sparks-wit: Sparton 4940TV

ID: 381259
? sparks-wit: Sparton 4940TV 
09.Jul.15 18:49
50

Gregory Jones (USA)
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I have this model set and it did operate when I bought it 30 some odd years ago but had a very dim picture. The neat thing about this TV was that the displayed picture was a reverse image so it looked normal when viewed through the mirror in the lid. Anyhow, I finally replaced all the paper and electrolytic capacitors and now there is no raster. I suspect problem is in the high frequency H.V. Power supply. It consists of a 6SN7 hooked up for parallel operation of a oscillator tuned to about 300khz feeding into primary of the high voltage transformer which has a secondary consisting of multiple separated windings to minimize corona discharge. The power supply appears to be operating correctly but the resistance reading of the secondary is about 560 ohms and that seemed a little high. Does anyone know much about these long forgotten H.V. Power supplies used in early TV or have any info about this particular set? Any insight would be appreciated. 
 

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 2
Check the HV oscillator voltages 
10.Jul.15 22:52
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Thomas Albrecht (USA)
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Thomas Albrecht

Gregory,

Many early TVs used this type of RF high voltage supply.  While the resistance of the secondary sounds higher than most, it is most likely OK (and hard to see what type of failure would lead to a resistance in this range).  I'd start by checking the voltages on the 6SN7 HV oscillator and seeing whether anything is amiss there.  Check the voltages against the table provided in the Sams service literature you can download from the model page.

Tom

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