At the risk of repeating myself:
Terrykc wrote:... It is important to always degauss the set and check/adjust purity before touching any convergence adjustments ...
As far as degaussing is concerned, now that the set's own degaussing circuit is working properly, you can use it for a sanity check.
Check the purity as before, then switch off.
When switching on again from cold, the degaussing circuit should come into action again - you should be a slight mechanical 'brrr' as the ferrous part s of the set, including the shadow mask inside the CRT, vibrate at mains frequency - and, on warm up, the purity errors (if any) should be exactly the same as before. If there is a change, repeat as necessary.
When the original Positor failed, there is a faint chance that the debris created when it disintegrated could have temporarily bridged some of the internal contacts giving a brief pulse of energy to the degaussing coils. The rapid collapse of the magnetic field at the end of the pulse would make the circuit work in reverse, actually magnetising the tube and the shield to a much higher level than the circuit is normally expected to overcome, thus repeating the operation might
be necessary to completely degauss the set.
I know I'm probably going to extremes here but I'm trying to cover every possible situation that might occur.
The fist time you carry out the purity and convergence procedures you can expect to take quite a long time, so it is worthwhile making sure that your efforts won't be in vain - as you would find out the next time you turned on the set!