Colordreams Attempt #2

Rev C board

Circuit Diagram

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.

Circuit Operation:

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!) to -.6V.

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.

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