Testing CRTs

[techknight]

For the heck of it, I decided to test a compact mac CRT, a 12ATY4N from I think an SE or maybe a Plus. 

it had SEVERE burn-in and I kept it around just in case. So for giggles I decided to pull out my Sencore CR161 and test the tube. 

Once I realized it was a 12V tube and not a 6V tube, I got better readings. It honestly actually surprised me a bit. For such severe burn in, the tube still tests strong. 

The point? Well I dunno, I don't really have one. Just a wild thought. It'll make it easier for me to pick and choose CRTs I guess, or aid in a troubleshooting process. But it goes to show you that even though a tube has severe burn in, don't write it off just yet!

The CRT tester also does a "life test" which basically estimates how much cathode material is left, or "life" in the electron gun of the tube. It cuts the filament off while continuing to test emissions. If it starts dropping like a rock instantly, the tube doesnt have much time left in this world. However if the needle stays steady, or drops very very little within 5 to 10 seconds, its got another decade or so of use left in it. 

 

[EvieSigma]

Watching shango66 on YouTube taught me that old CRT tubes are surprisingly hardy...

 

[techknight]

Yep, he uses a beltron which is a much more sophisticated model. But for what I do, what I have is perfectly fine. 

 

[EvieSigma]

I personally haven't encountered a CRT with a tube problem so I'll save the investing in obsolete tools for then, and even then it would have to be a vacuum tube set or a sweet Trinitron for me to bother...

 

[techknight]

I have a few tube sets, but I have run into a couple of machines that had really weak CRTs. One of them I think was a performa which was so worn out the colors were bleeding and losing focus. 

Biggest reason I own the tester is for the 40s/50s sets where I need to know if the CRT is good before I touch them for restoration purposes. I dont service modern CRT sets anymore, otherwise this tester has a rejuv function were I could just "pop it" to get some extra life out of it for a little while. 

 

[EvieSigma]

I've never even found a set that old myself, the oldest set I've laid hands on is from 1982.

 

[CC_333]

I think the oldest set I ever touched was from the mid 70s (it was my mother's set). It *could've* been early 80s, though. We did also have a Toshiba set, and that one was definitely built in the early 80s; it was somewhat larger, and we used it for a good long while until the tuner died in 2002 or so (it would just display a grey screen). It got thrown out, but I kept the H-hold and brightness/contrast knobs for some strange reason.

Anyway, back to my mother's set. Toward the end of it's "life", it was having trouble where some of the colors would drop out, and we had to whack it on the top and sides to get it to display properly.

Last time I saw it was about 18 years ago. I don't know what happened to it. I assume it got thrown out.

In retrospect, I kinda wish we'd kept it.

c

 

[techknight]

yea considering all it needed was solder joints reflown on the CRT neck PCB at the socket. Common problem. 

Oldest set I have is a 1948 motorola VT-71. 

 

[PB145B]

Oldest set I have is a 1948 motorola VT-71. 

Dang, that’s so cool! I’d love to have one of those.

It seems like I read somewhere, once that some of those early TVs didn’t have the rubber cup over the end of the flyback cable and the claw was completely exposed. Is that one like that? Just curious.

 

[techknight]

Actually this one is an electrostatic set. it has no anode or flyback. 

its got an HV generator that applies +6KV to the internal plates. It doesnt have a yoke either. the electrostatic CRTs are similar to old oscilloscope CRTs. there are horizontal and vertical plates internal to the tube which have an electrostatic charge placed on them and is varied which deflects the beam. That electrostatic charge also accelerates the electrons to the face of the CRT. the anode is internal. 

But yes alot of old sets didnt have the cup at all. just a soldered high voltage wire do a squeeze pin. thats it. 

 

[PB145B]

Actually this one is an electrostatic set. it has no anode or flyback. 

its got an HV generator that applies +6KV to the internal plates. It doesnt have a yoke either. the electrostatic CRTs are similar to old oscilloscope CRTs. there are horizontal and vertical plates internal to the tube which have an electrostatic charge placed on them and is varied which deflects the beam. That electrostatic charge also accelerates the electrons to the face of the CRT. the anode is internal. 

But yes alot of old sets didnt have the cup at all. just a soldered high voltage wire do a squeeze pin. thats it. 

That's very interesting. Never heard of an electrostatic CRT before.

 

[techknight]

Yep, they are out there. Scope clocks use them.