Colordreams Attempt #2
Rev C board
How it works:
Attempt #2. Yeesh. This thing is a mess! They really went Rube Goldberg on it. I think
some old guy just threw a bunch of parts from old radios together or something when he
designed it. There appears to be more bogus connections (the resistor between the two
data lines for example) on this one. Software must operate this lockout defeat during
startup for it to work properly.
This board uses an ICL 7660 to generate -5V which is then fed to the lockout pins on the board
through some weird-ass circuitry. The circuit connected to D3 is pretty basic, when D3 goes
high, pin 35 gets a -0.6V jolt. The diode clamps the -5V (through the transistor- no resistor!)
The other half, however, is truly bizarre. The first two transistors are pretty normal-
the collector of the second transistor will go to -5V if D2 is pulled high via the mapper,
but then it gets weird. The base of the next transistor is biased to -.3V via the germanium
diode (these have a voltage drop of around .3V, as opposed to .6V on silicon). Why this was
done is anyone's guess. As far as I can tell, shorting the diode out would do the same thing.
There are two 1K resistors in series, because they are in a resistor network.
Long story short- Pulling pin D2 low turns the -5V on to pin 70. I guess the weird shit in
the circuit is to thwart reverse engineering.