None physically look bad. Next step would be to remove the timing board and then test each cap. But as much as I'd hate to give up on this, there is only so much reward for the effort employed.
These Goldstar model is supposed to be reliable. I have my own scope and multimeters, and switch cleaner. Yes, it has the usual stacked wafer switch for TB setting.
I get a stable dot that doesn't wander when I have it set to X-Y mode and no inputs. I'm pretty sure the problem is in the sweep generator and not the horizontal amp.
I'd bet on it being dirty/oxidised switch contacts or a dirty pot contact. Or some other iffy contact. Other factors are possible but a lot less likely.
NT
I have the original printed manual which has the schematics. VR601 sweeper shows a voltage that varies in amplitude in harmony with the TB frequency drifting.
Can you work back to the source of this voltage and when it starts drifting? You seem very close to a cure.
My suggestion would be to check that pot and any trimpots there might be inside for signs of damage or erratic behaviour. It may work OK when at the lesser used end of its track and be worn or damaged at the other.
If all else fails use it in X-Y mode and build a new timebase sweep.
HV stability? It may just be a high resistance HV connection near to the front of the tube. All scopes I've worked on have a square test wave output. Connect this to the input, select the appropriate timebase and vertical scaling. Does the vertical scaling also change in the same way as the timebase variation. If so suspect the HV supply. However if the HV is varying I would expect the trace brightness to vary as well, possibly more noticeable if the brightness is turned down.
Warning: bleed resistors across a scopes HV supply have a habit of going open circuit so just make sure that your hand is not going to hit anything when you remove it at speed after getting the shock :)
Once whilst working on an old valve based Tektronic scope I rested my arm across a valve base. I ended up with two cauterised holes as the voltage on one pin discharged to another through my flesh.
Vertical scaling and brightness are stable.
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