Apple B&W CRT monitor popping and clicking

Phil Blundell pb at
Tue Apr 3 15:48:11 CDT 2018

On Tue, 2018-04-03 at 21:07 +0100, Ian Frost via cctalk wrote:
> Recently booted up my B&W Powermac G3, all came up fine including the
> 17” CRT monitor. However after a second or so, the 
> monitor gives a ‘popping’ sound and the image on the screen expands
> then shrinks. This repeats every few seconds.
> I’m guessing the monitor is on the way out - but as this is a
> complete B&W system - I wondered what the likely cause is?

That sounds like some sort of problem with the EHT supply.  If the
picture is stable until the first pop and then it expands, that might
suggest that the EHT voltage is dropping because of some sort of sudden
spark discharge.  Does the image also get dimmer/fainter at the same
time?  Can you see any evidence of arcing inside the monitor when this
is happening?  

I guess it's not impossible that slightly-conductive dust might have
built up inside the monitor somewhere to the point where the EHT
insulation is compromised and it's flashing over.  If that's the
problem then a bit of a clean-up might be all it takes to fix it. 
Needless to say, be careful when cleaning around the EHT.

Conversely, if the image was to get initially smaller and then suddenly
expand with a "pop", that might suggest that the EHT supply regulation
has gone wrong and the voltage is going too high.  From what you've
described though it doesn't sound as though that's what is happening.


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