And what do the neon lights do? I’ve never seen them lit up before….
I assume it’s diverting current from somewhere?
The orange ‘light’ on the other hand is something I’ve never seen before. @cheesestraws is that something you’ve ever come across?
Slightly out of my comfort area here, treat this with that in mind please:
The neon bulbs - and they
are neon bulbs - are overvoltage protection. The way that neon bulbs work is that they sit there totally inert until there's more than their strike voltage across them, then they light up, and it's significantly easier to keep them alight than it is to start them going. So you have a fairly robust, way to clamp the voltage across the neon to the voltage you need to maintain to keep it lit with a surge of its strike voltage.
Here's the circuit diagram from Thomas E. Lee's summary of how the AB works (the same document that
@Phipli) linked above:
And while I'm not going to pretend to be an expert in CRT circuitry, you can kind of see from this that the neons (NE1, NE2) are in a position where they can prevent dangerously high voltages reaching the CRT.
So: are
both neons lit? I can't tell from the photos above. If only NE2 is lit, then that might point to something bad happening to the 30V feed, which is one of the taps off the flyback, and might actually point to a dodgy flyback.
(now go back and read my comment about being out of my comfort zone to understand the size of the error bars on this)
Or is it indicative of a fault - defer to someone who actually has something this old or runs their early macs with the case off.
It's indicative of a fairly big fault.
