CRT - Confirm Dead (not an obvious fault)

[mumoto]

Hello,

I've just restored an old (80s) computer/monitor,  was working just fine until suddenly horrendous noise and lost the screen.  I initially thought it was a belt slipping on the drive or similar causing the machine to crash but no, it is was actually sparks inside the CRT tube.

It fires up luminous electric blue (don't want to look too closely incase I'm giving my eyes a nice dose of UVC), the phosphor doesn't glow,  just noise and sparking inside the neck of the tube/electron gun. This is a single-gun monochrome monitor.

With little CRT knowledge beyond basic principals is it safe to assume this is dead, or could it be a simple fix? With obvious caveats about the dangers of CRTs etc.

It's an old Amstrad PCW so there are millions of them, no great shame just annoyed having retrobrited it and carried out various other repairs!

Thanks,
BHS

[inse]

I only encountered noisy arcing when the FBT or multiplier was cracked.
Arcing inside the tube only when the vacuum had gone…
That’s as much as I remember
You could pull off the CRT board and see what happens

[Runco990]

It SOUNDS like the CRT cracked and is open to air. 

However, it could also be the ground strap from chassis to the outside of the CRT.  CHeck that first.
The outside of the tube must be grounded, as the outside and inside coating form a high voltage capacitor.
Sparks can ensue.

[TomKatt]

It is possible for the gun assembly in the neck to short, usually due to mechanical shock or over voltages, but sometimes something just "gives out".

In any event, arcing inside the neck is not a good sign and typically means the crt is shot.

[Chris56000]

 Hi !

An easy and safe way to check is to disconnect the CRT heater leads (usually coloured brown and black) from the CRT base socket from tags 3 and 4, and connect a 12 V supply to the vacant terminals 3 and 4 from a d.c. power supply with the rest of the monitor switched off – you should be able to see the dull orange glow of the heater/cathode assembly when you power the heater separately in this way – if the CRT has air in it you will not be able to see any glow!

In any case, it is obvious from the symptoms described that your CRT has lost its.vacuum causing the internal arcing, and it's a goner!

Either another CRT or another monitor is the only way forward from here I'm afraid!

Chris Williams