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Old 22nd Nov 2021, 8:05 pm   #1
Niechcial,Steve
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Default Pye BT18- Line Cogging/Tearing

This set has very bad line tearing/cogging with weak line sync. Frame sync is fine. I've been right through the sync separator and line osc. stages- changed coupling caps and tried different valves, checked all resistor values etc. Sync pulses look good on a scope.
Any suggestions please? Thanks
Steve
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Old 22nd Nov 2021, 8:41 pm   #2
Heatercathodeshort
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Default Re: Pye BT18- Line Cogging/Tearing

There is an electrolytic capacitor that passes half way through the chassis next to the line board on the right by the tag strip on top of chassis. I think it decouples the screen of the video amp. Worth a try. The 2uf video coupling cap on the tag board will cause a low contrast picture when O/C but does not effect the sync. John.
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Old 22nd Nov 2021, 10:29 pm   #3
AdrianH
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Default Re: Pye BT18- Line Cogging/Tearing

Do you mean a Pye B18T ? Just asking as I picked up one from the BVWS auction, not started on it yet but will be looking at all possible posts about the TV.

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Old 22nd Nov 2021, 11:13 pm   #4
Niechcial,Steve
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Default Re: Pye BT18- Line Cogging/Tearing

Yes sorry- I meant B18T.
This was all a bit bizarre, There was no capacitor physically in the position you describe John, but C6A, 12 mfd, located behind the frame o/p trans does indeed de-couple the screen grid/anode of the video amp. It checked high esr and replacing it produced a great improvement. And now the bizarre bit. While I was doing all this I had my multimeter negative lead clipped on the chassis with the red lead dangling in space. As soon as I removed the clip from the chassis some of the bad cogging/tearing came back. Obviously the meter leads were acting as some kind of aerial, and whatever was being picked up was somehow assisting. This got me onto the aerial input to the set which is a balanced transformer with centre tap connected to chassis via a parallel R/C combination. It was designed to work with balanced twin feeder lead from the aerial. When you connect an ordinary co-ax cable across it, it is of course no longer balanced. Nevertheless contrast and noise were fine with the aerial connected this way, albeit with this cogging fault. I tried disconnecting the centre tap of the transformer from chassis, thus converting the balanced input into an unbalanced one, and that completely cured the fault. I doubt if the impedance is correct, but there is still plenty of gain and no noise.
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Old 22nd Nov 2021, 11:17 pm   #5
Niechcial,Steve
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Default Re: Pye BT18- Line Cogging/Tearing

The B18T is a solid little set Adrian but it has EF50s doing all sorts of functions, and some of them are choosey as to which position they will or will not work in. The valve bases are also particularly prone to bad contact. Once recapped, they usually work very well.
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Old 24th Nov 2021, 10:23 am   #6
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Default Re: Pye BT18- Line Cogging/Tearing

Yes of course you are right Steve. I am mixing up the D16T and the B18T. The D16T has the capacitor I mentioned.

There was a query in Practical television about the balanced aerial input and the use of co-ax cable. The conclusion was from experience that it didn't make any difference, even in fringe areas as to which type of downlead you used. I have tried both with the same results. John.
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