Looking for HP41CX Advice

[GreyUser]

The HP41 display drivers, lcd’s and their interconnects went through many design iterations over the years of the product lifecycle.

The original display driver chips (only on the early tall key model) were encapsulated with a black, molded square cap epoxied into place. These modules require a different timing cap on the motherboard. The HP circuit identifier is C2 and the original tall key driver needs a 0.01 uf cap. Later versions are 470 pf (as used on on the CL logic board). This change is documented in the combined 41C/CV/CX service manual.

In a normal 41, the indicator this cap is the wrong value for the driver is that there will only be visible characters when you hold down any key. Release the key and lcd will go blank. I don’t know what the behavior would be when a 41CL board is installed.

Later boards were encapsulated with a black epoxy blob on p/n 00041-80003. This was followed by part number 00041-60146 which were encapsulated with a grey epoxy blob. Note that these two boards are of different physical sizes with the later -60146 part being wider. This can cause problems when attempting to put an older driver board together with a newer lcd.

The original tall key lcd’s had a greenish tint to the polarizer with not so great contrast. The later lcd’s have a neutral, clear polarizer which IMO provides much better contrast. I don’t know if there are any speed differences in the drivers, given the hardware differences, I guess this could be confirmed by looking at the lcd drive signals. I would agree that the older lcd’s do appear to have a slower visual response, it may be the lcd, the driver or a combination of both.

If I had a choice of which to turn into a CL, a tall key unit or a CX, it would be hands down the CX. With the tall key variant, you’ve got the lcd issues, the typical poor tactile feedback of the 1 and 3 keys, a case bottom that was designed for motherboard attachment with nuts and perhaps the worst feature, motherboard interconnects via a silicone elastomer that cannot handle the supply current. Hopefully that was fixed by HP service at some point with the later C/CV gold interconnect.

The original tall key 41C’s were quite the homing pigeons and most made the trip back Corvallis more than once. It is exceptionally rare to find one in the wild with all original parts.