HP27: lowest keyboard horizontal row not working

[isanchez]

Hi everybody,

I have an HP27 (Woodstock) in working condition.Yesterday I noticed that the “2” key did not work as it should, so I decided to clean the keyboard. I disassembled the calculator very carefully. I have done this hundreds of times.

After cleaning the keyboard and reassembled the calculator , all the lowest horizontal line of keys did not work at all, these are: “/”, “0”, “·” “SUM+”
(by the way, the “2” works now fine ;-D)

I tried again a coupe or three more times: disassembling, cleaning, assembling.

This morning I have tested the HP27 board in an HP25 and works fine.

I swapped the HP27 keyboard in with the HP25 calculator and still the lowest line of keys did not work at all, so it seems clear it is a problem with the keyboard.

Do you have any idea what could be this malfunction caused by? The continuity is checked in al (vertical)l lines, but the horizontal line sounds weird for me.


Thanks in advance,

Kind regards

Ignacio

[teenix]

(05-28-2019 11:32 AM)isanchez Wrote:  Hi everybody,


I swapped the HP27 keyboard in with the HP25 calculator and still the lowest line of keys did not work at all, so it seems clear it is a problem with the keyboard.

Do you have any idea what could be this malfunction caused by? The continuity is checked in al (vertical)l lines, but the horizontal line sounds weird for me.


Thanks in advance,

Kind regards

Ignacio

It sounds like the keyboard [row 7] is faulty. The pin for this row is on the lower keyboard connector, not sure if it is last on left side, or right side.

With the keyboard removed, you should be able to put an ohm meter on this pin and any column pin and see a short when the corresponding column key is pressed.

If this works, then it is possible that multiple repairs/cleanings have caused a problem with the connector on the bottom of the board or the matching CPU board connector.

If testing fails, closer examination of the key circuit board may be required.

cheers

Tony

[isanchez]

thanks!

Check done! I have checked the continuity of the keys (pressed) and the upper contacts (9), not the bottom.

considering the upper contacts (C) from left to right, from 1 to 9
Considering the keys rows (R) form upper to bottom, from 1 to 7
R1-C5
R2-C7
R3-C8
R4-C6
R5-C4
R6-C3
R7 (problematic row)-No contact (should be the C2, I suppose)

Is there any feasible repair?
Thanks
Regards

[teenix]

(05-28-2019 01:54 PM)isanchez Wrote:  thanks!

Check done! I have checked the continuity of the keys (pressed) and the upper contacts (9), not the bottom.

considering the upper contacts (C) from left to right, from 1 to 9
Considering the keys rows (R) form upper to bottom, from 1 to 7
R1-C5
R2-C7
R3-C8
R4-C6
R5-C4
R6-C3
R7 (problematic row)-No contact (should be the C2, I suppose)

Is there any feasible repair?
Thanks
Regards

I would look closely at the key circuit board and check for corrosion - aqua green powdery looking stuff on the circuit board tracks. It's possible that a track has been eaten away. Possibly hard to find, but also look for cracks, especially around the pin areas that connect to the cpu board.

If this fails, you may need to open the keyboard which I believe is difficult to re-assemble. There are old posts available that describe this in more detail.

cheers

Tony

[isanchez]

Thanks Tony.

In the visible side it is all fine.
I suppose that it is in the inner side where the problem is located.

Kind regards

Ignacio