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_The Doctor__
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In Topic: 130 XE keyboard membrains
Mon Apr 2, 2012 1:07 PM
The type of keyboard used is known as a "Low-resistance contact", the
resistance being about 1000 ohms or so. As you use the keyboard, the
resistance of the contacts tend to go up. For the regular keyboard and
the RESET key, this increase in resistance causes no problems. But the
console keys (OPTION, SELECT, and START) are read by a different IC, and
the change in resistance will eventually keep the console keys from
working. (The HELP key is actually read as just another letter key).
The fix to the problem is to add just enough resistance in parallel to
the key so that it is high enough not to make the computer read the key
as pressed, but low enough that when the console key is pressed, the
computer will recognize it.
The original idea for this fix came from Alan Haskell from the book "Mods,
Fixes, and Upgrades" available from Best Electronics, 2021 The lameda,
Suite 290, San Jose, Ca.5126.
One minor problem with the fix, however - it wouldn't work on the 130XE
that was given to me to repair. After several hours of pulling out my
hair over this thing, (and anyone who has seen my balding head KNOWS I
can't afford to do too much of that!) I determined the problem.
The resistor value given - 3000 ohms - was too low, for this machine at
least. This value was just slightly above what the computer registered
as a key pressed. Any random electrical noise would cause the computer
to read the key as pressed, which would cause problems with the BBS
program that was being used. A higher resistor value was needed.
Theres no "correct" resistor value to use, as it varies between different
130XEs. You may need to do some testing (as I did) to make sure it works
properly.
What You Need:
Soldering Iron and Solder
Wire Clippers
(3) 4700 Ohm Resistors, 1/4 watt
A small Phillips screwdriver
Needle-Nosed Pliers
How to do it:
1) Unplug all of the wires from the computer.
2) Turn the computer over and remove the four screws that hold the top
cover on.
3) Turn the computer back over and THEN take off the top cover.
4) Lift the keyboard up and forward and you should see the ribbon
connector at the lower right corner. Gently remove the ribbon from
the connector.
5) Remove the screws that hold the motherboard to the lower half of the
case. Lift the front part of the motherboard up and then forward to
remove it from the case.
6) Straighten the tabs that hold the top and bottom shields on and remove
the shields.
7) Turn the board over with the keyboard connector facing to the front.
The connector pins are numbered from right to left. Pin #3 is the
ground connection, and Pins #21, 22, +23 are the pins for START,
SELECT, and OPTION keys, respectively. These are the connections you
need to make for the repair.
Take the three resistors and solder the wire from one end of one
resistor and solder it to the second resistor, at the spot where the
wire comes out from the connector, being sure to cover them with a
short piece of insulation as well.
9) Check your wiring to be sure that there are no shorts!
10)Use as little solder as possible, and make the connection as fast as
you can, using as little heat as possible. Place a short piece of
electrical tape under on the board under the resistors, if needed,
and press the resistors close to the board.
11)Reassemble the shields and check to see that the resistors are not
shorting against the lower shield.
12)Reattach the keyboard to the motherboard, taking care not to bend the
ribbon - it WILL crack. It helps to insert one edge first, then
carefully work the other edge into the connector.
13)To test the repair, power up the computer and in BASIC type:
10 PRINT PEEK(53279):GOTO 10
and type RUN. You should see a vertical row of 7's. Pressing OPTION will
give you 3's. SELECT will give you 5's and START will give you 6's. The
value should not change while any one key is held down. This should
return the normal function of the console keys.
Special Note for Techs:
You can use the following method to determine the exact resistor value
that you need. It might save you time and aggrivation:
What you need (in addition):
Multitester (digital best)
10K Multiturn Potentiometer
Some short pieces of thin wire
This should be done between steps #4 and #5 of the above procedure:
A) Connect one short piece of wire to the center pin of the pot, the
other to one of the other pins.
B) Solder the free end of one wire to the ground pin (Pin 3). These
connections will only be temporary. Solder the other free end to one
of the console key Pins (21,22, or 23).
C) Adjust the pot for maximum resistance.
D) Reconnect the power and monitor. Reconnect the keyboard. Turn on
the computer with the option key pressed - you should get the
diagnostic screen. Select the KEYBOARD TEST and hit START.
E) Adjust the pot until the tone just starts to sound intermittently.
Measure the resistance by connecting the probes to the center pin and
the unused pin on the pot. Subtract the measured value from the rated
value of the pot to get the proper value. Record it.
F) Adjust it again until the tone sounds continuously. Record the value
the same way as in step E.
G) Turn the computer off, and disconnect the cables and the keyboard.
Unsolder the wires from the keyboard connector.
H) The proper resistor value to use will be the closest value that is
both HIGHER than the highest value recorded, but around DOUBLE the
lower value. The resistors you will use will probably be between 3000
and 5000 ohms. Continue on to step #5 as above.
I dug this up out of an old BBS..... and it has solved all but one out 20+ supposedly broken trace mylars I have been confronted with... if you do cause a crack it is normally on the short strip from the keyboard to the main board I just paint it and put it in when the traced parts are still damp... if you make the the trace to thick or put it in while completely dried when it bends it will crack again ... less is more!
resistance being about 1000 ohms or so. As you use the keyboard, the
resistance of the contacts tend to go up. For the regular keyboard and
the RESET key, this increase in resistance causes no problems. But the
console keys (OPTION, SELECT, and START) are read by a different IC, and
the change in resistance will eventually keep the console keys from
working. (The HELP key is actually read as just another letter key).
The fix to the problem is to add just enough resistance in parallel to
the key so that it is high enough not to make the computer read the key
as pressed, but low enough that when the console key is pressed, the
computer will recognize it.
The original idea for this fix came from Alan Haskell from the book "Mods,
Fixes, and Upgrades" available from Best Electronics, 2021 The lameda,
Suite 290, San Jose, Ca.5126.
One minor problem with the fix, however - it wouldn't work on the 130XE
that was given to me to repair. After several hours of pulling out my
hair over this thing, (and anyone who has seen my balding head KNOWS I
can't afford to do too much of that!) I determined the problem.
The resistor value given - 3000 ohms - was too low, for this machine at
least. This value was just slightly above what the computer registered
as a key pressed. Any random electrical noise would cause the computer
to read the key as pressed, which would cause problems with the BBS
program that was being used. A higher resistor value was needed.
Theres no "correct" resistor value to use, as it varies between different
130XEs. You may need to do some testing (as I did) to make sure it works
properly.
What You Need:
Soldering Iron and Solder
Wire Clippers
(3) 4700 Ohm Resistors, 1/4 watt
A small Phillips screwdriver
Needle-Nosed Pliers
How to do it:
1) Unplug all of the wires from the computer.
2) Turn the computer over and remove the four screws that hold the top
cover on.
3) Turn the computer back over and THEN take off the top cover.
4) Lift the keyboard up and forward and you should see the ribbon
connector at the lower right corner. Gently remove the ribbon from
the connector.
5) Remove the screws that hold the motherboard to the lower half of the
case. Lift the front part of the motherboard up and then forward to
remove it from the case.
6) Straighten the tabs that hold the top and bottom shields on and remove
the shields.
7) Turn the board over with the keyboard connector facing to the front.
The connector pins are numbered from right to left. Pin #3 is the
ground connection, and Pins #21, 22, +23 are the pins for START,
SELECT, and OPTION keys, respectively. These are the connections you
need to make for the repair.
Take the three resistors and solder the wire from one end of oneresistor and solder it to the second resistor, at the spot where the
wire comes out from the connector, being sure to cover them with a
short piece of insulation as well.
9) Check your wiring to be sure that there are no shorts!
10)Use as little solder as possible, and make the connection as fast as
you can, using as little heat as possible. Place a short piece of
electrical tape under on the board under the resistors, if needed,
and press the resistors close to the board.
11)Reassemble the shields and check to see that the resistors are not
shorting against the lower shield.
12)Reattach the keyboard to the motherboard, taking care not to bend the
ribbon - it WILL crack. It helps to insert one edge first, then
carefully work the other edge into the connector.
13)To test the repair, power up the computer and in BASIC type:
10 PRINT PEEK(53279):GOTO 10
and type RUN. You should see a vertical row of 7's. Pressing OPTION will
give you 3's. SELECT will give you 5's and START will give you 6's. The
value should not change while any one key is held down. This should
return the normal function of the console keys.
Special Note for Techs:
You can use the following method to determine the exact resistor value
that you need. It might save you time and aggrivation:
What you need (in addition):
Multitester (digital best)
10K Multiturn Potentiometer
Some short pieces of thin wire
This should be done between steps #4 and #5 of the above procedure:
A) Connect one short piece of wire to the center pin of the pot, the
other to one of the other pins.
B) Solder the free end of one wire to the ground pin (Pin 3). These
connections will only be temporary. Solder the other free end to one
of the console key Pins (21,22, or 23).
C) Adjust the pot for maximum resistance.
D) Reconnect the power and monitor. Reconnect the keyboard. Turn on
the computer with the option key pressed - you should get the
diagnostic screen. Select the KEYBOARD TEST and hit START.
E) Adjust the pot until the tone just starts to sound intermittently.
Measure the resistance by connecting the probes to the center pin and
the unused pin on the pot. Subtract the measured value from the rated
value of the pot to get the proper value. Record it.
F) Adjust it again until the tone sounds continuously. Record the value
the same way as in step E.
G) Turn the computer off, and disconnect the cables and the keyboard.
Unsolder the wires from the keyboard connector.
H) The proper resistor value to use will be the closest value that is
both HIGHER than the highest value recorded, but around DOUBLE the
lower value. The resistors you will use will probably be between 3000
and 5000 ohms. Continue on to step #5 as above.
I dug this up out of an old BBS..... and it has solved all but one out 20+ supposedly broken trace mylars I have been confronted with... if you do cause a crack it is normally on the short strip from the keyboard to the main board I just paint it and put it in when the traced parts are still damp... if you make the the trace to thick or put it in while completely dried when it bends it will crack again ... less is more!
