128 YOUR CARTRIDGE

Stolen From RVUG, New Jersey, Who Got It Off Q-Link

Edited by Bob Nunn, For MCUC Magazine

Many of you that have upgraded to a 128 still would like to use cartridges to
perform function while in 64 mode, but hate to wear out the cartridge port
plugging and unplugging the cartridge to go to 128 mode.  Here is a simple
modification to the cartridge that allows it to be switched out.

This modification requires removing a portion of the printed circuit board.
Those uncomfortable modifying their equipment should leave this to
professionals. We make no claims about how safe this modification is. You
break it, you own it.

The only hardware needed for this conversion is a SPST toggle switch and a
couple of inches of fine hook up wire available almost everywhere. The switch
must be small enough to mount in the lower half of the cartridge, so buy a
tiny one.

1. With a sharp knife, make a cut through the paper label on the closed end of
the cartridge. (I suggest a hairdryer to heat up the label and then carefully
peel it off. You can paste it back on this way.)

2. Remove the phillips head screw that holds the case together.

3. Carefully pry the top and bottom halves of the case apart, working evenly
around the three edges.

4. Lift the top half of the case off and set aside.

5. Remove the PC board and set it aside, reserving one pint of stock for
later. (Maybe this is the wrong recipe?)

6. With a small bit, or pointed implement, (like a knife) cut a small hole
(for the switch) in the right side of the bottom half of the case. Make sure
you are as near the rear of the case as the switch will allow.

7. Solder the fine hookup wire to the switch and mount it in the hole in the
case with the toggle action from front to rear.

8. Place the PC board back in the case just like it was before you messed with
it. (Discard any leftover stock.)

9. Count in from left to right nine edge connectors. This is the EXROM line,
and the one to be modified.  The trace from this connector should run
diagonally left, then straight back to the first pin on the right side of the
smaller of the two IC's. (You need to make durn sure you are working on the
EXROM trace, different cartridges will vary!) Page 351 of the 128 user manual
has a diagram of the cartridge port and clearly identifies the EXROM line.

10. Cut and remove a hairline section of trace, using a sharp knife.

11. Solder the wires from the switch to this trace, one per side it doesn't
matter which. You should remove the protective coating from the surface before
attempting to solder. (I suggest you tin the wires and then tin the cleaned
surface, apply minimal heat and solder the wires.)

12. Lay the wires so that they will not interfere with the case parts, or the
screws.

13. Carefully, press the two case halves back together and install the holding
screw. 

14. Thats it! Replace the label, insert the cartridge and then test it out.

One more observation.. if you have several cartridges that you use frequently
on your system you may want to consider adding the switch to the 128 instead.
Suitable locations for mounting the switch are available. Follow the same
directions in terms of cutting the trace except this time it will be inside
the 128.  The same instructions for soldering hold true here.

Caution should be used making any hardware modifications. If you are in doubt
as to which trace is the EXROM line, I recommend that you not perform this
modification. In any case I haven't tested this personally, but feel that the
information would be helpful to those who wish to experiment and improve their
systems. The advice is sound as far as it goes with my limited electronic
knowledge taken into consideration.
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