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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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20th May 2015, 10:38 pm | #1 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Murphy "Astra" V659X. Picture foldover.
I've heard reports from other owners of this type of set about the problem of fold over at the left side of the picture. The fault can be removed if the line hold is carefully adjusted but then there is the problem of hooking at the top of the picture. The set being discussed is the version with direct line sync.
I've redrawn the line sync circuit and added waveforms. The negative going sync pulses are differentiated and supplied to the grid of the 6/30L2 line oscillator. It could be that the oscillator is locking up to either the negative going leading edge of the sync pulse or alternately the positive going trailing edge. I could try connecting a diode across R353 to remove the positive going spike. DFWB. |
21st May 2015, 9:08 pm | #2 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Murphy "Astra" V659X. Picture foldover.
Hello David,
When I was first in the trade as a lad, this series of Murphy chassis had this line foldover fault from new. There was mention of it in the 'Murphy Service News' when the receivers were still under guarantee. I now have an excellent example of a 759X by courtesy of another Forum member but this also has slight fold over identical to yours. I don't think it was officially cured but very few customers noticed. John. |
21st May 2015, 10:04 pm | #3 |
Dekatron
Join Date: Jun 2013
Location: East Sussex, UK.
Posts: 3,315
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Re: Murphy "Astra" V659X. Picture foldover.
I also remember that fault and there was a mod, but can't remember what it was. Probably was in Murphy News if anyone has a copy.
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22nd May 2015, 12:32 am | #4 |
Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Re: Murphy "Astra" V659X. Picture foldover.
The oscilloscope confirms the waveforms shown on the attached circuit diagram on my first post about this subject.
The differentiation capacitor receives 50 volt negative going sync pulses. As a result of the insertion of the resistor R352 the amplitude of the negative going leading edge of the sync pulse present at the grid of the line oscillator valve is only 5 volts. The cathode coupled multivibrator requires only a low amplitude sync pulse for satisfactory synchronisation. I have connected a 1N4148 diode across R353. The anode is connected to the grid end of the resistor and the cathode to ground. The diode conducts on the positive going part of the differentiated sync pulse. The result from adding this component? No improvement whatsoever. The foldover is still present on the test card but as John states it is not so noticeable on normal pictures. DFWB. Last edited by FERNSEH; 22nd May 2015 at 12:37 am. |
22nd May 2015, 11:05 am | #5 |
Octode
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,958
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Re: Murphy "Astra" V659X. Picture foldover.
David
I'm not a TV professional, so will bow to your experience, but I have been wondering about the cause of line fold-over. I think it is caused by the line scan current at the start of the scan being in the 'wrong' direction. This current then reduces to zero over the first few inches of scan and then when it finally reverses polarity the scan proceeds normally for the remaining part of the sweep and produces a normal picture. So where does this initial 'wrong polarity' scan current come from? Is it possible that the line flyback current in the scan coils has not finished by the time that the scan is re-started and this is causing the fold-over, rather than the line oscillator mis-triggering on the wrong edge of the sync pulse? Ron |
22nd May 2015, 6:09 pm | #6 |
Rest in Peace
Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
Posts: 3,944
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Re: Murphy "Astra" V659X. Picture foldover.
It looks to me that in your second picture there is slight foldover on the right of the picture when the line hold is used to push the picture over. This suggests that the flyback is taking too long. If this is a design fault from new it looks like it won't be possible to solve it without upsetting the EHT voltage.
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