Your MGA uses an RCA bonded yoke CRT. Yoke is glued on
& the neck magnets are a piece of grey tape. Do NOT remove the tape !
Three possable problems with purity.
1) CRT is heavily magnetized & the DGS circuit cant handle it.
It will improve with time ( cold restarts).
2) There is a cold joint in the DGS or a bad DGS thermistor.
& the DGS circuit is not working right. Thermistor usually abt a
1/2" black square part made by TDK. Either 2 or 3 legs.
3) The set fell & the shadow mask broke loose. Nothing you can do
about that. Usually if you hit the set hard the purity problem will shift.
Beyond that the RCA CRT was a common jug. Many Japan & US
set used them & you can swap one out in minutes.
Buzz most likely from the yoke vert winding. NAP sets were a big problem with it. We sprayed it with something it a rattle can but I dont remember what ! Someone here will.
To track down buzzes & screams use a paper towel tube as a stethiscope or a plastic rod to poke at things.
good luck
73 Zeno
LFOD !