Chroma Clock Generator (Neo Geo AES)
The chroma clock is a self-oscillating circuit using a Hex Inverter 74HC04.
The clock circuit has the load capacitors, a trim capacitor for adjustment of the load, the load resistor, and the Hex Inverter which provides the voltage needed to drive the crystal.
The Hex Inverter pinout is as follows.
Confirm pin 14 has 5V, and ground has continuity to the power ground.
If that is ok, anything passed into the inputs should come out inverted on the outputs.
Measuring the first clock input on pin 13 shows the following:
Then the output (pin 12), and the next two stages, pins 11 and 10 all show the same thing (a slightly attenuated clock signal).
This is then fed into the CXA Video Encoder chip on pin 6 as the XIN through a few components. This output signal is labelled DIVI in the schematics.
Swap out the crystal or passive components with known good ones if you are still having issues.
A simple check if the circuit overall, is to measure the resistance over the 1M resistor. In circuit it should measure around 730k.
If the resistance is different it is often a clue one of the components is bad.
A faulty hex inverter resulted in this value measuring 330k to ground on the input (pin 13) when in circuit.
If the hex inverter is good on the extra unused circuits (input 0V output 5V or vice versa) it is not 100% guarantee the circuits on pins 13/12 and 11/10 are good as often the unused circuits remain ok.
The trim capacitor (orange) lowers the amplitude of the crystal. If you suspect it is bad, or are just diagnosing, you can remove it completely and you will simply get the highest amplitude sine wave.