I sympathise with you trying to follow the circuit diagram as it is drawn.
The way it is laid out makes it very difficult to see how the control circuit operates.
Some months ago I carried out some repairs to a Topward 6603D power supply, (The 60 volt 3Amp version), which is identical to the 6303D except for the transformer secondary voltage and additional series pass regulation transistors.
I ended up re-drawing some parts of the circuit diagram so that it was easier for me to see how the control circuit operates.
It may take a few days, but I can try to find my notes and hand re-drawn circuits to copy for you if that may help.
Regards.