A forum member has asked if there is any way of knowing the condition of the CRT in an old TV. This is important to know because if the CRT is bad the TV cannot be made to work - no more CRTs or CRT rebuilds are now available.

I told him that, short of obtaining a CRT tester (rare, expensive), the only way of knowing is to get the chassis running - sometimes a lot of work that could turn out to be for nothing.

Thinking about this, I reasoned that what we really need is a quick way to test CRT emission. So I came up with this really simple setup:

Almost all mono CRTs use the same pinouts for heater, cathode and G1. It's the same for 70 degree, 90 degree and 110 degree tubes which covers all B&W TV in Australia from 1956 until 1976. This makes a simple relative emission test easy.

To use it, connect up a 6 volt 1 amp DC power supply, a 1k resistor and a voltmeter as shown. Use insulated alligator clips to connect to the tube's pins.

Turn on the power supply and watch the meter.

If the tube is good you should get a reading within 6 seconds and by 10 seconds it should be within 10% of its final value of between 3 and 4 volts. If it takes longer than this and doesn't get above, say, 2 volts, you have a low emission tube.

For the final reading, regard 4 volts as 100%. 1 volt will be 25% - that should be a good indication of how the tube will perform in terms of brightness. That might still be better than nothing - you can decide.

The time a tube takes to reach 90% of final emission is a very good indicator of how it will perform when the chassis is restored to full working order.

Of course, low emission is not the only cause of tube failure, there might be shorts in the gun or burn marks in the phosphor for example. But these are uncommon, emission gives you 90% of the story!

Boy this sounds good Ian !
It also sounds easy .
The unfortunate truth about our TV hobby is there's no spare CRTs or FB
So this test you have come up with sounds good!
Pete

Ps are CRTs interchangeable
Eg, 21 inch pye work in a 21 inch HMV?

I'm going try this on a little 17 inch I've got put away that I would like to know if the CRT is any good.
It's a W101 in need of a good coat of Aquadag and the chassis is mostly poly with 1970s caps. I picked it up in Hunters Hill and it was a one owner since new.

Yes Pete there is a great deal of interchangeability with CRTs. All you really need to know is the screen size and the deflection angle. If you've got a really early 70 degree tube with mag focus and the replacement you want to use has electrostatic focus, just remove the focus magnets and connect the focus pin to about 200v.

Many early CRTs had non-aluminised screens and an ion trap. You can replace these with later CRTs, just discard the ion trap magnet.

If you have to, you can even replace a 70 degree CRT with a 90 degree one - you may also have to replace the yoke though.

For example, your Philips has a 21" 90 degree CRT. That's pretty much all you need to know.

By the way it tests 100%!

Wonderful! "" them's the words we want to hear""
😆 also ,its good to know that some CRTs are interchangeable because I often have tellies kicking around the place at home, that don"t appeal to me enough to restore, so they can be spare CRTs and parts.
Great test you came up with Ian.

Pete

Ian,
What power supply did you use.
.6 volt 1 amp? DC.
Was this a power supply from jay car etc etc, that you bought 1 amp.
Pete

Depends on what you have in the junk box. I don't know where mine came from. As long as it's 6 volts and can supply the heater current, which is normally either 600mA or sometimes only 300mA. Four AA alkalines will work.

Thanks Ian,