fixing a dead crt ?

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fixing a dead crt ?

Postby Harry Dalek » Thu Jan 27, 2011 8:18 pm

Hi i struck a major nasty problem on my viewfinder part of my project i was running my crt on 5v for testing i got to the stage of wanting the focus nice and sharp so guessed like most valves it should be running at 6.3v ...it worked ...well then it died ! :0 i have an open heater ...

OK i still have another little crt or 2 to play with so i found the heater on this one and slowly rased the voltage till it glowed ..which was only 3 or 4 volts ..then it died !!!! arrrrrrrrrrrrrrrrrrrrrrrrrrrrr

another open heater Ok i tried a little tip i recalled somewhere stored in my 2gig memory stick in my head if you run a eht across the heater sometimes it will weld it back ...well it worked for the tiny tube the larger older one i was using seems not perhaps not enough eht who knows ..

But any case it has me wondering could a damaged crt still work as a cold cathode crt ?

By perhaps by running a negitive eht to the damaged heater would this not send electrons to the tubes anode it would need 2 ehts?
I am also wondering if it could work on a much lowwer voltage .
I think it would work but not sure about sparking to the cathode and second anode .....mmmmmmm
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Postby Steve Anderson » Thu Jan 27, 2011 10:44 pm

Harry, most small electrostatic deflection CRTs either use 4V or 6.3V heaters, there might be a few around at 5V, but they are few and far between. The older, pre-WWII and very shortly after were when most of the 4V ones were made, from then on 6.3V became the norm except in a few odd cases. 5V is a rarity. In a lot of these tubes the cathode temperature is slightly lower than a standard receiving tube/valve sometimes the heater glow cannot even be seen, often the shape of the cathode precludes this too...rather like a very small thimble or cup.

One of those cases where I bet you wished you had downloaded the datasheets for them first! There's plenty of sites with them on, or ask me, there's a good chance I'll have it....unless it's a magnetically-deflected CRT...I avoid them. Period.

As for using them cold-cathode, forget it, it needs a noble gas for that...which is not air. Arcing the heater back together? Many stories from the 40s/50s/60s about this, but I don't rate it's success rate as high, at least in the long term.

Sadly I have to say you've got two(?) lovely ornaments now, that's all.

Steve A.
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Postby Harry Dalek » Thu Jan 27, 2011 11:22 pm

Hi Steve

I was using old camcorder viewfinder crts so they are the magnetically deflected .
My first one ran rather good on 5v couldn't see the glow but did on 6v and didn't like that at all for long darn ..

Can i ask why you dislike magnetically deflected crts ....my only dislike i can't google info on them ...more than likly why you stay away from them .

I think you know about Nyle Steiner home made crts hes cold cathode crt
runs 2000 to 4000 negitive on the cold cathode rod and the anode is just grounded ,reading he must have a small amount of air in the tube depending on how well hes vacuum pump works seems to work better the colser to a vacuum he gets ...now if the dead crt must be better than a home made one and closer to a vacuum than hes so i might just hook up hes simple one resistor and eht circuit and see ..if hes work has me wondering /

Perhaps hes only worked due to a small amount of air in the tube not the gas you are talking about but he has cathode rays .

I really am not sure how practical it is but the experiment interests me .


Yes i expect the heater weld is pot luck but seemed to work on the tiny crt
a few sec's of a few thousand volts made that little bugger work again ...who knows as you say for a while at least.

Thanks for the tip on electrostatic crts ..i have a big one i want to get going some time in time .
Last edited by Harry Dalek on Fri Jan 28, 2011 12:11 am, edited 1 time in total.
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Postby Steve Anderson » Fri Jan 28, 2011 12:07 am

harry dalek wrote:Can i ask why you dislike magnetically deflected crts ....my only dislike i can't google info on them ...more than likly why you stay away from them .


Well, it's not so much the lack of data but a lack of flexibility. With a magnetically-deflected CRT you've got those ruddy coils in the way. Fine. At one sweep rate. If you want to vary sweep speeds it becomes a nightmare. If you want wide bandwidth, forget it. That's the two main reasons oscilloscopes stayed with electrostatic CRTs, it would have been great to have had a CRT 'scope with a screen the size of a laptop screen...but can you imagine the size and weight?

With electrostatic deflection you only have to deal with a small amount of capacitance between the plates...unless into 100's of MHz where lead inductance can become an issue. Even so a few pf at MHz frequencies does require some current 'grunt' to be delivered. But at our frequencies...forget it, zero power.

I have had two Eizo Flexscan PC CRT monitors over the years (but gone flat-panel now) and due to the requirement to operate at several frame and line frequencies the timebase circuits were so so so complex. The CRT EHT also had to be separately derived. No 'blooming' on those displays with picture brightness changes. They were very good (and expensive) but I think there were more brains in the deflection circuits than the PC they were meant to display!

OK, with electrostatic CRTs you have to deal with 200-400V for the deflection amps...big deal...all CRTs require high volts one way or another...and from what I've seen that's not something that worries you too much!

What's better? 12V/1A or 240V/50mA? Same power, both solid-state compatible or if you're a bit of a tube-head like me...time to wind a heater tranny.

Steve A.
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Postby Harry Dalek » Sat Jan 29, 2011 3:38 pm

Hi steve
i can see they both have their good and bad points ,i have played with both over the years .
Most interesting was a slow scan p7 phosphor first tv from scratch project i made ...
You were saying about the deflection coils being a pain this thing needed a focusing coil ! which didn't come with it so i had to make one ...arrrr winding a transformer would of been easier ....the focusing coil worked great for a while then it started to get warm and lost a bit of magnetism . Glad focusing is all elecrostatic on both types of normal crt's.

Sounds like that monitor of yours must of been nasty if you were having trouble .

I just fixed my Hp lap top i tell you it was one of the most unenjoyable electronic repair jobs i have ever done in my life i never want to pull a lap top apart to the point of having to take out the mother board again ...my poor old eyes are not up to miro electronics only good thing was its repaired and typing this on it ...i have warned it its only in the bin if it gives me any more trouble .
:x :
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