Subject: Adding adjustable composite video output to a TrackStar
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From: Rubywand <rubywand@swbell.net>
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Date: Sat, 27 Feb 1999 04:03:37 -0600
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     When installing a TrackStar E, it was a chap to discover that composite
video from the board would not work with either of our composite monitors-- i.e.
an Amdek Color-1 and a surplus B/W monitor. The monitor displays indicated that
the video level from TrackStar's composite output was probably too high.

     On an Apple II+, this would be no special problem because there is a
mini-pot to adjust the video output level. If either monitor had an adjustment
for video level input, it would not be a problem, either.


     A check of the TrackStar board revealed that composite video comes from the
emitter of a transistor connected to ground via a 75 Ohm resistor. So; the level
is fixed. This output goes through a small RF choke to the RCA socket on the
back of the board.

     The cure is to replace the 75 Ohm resistor with a pot. If you run into a
similar problem using a composite monitor with your TrackStar E, here are
step-by-step directions for swapping in a Video Output adjustment ...


1. On the back of the board, find the trace going to the 75 Ohm resistor and the
choke. (The choke is close to the RCA socket. The 75 Ohm resistor is near the
choke.)  Cut the portion of the trace going to the choke.

2. De-solder and remove the 75 Ohm resistor. (You can snip it off if you like.)

3. On a 200-300 Ohm potentiometer-- I used a, roughly, 250 Ohm mini-pot--
connect three leads. (The length of the leads depends upon whether you have some
place on the back of the PC to mount the pot or just plan to put some tape
around the pot and let it flop around near the board. I don't know how long the
leads can be before noise pickup is a problem. It seems like a good idea to keep
length within about 8" or so.)

Use three different color wires-- say White, Green, Black. Viewing the pot from
the back, connect White to the Left tab, Green to the middle tab, and Black to
the right tab. (Mainly, the Green wire needs to be on the middle tab.)

4. Solder the Green wire to the end of the choke which used to be connected to
the 75 Ohm resistor-- i.e. the end _not_ going to the RCA socket.

Solder the White wire to the place the 75 Ohm resistor used to be connected
which used to connect to the choke before the trace to the choke was cut-- i.e.
this is the point which goes to the emitter of a nearby transistor.

Solder the Black wire to the other place the 75 Ohm resistor used to be
connected. This is the Ground end.

Note: Mixing up the White and Black leads is okay. The only reason for
connecting them a certain way is so that a clockwise turn of the pot = increased
Video Output.

The circuit you end up with looks like this ...


                              |----
 Pot                         /
  _______ White_____________/ emitter of transistor
 Z
 Z
 Z <----- Green -----------UUUUUUUUU-------- to RCA socket center pin
 Z                          Choke             Composite Video Output
 Z_______ Black____
                   |
                  _|_
                 Ground


5. Insert the board. Find some place safe to mount the pot or let it sit so no
leads or metal parts touch any circuit. Connect the monitor and power up the
computer. Start TrackStar and adjust the pot and monitor for a good display.



Rubywand