Blanknut display problem

[David Ramsey]

I recently acquired a minty blanknut 41; it's in perfect condition except that a couple of the display digits have segments that don't work (an "E" in that position will display as an "F", for example).

Sometimes you can change the position of the problematic digit by flexing the calculator, so it's not an LCD problem.

Online and here, I've seen references to the pressure connector inside the calc that connects the CPU board to the rest of the calculator. Cleaning these contacts, or even replacing them with soldered wire, fixes a host of problems, but I don't know if any are related to the display.

Suggestions?

[Diego Diaz]

Hi David,

Acording to the described sympthom, your 41 has more likely developed the "Display Driver to LCD polymer strip stress syndrome".

In short, there are a couple of conductive polymer strips that connect the LCD itself with the Display Driver board. The whole thing is tightly held together by a couple of U shaped steel frames that ensure appropriate contact pressure.

However, moist, corrosion or aging can lead this contacts to get dirty, worn or loosen; causing the display to behaves like yours: missing segments, irregular opacity or shades.

It can be repaired, but removing, dissasembling and (even more) re-assembling the display is not a trivial task, and chances are that the LCD get broken if it's not handled with extreme care.

Obviously, replacing the whole display is a better option in case you get a suitable spare part

Hope this helps.

Diego.

Edited: 30 Aug 2008, 3:08 p.m.

[Randy Sloyer]

Assuming the unit is fullnut - if not, it's game over, there is nothing you can do for this on a halfnut.

It is one of the hardest problems to troubleshoot... as taking displays and driver modules apart and putting them back together is risky at best. You're better off just replacing the LCD and its driver as a single part from a working unit.

That soldered wire article has nothing to do with display problems and has, IMO, destroyed far more 41's than it has fixed. Once you go down that road, it's a one way trip to the trash can.

[David Ramsey]

It is a fullnut, and I'm pretty sure I have another one around I can use for parts. I assume display replacement will be obvious when I've opened the unit up...?

[Randy Sloyer]

Yes, obvious. Sixteen connections will need to be unsoldered, a desoldering station is best, fine desoldering braid second best. Avoid spring loaded desoldering pumps, the traces and tabs are too fragile for those tools.

PS: Don't forget the "display shield". It's that drop-in little plastic strip along the top of the display. It helps prevent premature fracturing of the connections.

[Diego Diaz]

Yep, I have assumed that the unit is a fullnut, otherwise... well, you've said it better than me :-)

I also adhere to the comments on the "wiring CPU board fix". Not the way I like the things done.

David, back on the Fullnut display replacement, please be aware that earlier versions does mount a different (not compatible) part; so pay attention to see if the serial numbers of both machines are close enough. Also check that they're physically similar.

Good luck.

Diego.

[Randy Sloyer]

Diego,

Good point on the early version... as there are at least three different versions of the driver module.

Very early 41C's modules will not be compatible. These would only be in 19xx date code tall key units and have more than 16 connection points.

There was also an external difference in the modules. Original driver modules used a .01uf cap in the display timing oscillator which is located on the logic board. Later rev F display drivers used a 470pf cap. Having the wrong cap will give display problems, usually the display being blank until you press a key.

[David Ramsey]

The seller's offered me a choice of a refund or price adjustment. Do you guys think this is a reasonable repair risk (it may exceed my limited soldering skills; I have replaced 5-digit display modules in the first generation calcs a couple of times, but then I could use a solder sucker...)? If so, anyone want to take a crack at it?