Analog scope won't auto trigger anymore after sitting unused for a few month

[Noxy]

As a novice, I've got myself an old analog oscope to experiment (basically just to fry that instead of the digital one I already own ).
It's a cheap entry level advanced/gould OS1000A which was working just fine when I first got it but today, after sitting unused for a few months, the AUTO trigger doesn't work anymore.
In details, when the trig level is adjusted within the range of the input signal it trigger just fine, otherwise the screen goes blank as it would in normal mode.
Instead, if I set the trig knob to free run, it free run as usual.

Repairing an oscilloscope is definitely out of range for my level but this one is a relatively simple model and also the issue doesn't seem too complex.

How is the free run behaviour obtained in an analog oscilloscope?
Of course the beam is swept across the screen at a certain frequency but where is such signal taken from? Is it usually generated by dedicated circuitry on the trigger board or it's derived from other sources?
Either ways, since I can get the oscilloscope to trigger and free run by working on the controls, is safe to assume that both circuits are working fine, isn't it?
Perhaps the issue isn't far from the trigger knob.
What do you guys think ?

[CaptDon]

Sometimes there is a free running oscillator that is gated on or off
by the trigger circuits. If no trigger is sensed the free running oscillator
is allowed to start running and it will trigger the sweep. It is injected
after the trigger sensing circuits so that the scope won't think it has
a 'valid' trigger. Some scopes have a sort of slide bias on the sweep
oscillator, If no valid trigger is detected the 'triggered sweep oscillator'
will begin to have its stability slide toward 'free run'. When a valid
trigger becomes present the bias will shift back to 'gate on trigger
only'. The time constant of the slide bias is usually about 1 second
which is why you have to select 'normal mode' if you are viewing a
trigger source which only occurs  at two second intervals or longer.
The scope would revert to 'auto' and start sweeping and not be ready
for a valid trigger to sync properly.