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 Replacing a vertical blocking osc. transformer made of unobtainium
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 Return to top of page · Post #: 1 · Written at 3:46:02 PM on 22 August 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

Vertical blocking oscillator transformers are a common failure item in 50s and 60s valve TVs due to the fine wire used in their construction.
Replacements are often hard to get.

Here is a way of replacing a transformer with just a few passive components. I've had to do this a few times lately so I thought I'd share this method.

Blocking Oscillator


 
 Return to top of page · Post #: 2 · Written at 9:19:32 PM on 22 August 2020.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 880

Hey Ian, did you forget to post something or is it a inclusion by Brad pending?

Fred.


 
 Return to top of page · Post #: 3 · Written at 11:42:57 AM on 23 August 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

The 2nd option!


 
 Return to top of page · Post #: 4 · Written at 4:18:05 PM on 24 August 2020.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6348

Photo uploaded.

Suggestion: It is best to include the phrase [photos pending] as MonochromeTV does. Smile


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 5 · Written at 6:41:18 PM on 24 August 2020.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 880

Ian, Aha!

So its a positive feedback loop via the plate to grid to oscillate, and the 15nf tweaks the phase shift?

Fred.


 
 Return to top of page · Post #: 6 · Written at 7:13:41 PM on 24 August 2020.
Ian Robertson's Gravatar
 Location: Belrose, NSW
 Member since 31 December 2015
 Member #: 1844
 Postcount: 1904

Yes that's basically it.

The 15nF is mainly there to attenuate the hor scan pulses that find their way there via crosstalk in the yoke. If not removed they muck up the interlace.

The whole circuit (not all shown) is fascinating in that the op stage plate provides both positive and negative feedback. It works because of the long time constant blocking that generates the sawtooth waveform by slowly charging and quickly discharging a capacitor.


 
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