41C is ON, display is OFF, a key pressed display is ON --Repair Help

[TallKey]

Reviving a HP-41C, it is a very clean early low key Fullnut.

It needed the battery film replaced (replacement is from Diego). The circuit board gold traces look near new, zebra is in great shape, the posts and screw wells are in great shape.

Have used a 8x loupe to check for lint/dust at IO film and zebra strip.

Ohm meter checked some of the zebra to circuit endpoints and they checked 0.2-0.4 OHMs.

Hopeful someone has some good guesses/guidance to help repair this beauty. Fingers crossed it is not "your display driver has failed."

It powers ON and all keyboard keys test good. The key presses work normally, until the key is released and the display blanks. IE press 8 and see 8, release the 8 key and the display blanks. Press 8 multiple times it fills the mantissa, but releasing the 8 key and the display blanks.

More of the same:
CLX ON
MEMORY LOST (displayed briefly), then a blank display.

Press the TAN key, TAN is displayed followed by NULL, release the TAN key and the display blanks. Same behavior for all keyboard keys.

PRGM, ALFA and USER cause the display to display normally, then releasing the key causes the display to blank. ALFA and PRGM modes appear to work normally. The ON key turns it OFF.

All display segments appear to work, did not test the colon ":" key.


Have swapped spare 41c logic boards, same as above.

Thanks!

[Didier Lachieze]

According to this thread, this could be due to a defective display driver:

Quote: From page 7 of the service manual's updated service notes (HP41C-35)

4. Display lit only when a key is pressed (blanked in standby).

a. Replace display driver hybrid. (P/N 00041-60090)

[brouhaha]

(02-13-2023 11:01 PM)TallKey Wrote:  The key presses work normally, until the key is released and the display blanks.

Sounds possible that the DPWO (Display Power On) signal from the display hybrid might either not be generated correctly (bad display chip), or not get to the Nut CPU chip (bad connection, corrosion, cracked trace, etc.).

[mfleming]

One commonly occurring 41 display fault in past threads has been a bad solder joint on the display leads. Continuity checks seem to show all is well, but touching up each joint solves the problem. Worth a try first before replacing the display itself.

[TallKey]

(02-14-2023 03:55 AM)Didier Lachieze Wrote:  According to this thread, this could be due to a defective display driver:

Quote: From page 7 of the service manual's updated service notes (HP41C-35)

4. Display lit only when a key is pressed (blanked in standby).

a. Replace display driver hybrid. (P/N 00041-60090)

Thanks for reminding me of the this part of the service manual.

The service module gave me hope, but reading the service manual 2-57 (p2-12) probably means the module avoids STANDBY mode by keeping the display ON (PWO high).

Standby Test (p4-17). Any change in the display indicates that the display driver, oscillator capacitor (C2), or CPU is bad. Initiate Standby Test, display goes OFF, pressing any key, display ON, then STANDBY OK is displayed. C2 being bad on 2 CPU boards seems unlikely.

Service module installed, all the tests I can perform test OK but for the Standby Test. Standby test is supposed to cause the display to remain ON (it stays on when tested on the CV).

Too tested the contacts "Figure 4-5 keyboard PC board", service manual. Checked all 59 pads, with the zebra connector installed, 0.2 - 0.4 OHMs.

Have not attempted to retouch the solder contacts. They all look good and they feel solid.

The driver is the type with grey epoxy covering the 2 ICs. It is my understanding not any Fullnut display driver will work as a replacement. Is that correct?

See photo to date the display at hand.

I do not want to give up on this and part it out. It is physically in minty condition (the quad memory module needs a home).

If I had a known compatible & working display driver, I'd replace the driver.

   

[TallKey]

Replaced the driver board. Never want to do the it again, but probably will.

My problem is replacing the display clips. The are removed easily enough. Installation, for me, harrowing.

The front and back glass
surfaces of the LCD module each have a
plastic layer that is easily damaged,
and the contact fingers on the display
driver PCA are easily damaged.

The display driver board is pretty darn flimsy, Mr Manual.

Sadly the bad driver board looks just like the good board. Anyone know what causes driver boards to fail?

Thanks all who helped ID the problem and having the HP Service Manual at hand is a big help.

The donor 41C has a few display segment problems. Moved the known bad display driver to the donor 41C. Driver behavior, no change in the donor 41C.