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Picture Tube Isolation Transformer

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PostPosted: Wed Dec 05, 2007 6:13 am    Post subject: Picture Tube Isolation Transformer Reply with quote

Back in the old days they sold and isolation transformer that plugged between the picture tube socket and the picture tube base. These apparently are a thing of the past. I was wondering if anybody had one however that they would part with. My old TV set has a cathode to heater short on the red gun.
Your help would be greatly appreciated.
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Old Bassid

PostPosted: Wed Dec 05, 2007 8:15 am    Post subject: S/C cathode to heater red gun. Reply with quote

Lissen you old bugger- all you need to do is isolate present heater drive on crt base- then wind 3 turns of insul wire around lopt limb & connect same to heater conns on crt pcb- this floats the heater drive- there are other approaches which experienced techs could use- but this is the most appropriate for a person of your low intellect!.
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Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Mon Dec 10, 2007 7:58 am    Post subject: Reply with quote

Another method is taking a mains transformer from a wall cube, clock radio etc, that has a secondary voltage of, say 9 V.
Supply the primary with mains voltage from the tv's mains switch and feed the CRT heater from the secondary via a resistor which you adjust to get the correct heater voltage (usually 6,3 V).
Connect a bleeder resistor of some hundred kohms from one side of the heater to one of the CRT cathodes to avoid static charging if the short would not stay there.

If you are in doubt regarding the heater voltage, try to feed the heater from a bench power supply and increase the voltage slowly until it glows as usual, then read out the voltage and assume that the heater is intended for the nearest standard voltage (for example: 6,3 V , 11 V, 12,6 V).
When measuring the heater voltage from the tv's original heater supply, it can be difficult as it is often an impulse voltage from the LOPT. Then, a expensive DMM with "true RMS" readout is necessary to get a correct value.
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