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Old 21st Nov 2020, 8:08 pm   #1
high_vacuum_house
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Smile Marconiphone VT165 frame hold issues

Good evening,
I have been working on this 405 line receiver on and off for a few months and it now gives a good picture when stable...

I have an issue with the frame hold in which when you turn on from cold is always way off frequency and the frame hold needs twiddling to get it to lock. When it does lock it is stable. There is what I believe is cogging taking place and can be seen in the upper part of the raster and easily seen on the verticals either side of the circle on test card C as well as the castellation's at the top of the test card.

I have replaced all of the Hunts capacitors around the frame stage and linearity is good. I replaced R115 which is the sync signal into the frame stage. R117 is a bit high though I haven't replaced this yet. R116 reads a tad low though I think it is fine. I am struggling what to try next. Possibly change PCL82 sync separator if this is getting tired?

Many thanks,
Christopher Capener
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Old 21st Nov 2020, 10:48 pm   #2
FERNSEH
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Default Re: Marconiphone VT165 frame hold issues

Check C53 in the video amplifier stage. The 50uF elco is the cathode bypass capacitor of the PCL84.
C56 (0.1uF) sync coupling capacitor OK?

DFW.
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Old 21st Nov 2020, 11:19 pm   #3
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Default Re: Marconiphone VT165 frame hold issues

Both stock faults with this Thorn Chassis David. I think the Ferguson version is the 500 chassis. John.
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Old 22nd Nov 2020, 9:14 am   #4
high_vacuum_house
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Default Re: Marconiphone VT165 frame hold issues

Good morning,
Many thanks for your reply, C56 sync coupling capacitor has been changed. C53 has been changed in the past with an old RS 47uF capacitor. I will have a look at this one later today.

Christopher Capener
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Old 22nd Nov 2020, 11:29 am   #5
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Default Re: Marconiphone VT165 frame hold issues

Connect an oscilloscope to the grid and anode of the video output valve and observe the waveforms during the frame sync period.
The hooking at the top of the picture gives a clue that all is not well with the frame sync pulses.
Could be sync crushing.
The Thorn 500 series does not employ a frame sync integrator, instead the frame pulse is differentiated by coupling capacitor C89 (150pF)

DFWB.
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Old 22nd Nov 2020, 5:28 pm   #6
high_vacuum_house
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Default Re: Marconiphone VT165 frame hold issues

Good afternoon,
After changing C53 which when removed measured 50uF but had a high ESR was replaced and the line and frame hold controls were cleaned as were a bit crackly has mostly cured the problem. The verticals on test card C are now straight except for about 1/4” of the top castellations which are a little crooked but can live with that as the test card is nice and stable with good line and frame linearity and contrast.
Many thanks for your help,
Christopher Capener
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