Sony BVM20F1U ABL Circuit activating OVERLOAD alert.

[Mantrox]

Hey folks, complete novice here.
I'm trying to pinpoint a fault on this quite complex CRT monitor.

The unit, when you power it on, it light's up the OVERLOAD alert and doesn't turn on the tube.

In the service manual there's a description of the ABL (Automatic Brightness Limiter) circuit, and it seems that it is responsible for turning on the OVERLOAD alert when there's a specific problem.
Here's some snippets from the manual:





Can anybody help me in interpreting what this circuit is doing, and what steps could i take in order to diagnose the problem?


Some additional information:

Image of the block diagram of the whole monitor.

The service manual
https://cdn.retrorgb.com/assets/monitormanual/Sony_BVM_E_F_Service_Manual.pdf

There are 4 main boards that make up the overall monitor system:
BK - RGBS Video Input
BC - ISR (CPU\ROMS)
E - Deflection
G - Power Supply

There's also several smaller boards, including the ones mentioned on the diagram:
PC - Flyback Transformer Board
PA - I don't know what this one does exactly

I already have switched the G and E boards with other working ones with no effect.
I also recapped the PA board, since it seemed to be directly involved with the ABL circuit, also with no effect.

Thank you for your time.

[Mantrox]

No hope?