Rejuvenating CRT's

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Rejuvenating CRT's

Postby Harry Dalek » Thu Jun 08, 2017 9:54 pm

http://www.thevalvepage.com/teletech/crt_oc/crt_oc.htm

i suppose its worth a shot since its useless other wise ...i still wonder if induction heating coil base of the tube would work if controlled .
Something that comes to mind while working on to p7 CRT image testing i disconnected the heater voltage and i was surprised the thing worked fine several seconds after the heater had cooled made me wonder how little was needed here for the crt to work .
Last edited by Harry Dalek on Mon Jul 31, 2017 10:57 pm, edited 1 time in total.
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Re: Rejuvinating CRT's

Postby McGee2021 » Fri Jun 09, 2017 6:42 am

I always like this part of the website:

http://www.thevalvepage.com/tvmanu/bair ... on1937.htm
John Logie Baird was obviously the man who sowed the seeds but did not reap the harvest.
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Re: Rejuvinating CRT's

Postby Steve Anderson » Fri Jun 09, 2017 9:10 am

Harry Dalek wrote:...I disconnected the heater voltage and i was surprised the thing worked fine several seconds after the heater had cooled.

Yep, the heater cools down quite quickly, that's why that red/orange glow stops. But the cathode in relative terms has a much greater thermal inertia or mass - hence the reason for all tubes/valves taking dozens of seconds to warm up and get going. It's the reverse when the heater is disconnected.

In most equipment the HT supplies will discharge quicker than the cathode cools, but in a CRT often the supplies will keep the thing going for quite a while as most CRTs draw such a small current. An average valve/tube table-top radio will draw 40-60mA from 200-300V, a 'scope type CRT is usually much less than 1mA.

One problem with the induction heating idea is focusing the heating to the cathode only, you do not want the grid to get hot such that it too starts emitting electrons. This was one of the design challenges in later and smaller valves/tubes. To get a high mu (amplification factor) the cathode-grid spacing needs to be very small. If ever you take apart an ECC83 dual triode you'll see what I mean. That has a mu of 100 and was about the limit, though the ECC803 did manage a mu of 130.

I don't recommend breaking a valve/tube deliberately, they do contain some nasty toxic chemicals.

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Re: Rejuvinating CRT's

Postby Harry Dalek » Fri Jun 09, 2017 10:24 pm

Steve Anderson wrote:Yep, the heater cools down quite quickly, that's why that red/orange glow stops. But the cathode in relative terms has a much greater thermal inertia or mass - hence the reason for all tubes/valves taking dozens of seconds to warm up and get going. It's the reverse when the heater is disconnected.


I should of really timed it better but did find it interesting i kept wondering why it was still working with the heater disconnected ,most of the time you have both circuit power supply and heater both switching on and off same time ..this time just the heater power was removed as i wanted to test the circuit while on and the crt off or heater off ...never saw the effect before i was surprised ....

In most equipment the HT supplies will discharge quicker than the cathode cools, but in a CRT often the supplies will keep the thing going for quite a while as most CRTs draw such a small current. An average valve/tube table-top radio will draw 40-60mA from 200-300V, a 'scope type CRT is usually much less than 1mA.


Well its an interesting effect indeed !

One problem with the induction heating idea is focusing the heating to the cathode only, you do not want the grid to get hot such that it too starts emitting electrons. This was one of the design challenges in later and smaller valves/tubes. To get a high mu (amplification factor) the cathode-grid spacing needs to be very small. If ever you take apart an ECC83 dual triode you'll see what I mean. That has a mu of 100 and was about the limit, though the ECC803 did manage a mu of 130.

I don't recommend breaking a valve/tube deliberately, they do contain some nasty toxic chemicals.

Steve A.


I have been thinking about this for a while the induction coil first idea was a flat coil around the base of the glass just below the cathode where you see the wires connections to parts of the tube problem i was thinking here it would also heat the various connections to the control grid anodes and such may be too much ? but you can control the induction you don't have to melt the thing perhaps there would be a sweet spot where the heater connections warm up enough to get the thing to boiled off electrons in thermionic emission but may be very slow to get going...Valves tend to be warm so i would think other parts of the valve can take being warm up also in this idea ...
Idea 2 is to actually to heat metal pins to the heater at the connections with small induction coil thermal temperature make its way up the wire to the broken heater which would be better as you could control it but i suppose you could connect those pins to longer rods and heat them with a flame if you had to Macgyver it as a last resort ...after that i am running out of ideas ....Laser pointed at the heater !!!!!!! :D
I do have a little crt with a broken heater but these unproven ideas don't need it to be broken just a method without using voltage to the heater if it works or not ...need to make that induction heater ! :shock:
My original thoughts were to try this on a triode or such wired up as a oscillator or amp and test .
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Re: Rejuvinating CRT's

Postby AncientBrit » Fri Jun 09, 2017 11:09 pm

Hi Harry,

Another variation on the idea of creating a spot weld is to charge a capacitor to say 100 v and connect across the heater.
The value of the capacitor needs to be quite small, start with say 2uf.
The idea is to make the weld without burning out the filament.
With 2 variables here there is much trial and error.
Start at the low end.

Good luck!

Cheers,

Graham
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Re: Rejuvinating CRT's

Postby Harry Dalek » Sat Jun 10, 2017 11:05 am

AncientBrit wrote:Hi Harry,

Another variation on the idea of creating a spot weld is to charge a capacitor to say 100 v and connect across the heater.
The value of the capacitor needs to be quite small, start with say 2uf.
The idea is to make the weld without burning out the filament.
With 2 variables here there is much trial and error.
Start at the low end.

Good luck!

Cheers,

Graham


Valves are starting to become rarer and if you have one that has a broken heater a way a procedure to fix it if for a while is better than throwing it away theses things cost a bit these days ...Thats a interesting idea bit safer than the first post i put up .
I want try a way of running a valve with out heater voltage i will look into the things needed give me an excuse to test this ..need a test bed and the valve socket i think i still have a few that are ceramic....good for thermal testing
It would be good to make a test circuit that works as is heater working valve as normal then do the tests on the heater the other ways that might show its possible to run a valve with a open heater the second step ...
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Re: Rejuvinating CRT's

Postby Harry Dalek » Mon Jun 12, 2017 11:17 pm

I found this interesting similar to Graham's idea in the weld
Theoretically you can repair a burned out tube if it meets certain criteria. The filiment's detached ends have to be positioned close enough to each other and the tube run in a series with other like tubes and enough voltage applied so the broken filliments actually " arc-weld" together (page 31, left page). The manual pictured was circa 1944 and in wartime, tubes were worth their weight in gold just as many other badly needed commodities.
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tube repair 002.jpg
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tube repair.jpg
tube repair.jpg (57.34 KiB) Viewed 584 times
tube repair 001.jpg
tube repair 001.jpg (64.5 KiB) Viewed 584 times
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Rejuvinating CRT's

Postby AncientBrit » Tue Jun 13, 2017 5:48 pm

Hi Harry,

My only observation from the circuit you found is that there appears to be no current limiting once the weld has been completed.
Using a capacitor as the voltage source limits the peak current as the capacitor discharges through the heater.

Returning to my original suggestion I believe you can use a fixed HV supply.
If you charge the capacitor via a high value resistor from one of your 'scope supplies and have a high impedance DVM across the capacitor you can discontinue the charging at an appropriate moment.
(Make the time constant large enough to allow for your reaction time)

Then quickly switch the capacitor directly across the heater.
If the weld 'makes' you will see the reading on the DVM rapidly drop to zero.
If not you will see the voltage decay slowly due to the loading of the DVM.

Cheers,

Graham
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Re: Rejuvinating CRT's

Postby Harry Dalek » Tue Jun 13, 2017 7:57 pm

AncientBrit wrote:Hi Harry,

My only observation from the circuit you found is that there appears to be no current limiting once the weld has been completed.
Using a capacitor as the voltage source limits the peak current as the capacitor discharges through the heater.

Returning to my original suggestion I believe you can use a fixed HV supply.
If you charge the capacitor via a high value resistor from one of your 'scope supplies and have a high impedance DVM across the capacitor you can discontinue the charging at an appropriate moment.
(Make the time constant large enough to allow for your reaction time)

Then quickly switch the capacitor directly across the heater.
If the weld 'makes' you will see the reading on the DVM rapidly drop to zero.
If not you will see the voltage decay slowly due to the loading of the DVM.

Cheers,
Graham


OH i Agree there Graham !...Your Capacitor procedure you posted up sounds far saver to me on a heater weld idea than the other ideas posted up ...just found it interesting it had crossed the minds of others in the past and they came up with a procedure for it what ever its worth .
Makes me also think all this on Playing with fire posts
viewtopic.php?f=8&t=1681&hilit=playing+with+fire
I am interested in seeing it work wonder if it would work on a light globe of various sizes voltage types .. that would be interesting as you could film it theres always a dead globe laying around .
The running of either a broken heater as is or one fine but with out voltage to its heater is the other thing that i would like to look into ...i got some parts together still thinking of a simple test bed for the experiments ...
Proof of concept would be nice
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Rejuvinating CRT's

Postby Harry Dalek » Mon Jul 31, 2017 10:56 pm

came across this on page's 189 and 190 on below PDF Rejuvenating dead crt's pretty much Grahams procedure .
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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