zero, so only a light pressure is necessary to restore function.

Sometimes it is possible to reseal the conductive cable with
caution. The conductive adhesive used to mechanically and
electrically bond the tape to the lcd glass ITO contacts is
either a thermoset or thermoplastic adhesive impregnated with
microscopic (10~40 micron) metal plated polymer balls. This
anisotropic film laminates the carbon ink cable to the glass
under heat and pressure. If the adhesive gives up the ghost
over time it might be worth a try to reheat the junction
cautiously, say with a thumb tack head heated by a low heat
soldering pencil. Keep moving it around while
applying pressure to reseat the junction. Remove heat while
maintaining pressure and movement. Starting off with a
conservative heat is a good idea as overheating the carbon
ink cable can melt the cable polyester film backer.


#19

Quote:
as a general repair of LCD issues (not HP specific) where a flexible 'printed' cable is bonded to glass or a PCB:
FWIW: Only HP calculators made after 2003 use this technology, beginning with the with the HP10Bii.
#20

Quote:
Is there any way to repair the LCD? I don't want to put a lot of money into it - so it would have to be a do it yourself repair...

Missing segments in a fullnut display is caused by one or more of the following three things:

  1. Defective LCD
  2. Defective Display Driver
  3. Bad interconnects between 1 & 2 (elastomeric zebras)

It is one of the more difficult problems in a 41 to repair and you risk LCD damage if done incorrectly. If cleaning the interconnects does not resolve the problem, you'll need another LCD and driver to troubleshoot.

If the calculator had signs of battery leakage, I would lean towards an interconnect problem on early 41's, especially if it has the round gold connectors. Those typically need to be replaced.

If the interconnects are white or pink, it is most likely a bad driver.


Edited: 19 Nov 2010, 4:50 p.m.


#21

Hi Randy,
I've checked the contacts and they look great.

BTW, I've used your company before for a repair and might do so again for this repair if it is something you could fix. I see from your web site that you are swamped with repairs and not taking any new orders at the moment - so I'll check back in a few weeks/months.

Best Regards,

Kevin

#22

"elastomeric zebras": is there a chance these have shrunk over time? this is the bane of many Fluke multimeters. have you tried GENTLY pressing back on the LCD to try and compress the gap that the zebra strips fill? in the past i've fixed these sorts of problems by shaving down surrounding supports to shrink a gap, as well as 'massaging' the zebra strips to stretch them a fraction in the required direction. on SOME Fluke meters the PCB has a thick solder tinning over all exposed pads, and it is possible to simply build up the pads with more solder.

can someone perchance provide a link to a photo?

cheers,
rob :-)


#23

Not only Fluke multimeters. Years ago the used to plage the HP48SX... mine lost 3 lines that way...


#24

Had nothing to do with the zebra connectors, it was due to a layout problem of the internal LCD traces... by Epson.

The root problem still exists today in all 48S/G series LCD's but the occurrence is extremely low. The condition is when all top row annunciators are on without excitation. Same root failure, difference symptom.


#25

It has to do with the elastomers, if you press it a bit more (LCD against the board) the lines show up again, at least in mine and a few others I saw at the time ('91, '92).


#26

Quote:
if you press it a bit more (LCD against the board) the lines show up again

Yes, that may be true but in the case of an early production 48SX (serial begins with 30) that is missing three vertical columns on the far left side of the screen, the root problem isn't the connectors, it's a design flaw of the LCD.


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