The Astrocade 24 Button Keypad Matrix

By MCM Design
April 2022

This Doc serves as a dual intent providing info to help an Astrocade user who:

Wants to rewire an upgraded remote 24 key keyboard to be compatible as a substitute for the Astrocade built-in keypad or
Desires to strip down the keypad to repair a keypad matrix issue.

THE KEYPAD MATRIX PHYSICAL LAYOUT

The 10 keypad matrix lines soldered to the motherboard along with the keypad ROW and COLUMN pointers are all labeled on page 1 of MCMs Doc "Astrocade Keypad Repair" posted on the Bally Alley. Here's a link to the Doc.

https://ballyalley.com/faqs/faqs.html#AstrocadeKeypadRepair

All 10 keypad matrix lines soldered to the motherboard are routed to the top of the plastic rectangular keypad support, which slides into the keypad support mounting on top of the Astrocade motherboard.

The 10 lines are laid out and embedded into the plastic support and are parallel to each other. Six copper overlays rest on top of the 10 embedded matrix lines. Each overlay is connected ONLY at one point to a specific matrix ROW line. See the attached 3 photos.

The 6 overlays, from the rear to the front of the keypad support, are electrically connected (soldered) to the specific matrix ROW line as indicated below.

Overlay         Connected To
                         Matrix Line

OVRLY 0         ROW 0
OVRLY 1         ROW 1
OVRLY 2         ROW 2
OVRLY 3         ROW 3
OVRLY 4         ROW 4
OVRLY 5         ROW 5

There are four 9mm in diameter copper ROW contacts on each overlay, which are centered below each of the respective 4 keypad ROW buttons. Each circular ROW contact flexes downward, when the keypad button is pushed, to make contact with its specific matrix COLUMN line.

ELECTRICAL KEYPAD BUTTON MATRIX CONNECTIONS

Below is an overview of each keypad button showing its ROW to COLUMN connection when the button is pushed down.

                    COL 3       COL 2        COL 1       COL 0

ROW 0          C               Up          Down         %

ROW 1         MR            MS            CH            Div

ROW 2          7                8                9               X

ROW 3          4                5                6              --

ROW 4          1                2                3              +

ROW 5         CE              0                 .              =

STRIPPING DOWN THE KEYPAD TO ACCESS ITS MATRIX FOR A REPAIR ISSUE

As indicated in the Doc mentioned above, the keypad can be checked for functionality using the lowest R x 1 range on an ohmmeter.

Attach a short wire with alligator clips to one test probe. Clip this lead to the ROW line number (soldered to the motherboard) you want to test, which will free you up to move the other test probe to a COLUMN line number with one hand and press the respective ROW button with the other hand.

Ideally, you should be reading close to zero ohms when a button is pushed. Aged contacts will result in higher resistance readings. If no resistance (infinity) is indicated, double check your ohmmeter connections. No resistance indicates you might have an issue (break) in the keypad matrix.

It is possible to access the keypad matrix by slowly, gently peeling back the black/aluminum combo layers on top of the matrix. Once you have completely exposed an overlay, tape it down to reduce the risk of damage during your "peel off" attempt. You may likely have to peel off the entire layers, depending on where the keypad functioning issue is located.

Be careful not to bend the circular contacts on the copper overlay or break the tiny, very fragile overlay solder connection to its ROW line.

If you do break a solder connection, it is possible to re-solder the broken connection.

To solder a broken overlay ROW connection:

Scrape off using a small flat edge screwdriver or use acetone to remove any sticky tape adhesive within the soldering area on the top of the overlay.
Scrape to clean the soldering area at the top of the ROW line and the top of the overlay, so the solder will stick easily.
Use a 20 or 25W soldering iron to solder a bead on top of the ROW line.
Tape down the overlay.
Solder the overlay to the ROW line.
Jiggle the overlay to test for a strong solder joint.
Finally, use an ohmmeter to confirm the overlay is now connected to the ROW line with zero ohms.

Once the keypad matrix is exposed, you can use your ohmmeter again to see if you can locate the keypad functional issue.

Check one row at a time. You'll likely have overlays that are bent in various places, no longer lay flat on the plastic support, etc.

Use a little piece of 3/4" wide masking tape just to hold down an overlay's loose end so it looks like the overlay is aligned. Avoid touching the tape's sticky surface with your fingers. Do NOT bend the tape around the plastic's side.

Test all 4 row contacts with an ohmmeter. Your goal is to get all 4 button contacts functioning, hopefully with a resistance of close to 1 ohm or less.

See those 4 circular cutouts in the plastic for each row? You might have to place some precut narrow strips (a few mm wide) of masking tape behind and in front of the circles to keep portions of the metal overlay from making undesirable contact with any of the embedded ROW and COLUMN lines in the plastic (see photo).

You may be able to reduce contact resistance by scraping clean the top of the COLUMN line within the cutout circle. To prevent damaging the overlay, avoid cleaning the bottom surface of the overlay.

When you are satisfied with the 4 row contacts performance, tape down the entire row with masking or electrical tape. The tape should extend beyond the width of the plastic so you can touch the sticky ends with your fingers.

Important!
Score (cut) off each tape end, using a sharp utility knife, at least 1 mm away from the plastic's two sides so the tape won't get "hung up" when you slide the plastic through its 6 mounting supports later on.

Repeat your ohmmeter tests and tape down procedure for the remaining rows.

All 6 overlays should now be covered over with masking or electrical tape. Add a strip of tape along  but 1 mm away from the left and right sides of the plastic.

Center and add a final layer of aluminum or duct tape down the length of the plastic. This final layer will cover much of the masking (or electrical) tape to help hold it in place. You can wrap it around the front end of the plastic support.

Optional:
Check all 24 row contacts with an ohmmeter one last time before you install the keypad onto the motherboard?

INSTALL THE REPAIRED KEYPAD

Now slide the plastic support through the 6 support guides that are part of the cassette connector that is screwed onto the motherboard.

Fasten the keypad assembly/cassette connector combo to the motherboard with 4 screws.

Turn the motherboard over and check that the END of the plastic keypad is butted right up against the 2 keypad end supports. If not, slide the keypad plastic right up against those 2 supports so all the keypad contacts will be centered under their keypad buttons.

End Of Document