Thread: Gec bt1091.
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Old 17th Mar 2018, 12:06 am   #19
FERNSEH
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Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
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Default Re: Gec bt1091.

The first attachment shows the smeary picture and the second the distorted video waveform at the cathode pin of the CRT.
The video waveform across the diode detector load resistor is perfect.
A fault in the white spot inverter was considered but it's working OK, simply remove the L63/6J5 valve to disable the circuit.
The CRT has no graphite coating, hence no EHT smoothing capacitor. Try fitting a 1000pF 20KV capacitor inside the EHT box. Still no improvement.
No faults could be found in the video amplifier.
The service manual for later production sets fitted with the L63 spot inverter triode informs us that the original diode white spot clipper should be disabled by removing the wire link between the anode of the video amplifier and the cathode of the diode.
In fact the wire was still in place. Clipping the wire out cured the fault.
So it would seem that this set has always displayed a very poor quality picture.
Until now that is, the picture is bright and well contrasted but the CRT focus is poor.

DFWB.
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