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Old 20th Dec 2016, 12:36 am   #1
1100 man
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Default How to modify Pye CT205 to direct sync

Evening all,
It being Christmas, I am hoping to find a bit of time to do some more work on the Pye.
Flywheel sync was a great innovation for receiving broadcast tv as it got round the problem of weak or noisy line sync pulses. What it doesn't do is cope with rapid timing errors in the sync pulses produced by VCR's- you just get a very bent or wobbly picture!
I plan to use the set with a 3v29 (or even worse 3v22) VCR which currently produces a very shaky picture on the CT205. I have tried to reduce the time constant by altering various cap and resistor values but without huge success.
As various other posts have said, these were never very good with VCR's.
So I would like to modify the line oscillator for direct sync and do away with the flywheel action all together.
I have tried, somewhat randomly, feeding sync pulses into various places and can get a very stable picture but displaced 3" relative to the start of the scan.
Can someone with a much better grasp of circuit theory than mine suggest ways to modify the circuit? (I have a 'scope, test gear and a good working knowledge of tv's)
As I understand it, the pentode section of the PCF802 is a voltage controlled oscillator. An error voltage is produced by comparing the incoming line sync pulse with a fed back line scan pulse. The triode section amplifies this error voltage and either speeds up or slows down the oscillator.
Any help to re- design the circuit would be greatly appreciated. I have attempted to attach the line oscillator part of the circuit but am not sure if the component id's will be legible.
Many thanks
Nick
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Old 20th Dec 2016, 2:08 am   #2
FERNSEH
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Default Re: How to modify Pye CT205 to direct sync

There might be difficulties here. The pentode section of V1 (PCF802) functions as a sine wave oscillator. The frequency and phase of the oscillator is controlled by the triode section which functions as a capacitive reactance valve.
You could try this: Short circuit the grid of the triode (V1A) to ground, that will remove the control voltage. Conduction through the triode and therefore the reactance function will still be controlled by the line hold control.
The differentiated line sync pulse should be disconnected from the junction of the discriminator diodes D40 and 41 and the sync coupling capacitor C203 connected to the control grid of the pentode section of V1. Without having the service manual to hand I cannot determine if the leading edge of the differentiated sync pulse is positive or negative.
It cannot be guaranteed if this modification will work and a more sensible approach to the VCR playback problem will be to shorten the time constant of the discriminator control voltage filter. Try reducing the value of C208 from 1mfd to 0.47 or even as low as 0.22mfd.
Direct sync for 625 TV is not recommended and I'd imagine the rather unstable VCR signal will cause vertical ragging.

DFWB.
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Old 20th Dec 2016, 9:48 am   #3
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Default Re: How to modify Pye CT205 to direct sync

The easiest solution is to play the VCR through DVD recorder used as a timebase corrector.You can usually set the recorder to record pause and monitor the composite out.
It's amazing how it can clean up the syncs.
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Old 20th Dec 2016, 11:52 pm   #4
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Default Re: How to modify Pye CT205 to direct sync

Many thanks for the detailed explanation, David, I will try the mods and report back.
Restoration73, I was hoping not to have to use a separate device, but may have to if all else fails.
Cheers Nick
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Old 21st Dec 2016, 12:12 pm   #5
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Default Re: How to modify Pye CT205 to direct sync

I'm with David when he says direct sync will cause ragged vertices.

Not sure of the circuit off hand but did the Philips G6 have a form of direct and flywheel sync circuit? The May 1982 issue of "Television" listed further minor modifications I carried out during restoration. Needless to say the results on VHS were absolutely first class on both systems.

The Pye hybrids were never particularly good on VHS replay. I would go as far as to say unwatchable in unmodified form.
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Old 22nd Dec 2016, 11:56 am   #6
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Default Re: How to modify Pye CT205 to direct sync

Good morning.
I have been doing some experiments and have some promising results. By grounding the grid of the triode and feeding the line sync pulses direct to the grid of the oscillator (as Fernseh suggested), I can get a remarkably stable picture even from a noisy VHS tape full of dropouts! No sign of ragged verticals, but the whole picture does shift slightly from side to side as the video's servo adjusts the head drum speed but I could live with that.
The main problem is that the oscillator is triggered by the wrong part of the sync pulse. The pulse is 40V negative going and obviously the sharp falling edge should trigger flyback and hence the start of the next scan. Looking at the sync pulse and the output from the oscillator on the 'scope, It seems to be the rising edge doing the triggering and consequently the scan starts too late and finishes after the picture information. I therefore lose the first black bar of the stairstep pattern.
I need to get the thing to trigger from the falling edge so I might try inverting the sync pulse with a transistor and see if that works.
I have also been trying mods to the time constant of the existing flywheel circuit but with no success. Reducing the value of the caps, especially the 1 mfd causes all sorts of problems with pulling of various parts of the picture.

QUOTE
"The Pye hybrids were never particularly good on VHS replay. I would go as far as to say unwatchable in unmodified form."
I guess no one managed to sucessfuly modify them at the time and so far, the direct sync route seems more hopeful.
I will persevere with this approach.
Many thanks for the input
Cheers Nick
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Old 22nd Dec 2016, 2:07 pm   #7
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Default Re: How to modify Pye CT205 to direct sync

I'm absolutely sure we used to use a 26" Pye hybrid (697 chassis) with a VHS video recorder (Ferguson 3V30). I remember the 'Visionhire' installer turning his nose up at it, insisting that it couldn't possibly work, but it was absolutely fine. It's quite possible that we modified the line timebase, probably by changing the time constant of the flywheel circuit, but I don't remember any problems viewing VHS tapes on the Pye set.

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Old 22nd Dec 2016, 4:32 pm   #8
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Default Re: How to modify Pye CT205 to direct sync

An alternative idea to the sync inverter would be an additional sync separator supplied from the emitter of the video phase inverter transistor.
The negative going syncs can drive a pnp transistor of the BC557 type. The syncs present at the collector will be positive and of the correct polarity to trigger the sine wave oscillator.
In essence the new sync separator will be upside down version of the receiver's sync separator stage.

DFWB.
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