I was told to play with the trim knobs which bias the circuit but that didn't seem to have an effect other than varying the tonality of the phasing but not the intensity
It's a good start. (Unfortunately the side-effect is you have to reset the pot.)
The fact it didn't work points to a problem with the Low Frequency Oscillator (LFO).
That's the part of the circuit around IC104, C113, C114 the Width pot, the Speed-pot.
There two common problems:
- If the capacitors are electrolytics, the capacitors C113 and C114 could dried out.
If it's tantalum it's probably not the issue.
- the other is either the Width pot or Speed-pot has become faulty, or a wire has come off.
For this case: set the two pots to maximum and with a wire short points 12 and 8 on the Width pot,
then points 7 and 9 on the Speed pot. If it comes to life you know where the problem is.
Then you can try cleaning the pots.
Make sure that schematic matches your board.
If you have a square switch, which is a two-contact push-button it probably is. I remember there's a few variants of phasers which look quite similar from that era.
[EDIT: I think the issue was the round switch era.]