[Premium-Rx] Cubic R-3030

[Tisha Hayes]

Interesting, I just did a long write-up on this on one of the other lists
(maybe Harris or one of the mil receiver sites in the past day or two) and
you are correct about the debounce.

Back in the 80's I designed this test-set that had a flat front with
membrane keys for data entry and optioning. After a few years they were
coming back with extremely worn out "enter" keys and the trouble ticket was
about multiple key entries when pressing keys. I had to go back and design
a little adder board that went between the display panel and the logic card
that put an RC circuit debounce onto the 4 x 8 scan lines of the keyboard
to logic board interface. After I finished the design one of the service
techs came in and saiid, "well, I have just been changing this 2.2 uFd cap
to a 4.7 uFd cap"... it was the capacitor that made up the RC circuit on
the keyboard decoder circuit. Thank goodness someone was actually looking
at what I had designed and wondered what that capacitor was for, he had the
fortitude to go read the chip data-sheet and saw what it was for.

Either solution would work, his was better for it's elegance. In the future
I added RC integrators and Schotky buffers so I had some EMI isolation and
was always pulling the line to VCC with a resistor so it was not at a high
impedance state on the logic lines.




--------quoted------------
...  There are two issues with the keypads, firstly that the value of the
keybounce suppression cap in the interface module is too low, and secondly
that
the keypad plastic overlays become detached from the keypads and bulge in
the middle because they are held in place by machine cut double sided tape
that  has a relatively small contact area on the thin frames separating the
individual  keys.

Issue one is dealt with by increasing the value of C2 in the Panel
Interface Module from 3.3uF to somewhere closer to 10uF.
This cap goes from pin 7 of U6, the 74C923 Keyboard Encoder to ground, and
I found it easier to leave the existing capacitor in place and fit another
in parallel across the top of the IC and soldered to the actual IC pins. I
used 6.8uF but 4.7 uF would probably be ok too, or perhaps it was the other
way  round it was a few years ago now:-)
----------------------------

-- 
Ms. Tisha Hayes/ AA4HA
- Invisible airwaves crackle with life, bright antennae bristle with the
energy.
Emotional feedback on a timeless wavelength.
Bearing a gift beyond price, almost free.
Rush, 2112