I have found the problem.
As mentioned at the beginning of this thread, the scope had been working after the initial repair, and it was only after upgrading the two bridge rectifiers that the problems started.
Following the strange readings last night across CR1721 and its transformer centre tap, I decided to remove the two bridge AC wires and take readings. These were 14¦¸ across the wires, yet 29¦¸ across one of the wires and transformer middle, with 14¦¸ from the other AC wire.
The schematic didn't give any clues....
Fortunately, however, when I bought the scope, I took detailed photos covering every area of each board, detailing all components and connections.
What had happened is that when the bridge rectifiers were renewed, one of the CR1721 AC wires had inadvertently been swapped with one of the neighbouring wires which had originally connected to the centre of C1723, whch then links to the transformer centre tap.
Thank God I took those photos, but I owe you all an apology for leading everyone on a wild goose chase. my only hope now is that there has not been any serious damage done.
A quick check on the rails reads;
+140v ... 201v
+65 ... 94v
+15 ... 0.8v (kinda expected following the exploding part last night)
+5v ... 7.21v
-15v ... -20.1v
-8v ... -10.2v
After everything that has been tested on this scope during this marathon thread, I shall never again feel intimidated by the prospect of deep-diving into a vintage Tek scope.
Nothing compensates for experience and certain things which I had previously studied yet never really grasped their practical application, have now become standard to by knowledge bank.
I would like to sincerely thank everyone who has contributed, with particular thanks to Ozan, Harvey, Jim and Roger, with bonus points going to Harvey for his patience and homework given.
Much respect,
James
James55