KB QVP20: bad interlace. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 9th Dec 2015, 4:42 pm

I've sorted out most of the problems with this Kolster-Brandes 17" portable TV including taming the tricky frame timebase.
However, one serious fault remains, bad interlace, in fact there is no interlace whatsoever, the set displays a coarse 188 line picture.
Going way back to the days when these sets were new, a known cause of bad interlacing was due to a defect in the scanning coils.

DFWB.

Heatercathodeshort · 9th Dec 2015, 7:57 pm

Hello David,
I came across this problem a lot when I was in my early days in the trade. KB certainly had a problem with the scan coils in the Q and R series 110 degree models. Modified scan coils were available at a price and most customers could not see the fault even when pointed out to them...
Just a thought. Make sure the line lin sleeve is in the correct axis. If twisted it can act as a coupler between the line and frame coils. Such a pity this fault existed. How they got out of Footscray is beyond me. The QVP30 is a much nicer design than the hideous PVP20. The country was throwing off the restrictions of WW2 and space travel with the 'Sputnik' was all the rage. Strange but happy times. John.

Nuvistor · 9th Dec 2015, 8:03 pm

Was there a set were you had to loosen the fixing of the scan coils and move the line and frame coils around to get the best interlace? I may be thinking of another problem caused by the scan coils but it was interaction between the coils that caused some fault or other.

Frank

FERNSEH · 9th Dec 2015, 8:59 pm

The line linearity sleeve has been repositioned on the tube neck. The only noticeable result is there is more picture width but the interlace fault still persists.
The frame sync pulse at the anode of the 12AX7 frame pulse clipper/amplifier is almost 80 volts. The valve is OK.
The attachment shows the delayed frame sync pulse.
I'm pretty certain the fault is the result of the line scan waveform entering the frame scan coils.

The line pairing could be tolerated if was not for the 25Hz flicker.

DFWB.

Heatercathodeshort · 10th Dec 2015, 9:42 am

Quote:

Originally Posted by FERNSEH

The line pairing could be tolerated if was not for the 25Hz flicker.DFWB.

Nothing changes. Baird had this problem in 1936/7..

Just a trace of line waveform on that sync pulse David but that tiny amount would have little effect on interlace.
You have to put up with that when your into pink horrors. John.

FERNSEH · 15th Dec 2015, 9:29 pm

I'm absolutely certain that the scan coils are the cause of the bad interlace.
Too confirm that the frame sync relationship with the line sync is correct, that is, the half line off-set on each successive picture field.

The first attachment shows the odd field ending on line 202 and the second the even field ending on line 405.
The oscilloscope was synchronised on the frame sync pulse present at the anode of the triode frame pulse clipper.

Another test was conducted with the scope synchronised from the frame flyback pulse. Again, the half line off-set was apparent.

DFWB.

1955APREN · 15th Dec 2015, 11:50 pm

I might be wrong about this but did some makers ie Sobell and Baird fit a vdr across the primary winding of the frame output transformer to over come this fault. regards Derrick

FERNSEH · 16th Dec 2015, 1:29 am

The reason why a VDR is connected across the primary of the frame output transformer is to limit the amplitude of the flyback pulse.
For the second sync waveform test the scope was triggered from the blanking pulse present at the slider of the brightness control. The blanking pulse is simply the flyback pulse which is generated in the frame output transformer.

DFWB.

Heatercathodeshort · 16th Dec 2015, 8:30 pm

Oh David! I wish you wouldn't post waveforms like that..It brings it all back from 1964!
I seem to remember attempting to fit a pair of standard 110 degree scan coils in a hope to clear up the interlace problem.
It sure did that but the picture ended up with very bad linearity due to mismatch. It may be possible to fit a frame output transformer and matching scan coils from a scrap Bush but then you have the mechanical fixing problem in this model and the fear of a poor line mismatch. You simply can't win either way and I'm afraid you will just have to put up with it. Regards, John.

9th Dec 2015, 4:42 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	KB QVP20: bad interlace. I've sorted out most of the problems with this Kolster-Brandes 17" portable TV including taming the tricky frame timebase. However, one serious fault remains, bad interlace, in fact there is no interlace whatsoever, the set displays a coarse 188 line picture. Going way back to the days when these sets were new, a known cause of bad interlacing was due to a defect in the scanning coils. DFWB. Attached Thumbnails

9th Dec 2015, 7:57 pm	#2
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: KB QVP20: bad interlace. Hello David, I came across this problem a lot when I was in my early days in the trade. KB certainly had a problem with the scan coils in the Q and R series 110 degree models. Modified scan coils were available at a price and most customers could not see the fault even when pointed out to them... Just a thought. Make sure the line lin sleeve is in the correct axis. If twisted it can act as a coupler between the line and frame coils. Such a pity this fault existed. How they got out of Footscray is beyond me. The QVP30 is a much nicer design than the hideous PVP20. The country was throwing off the restrictions of WW2 and space travel with the 'Sputnik' was all the rage. Strange but happy times. John.

9th Dec 2015, 8:03 pm	#3
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: KB QVP20: bad interlace. Was there a set were you had to loosen the fixing of the scan coils and move the line and frame coils around to get the best interlace? I may be thinking of another problem caused by the scan coils but it was interaction between the coils that caused some fault or other. Frank

9th Dec 2015, 8:59 pm	#4
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: KB QVP20: bad interlace. The line linearity sleeve has been repositioned on the tube neck. The only noticeable result is there is more picture width but the interlace fault still persists. The frame sync pulse at the anode of the 12AX7 frame pulse clipper/amplifier is almost 80 volts. The valve is OK. The attachment shows the delayed frame sync pulse. I'm pretty certain the fault is the result of the line scan waveform entering the frame scan coils. The line pairing could be tolerated if was not for the 25Hz flicker. DFWB. Attached Thumbnails

15th Dec 2015, 9:29 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: KB QVP20: bad interlace. I'm absolutely certain that the scan coils are the cause of the bad interlace. Too confirm that the frame sync relationship with the line sync is correct, that is, the half line off-set on each successive picture field. The first attachment shows the odd field ending on line 202 and the second the even field ending on line 405. The oscilloscope was synchronised on the frame sync pulse present at the anode of the triode frame pulse clipper. Another test was conducted with the scope synchronised from the frame flyback pulse. Again, the half line off-set was apparent. DFWB. Attached Thumbnails

15th Dec 2015, 11:50 pm	#7
1955APREN Hexode Join Date: Nov 2010 Location: Market Drayton, Shropshire, UK. Posts: 485	Re: KB QVP20: bad interlace. I might be wrong about this but did some makers ie Sobell and Baird fit a vdr across the primary winding of the frame output transformer to over come this fault. regards Derrick

16th Dec 2015, 1:29 am	#8
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: KB QVP20: bad interlace. The reason why a VDR is connected across the primary of the frame output transformer is to limit the amplitude of the flyback pulse. For the second sync waveform test the scope was triggered from the blanking pulse present at the slider of the brightness control. The blanking pulse is simply the flyback pulse which is generated in the frame output transformer. DFWB.

16th Dec 2015, 8:30 pm	#9
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: KB QVP20: bad interlace. Oh David! I wish you wouldn't post waveforms like that..It brings it all back from 1964! I seem to remember attempting to fit a pair of standard 110 degree scan coils in a hope to clear up the interlace problem. It sure did that but the picture ended up with very bad linearity due to mismatch. It may be possible to fit a frame output transformer and matching scan coils from a scrap Bush but then you have the mechanical fixing problem in this model and the fear of a poor line mismatch. You simply can't win either way and I'm afraid you will just have to put up with it. Regards, John.