So finally this is fixed.
I rebuilt the X deflection according to the schematic for a G05-801 and replaced Q704/Q705 with MJE15031 and Q706 with MJE15030.
At this point the R720 stopped trying to burn and this is what I got on screen:
No, the camera is not out of focus, you can see my t-shirt is in focus in the background, the best description I can give is it looking like someone scrubbing a pencil on the screen while drawing.
The Z Blanking is obviously working properly.
I changed the transistors and it still happened, double checked i'd crossed the legs over properly etc.
Then I decided to look at another deflection board.
This is the Q704/705 section in question:
The original parts on here are MPSU55 and MPSU56 which have different characteristics to MPSU57 and MPSU07.
Yet, the schematic clearly defined Q704 / Q705 as MPSU57 so i'd subbed in according to various conversations we've had about this.
I checked another couple of known working G05-801's and sure enough they had MPSU55 and 56 on in those places.
I looked at the spec sheet for those and found the hFE was different to MJE15031 which I wondered if that was the issue.
Anyway, I messaged Colin and he gave me the electronic version of a punch in the arm to remind me about BD237/BD238's.
I looked at the specs of BD237/238 and they look the same to me as their MJE...30/31 counterparts.
But, I put 2 x BD238 in Q704 and Q705 with the legs crossed over appropriately and...
No vector scrubbing.
I am left back with one issue which is some jagged tops to some of the vectors and the occasional problem with the brightness pot but I can come back to those later.
So I can confirm that BD237/BD238 work in G05's in place of the schematic mentioned MPSU07/MPSU57.
The only thing I can think of is that the deflection boards I looked at all had 1980 on them, the manuals only go up as far as 1979 - did Atari / Electrohome make some change due to parts supply which involved changing another passive part that biases the transistor differently.
