CRT repair safety

[D.M. Procida]

I've fixed some iMac CRTs by replacing the burnt-out flyback transformer. How much danger if any is there of doing something to it so that it produces excessive X-radiation?

Thanks,

Daniele

[Grunff]

If the flyback is outputting a significantly higher voltage than the original, then you will get more x-rays produced, and of a shorter wavelength (higher energy). Where did you source the flybacks? Were they a direct replacement?

[NikV]

As I understand it the flyback transformer just directs the beam of electrons, the accelerating voltage of the main guns increase the energy of the beam - increasing this will increase the proportion of x-rays given off by the screen.

Nik V

[Grunff]

I'm not sure what you mean, but the flyback provides the ~20kV accelerating potential.

[Dave]

...snipped

X-rays are emitted as the e-beam impacts on the phosphor (and maybe the mask) but they are low energy ("soft") and are stopped by the glass. The normal EHT (around 25kV) would have to increase significantly to generate externally detectable X-rays. Usually there is a protection circuit which indirectly monitors the EHT and will shut things down if it gets too high, if this fails the CRT will probably flashover. Bottom line: no worries.

Dave S

[Simon Barr]

I wouldn't worry about it. I used to repair monitors for a living some years back and we *never* used to check the output. All our repairs had to be carried out according to a repair manual and I don't recall any of them mentioning this. I would expect something as new as a IMac to have lots of paranoia circuits on the HT side anyway, so if something was over voltage it would shutdown.

I also remeber someone telling me once that the majority of the radiation from a monitor comes out of the back anyway. The CRT front is very thick, maybe 1.5cm and blocks most radiation.

[Peter Scott]

Agreed. I am sure I remember that to get hard X-rays capable of damage to human tissue, you need getting on for 100kV. I think you need a massive metal target as well to deal with the energies.

Peter Scott

[Terry D]

As a radiation worker for many years, I have never detected any measurable radiation levels from a TV screen, using monitors capable of detecting down to 60Kev. If any radiation level was detected on the surface of the screen the inverse square law would result in a negligible level at normal viewing distances.

Terry D.

[D.M. Procida]

Executive summary: no, there is no danger.

Thanks for all the reassuring answers.

Daniele

[The Natural Philosopher]

You haven't fixed teh CRT. You have fixed the monitor.

We are all mutants anyway. A little extra EHT won;t make it any worse...

[N. Thornton]

Is it not inevitable that a 60keV plus detector would not detect anything produced by a 19-24kV screen?

Regards, NT