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Old 11th Feb 2024, 1:31 pm   #641
McMurdo
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Default Re: Baird T5 restoration project.

I'm probably just repeating the meaning of Ed's comment but might it be an idea to add a little something to bring the heater up slowly each time, either constant current or a generous slow start to be kind to the weld?
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Old 11th Feb 2024, 5:59 pm   #642
FERNSEH
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Default Re: Baird T5 restoration project.

Quote:
Originally Posted by McMurdo View Post
I'm probably just repeating the meaning of Ed's comment but might it be an idea to add a little something to bring the heater up slowly each time, either constant current or a generous slow start to be kind to the weld?
Hi Kevin,
if the CRT is to survive some means of limiting the current surge in the tube filament on switch-on.

Murphy310 wrote: "Not wishing to be a pessimist I doubt this will be a lasting repair especially when the set is transported.
It's really difficult to weld a heater/filament as the required welding current would be well above the filament current for it to be a success, but of course that current would blow the heater out of the window."

How about this? A capacitor charged up to say, 5000volts. Discharge the capacitor across the heater or filament pins of the CRT.
q = CV. (a) the energy sored in the associated electric field is 1/2 joule when the PD is 1 volt.
(b) the PD is one volt when the charge is 1 coulomb.

W = 1/2CVsquared joules. Instantaneous discharge current will be a very high value within the short discharge time.
There is an AW43-80 CRT upstairs which had an OC heater. A blast of EHT fixed the heater for a short while but the "repair" didn't last long. Try the capacitor discharge method.

If 5KV doesn't work a voltage doubler can be tried.

DFWB

Last edited by FERNSEH; 11th Feb 2024 at 6:06 pm.
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Old 11th Feb 2024, 11:02 pm   #643
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Default Re: Baird T5 restoration project.

Hi David
Yes exactly what I thought was to charge up a capacitor with 5 or 6kv. The problem is if you go too far and blast material off the "loose" ends. I remember shorting out a 1uf 5kv cap fully charged, one hell of a bang and the screwdriver was firmly welded to the capacitor! I'd be thinking of 0.1uf maximum at 5kv.

If successful a simple limiting resistor in series with the heater feed would limit the surge and if the AC supply is too low for that a bridge rectifier made with high current Schottky diodes and a meaty reservoir cap will give you more headroom assuming it's a separate winding on the mains transformer.
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Old 12th Feb 2024, 5:56 am   #644
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Default Re: Baird T5 restoration project.

Hi David, I have some NTC's at home from SMPS use the are about 1R when hot, more when cold. It will mean a higher heater voltage feed

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Old 12th Feb 2024, 9:50 am   #645
radiomarktv
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Default Re: Baird T5 restoration project.

I would go with the NTC soft start reference to Trevor and Ed’s comments.

You had the set running ok for a while after the first attempt and it seems the next cold switch on surge was too much for the tube.

Mark.
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Old 12th Feb 2024, 9:29 pm   #646
FERNSEH
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Default Re: Baird T5 restoration project.

I have to report the CRT has gone OC heater again.
Will try the capacitor discharge method to weld the filament but to be honest it looks like it'll have to be a Cossor 65K for this set.

DFWB.
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Old 22nd Feb 2024, 11:16 pm   #647
FERNSEH
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Default Re: Baird T5 restoration project.

This Cossor 65K will go into the Baird T5 after a number of reversible modifications have been carried out.

DFWB.
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