Been a while since ive worked on an SS machine. But... the GI is straight from the transformer yes? Its the 6.5VAC line that goes through TP4 and fuse F5 ( 20amps).
If the potential difference between the two sides of the transformer output is only 3V then I would test both sides of the 6.5VAC line AT the transformer and read the voltage there. If good.. move along to the fuse etc.
If bad then you have either a drop at the high voltage or an increase at the low ( AC ground 0V). If you measure both of these with reference to ground then you may find a reading such as : 9.5V at high and 3V at AC ground.. gives you a potential difference of 6.5VAC.
Often a stray whisper of another cct is touching part of the GI and throwing the PD out.
