Just had a thought, can my homemade crt to amp cabling be too long and cause a voltage drop? Reason I ask is that I had no idea what length of wiring was required and so purchased about 3m of wires of which I have left intact until I was sure of things working. So my interface cable is at least over 2m long

Update I measured 3.4vdc from the chassis where the orange wire/sync connects to