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Old 9th Oct 2020, 12:43 pm   #1
Mikey405
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Default Poorly Philips A4 Chassis (AKA Pye 717)

Hi all.

After spending many man-days on a Philips G18C571/01 (Basically a Pye CT200/1 in a small G8-a-like cabinet with a few very slight differences to the circuit), Tas (of this forum) and I are rather stumped by an odd line-phase problem.

The picture is shifted on the raster a long way to the right. (The attached image has the brightness turned up and the contrast turned down in order to show where the raster starts on the left hand side.)

There is a "line phase" control on these sets which alters the "Pulse Width" circuit inside the TBA920 line oscillator chip but this control has very little effect on the picture - in fact it has virtually no effect whatsoever until the control is turned almost fully clockwise, and then the oscillator stops oscillating.

The voltages around the chip are mostly about 0.8 volts high (measured with a DMM) compared with those shown on the diagram.

Interestingly, the voltage on pin 3 (the phase control) is about 2 volts too high. If R614 (3K3) is increased in value to 4K7 then the phase control has more effect and the picture can be centred (the voltage on pin 3 is then roughly correct). This, however, isn't really cricket - and with the component changed to the wrong value, there is a "wobble" on the verticals, which isn't there with the correct value resistor.

The feedback from the line output transformer is fed directly from winding 664 through R512 to the chip and there's nothing much which might alter its phase. There is some kind of phase-changing network, being C610, R611 and C609 which are fed from the sync separator portion of the IC and then fed back into the line oscillator phase detector via pin 6. These components have all been replaced without effect.

All the waveforms around the IC look exactly like those in the manual, the chip itself has been replaced along with almost all directly-connected components - especially any which may have an effect on the phase component of the line oscillator - and still the fault eludes us.

So... Does anyone have any ideas?

Answers should be addressed to Tas, as the set is now back down with him - although I shall be following with interest.

Thanks all.

Kind regards.

From Mike.
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Old 9th Oct 2020, 12:50 pm   #2
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

You don't fool us with your A4 nonsense - it's still a Pye CT200!
Although you've replaced the IC, ISTR the mono 320 did do this when the wrong spec IC had been fitted. I think it had some sort of suffix on it. However I'm clutching at the same straws you've been clutching at, I suspect.
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Old 9th Oct 2020, 7:13 pm   #3
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

As an alternative thought...could there be some sort of DC shift on the scan coils?
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Old 9th Oct 2020, 10:11 pm   #4
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Hi Glyn and SB.

Re the Philips "A4" thing - I thought you'd like that. And regarding the TBA chip. that would have been an excellent suggestion but for the fact that it was the original device fitted to the set.

Tim J. (Studio 263) of this forum had an interesting suggestion re the sync separator triggering on the burst rather than the sync pulse so a check of the video DC levels into the chip may reveal something.

Regarding DC in the scan coils, the raster itself is correctly centred, SB, it's just the picture within the raster. I think they used to call it "False Line Lock" in old books.

Thanks all.

Kind regards.

From Mike.
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Old 9th Oct 2020, 11:17 pm   #5
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Isn't there a line phasing circuit in the TBA920, essentially a picture shift control? I can remember having this fault in the Invicta version of this set. Need to look up the internal architecture to find the phasing part.

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Old 10th Oct 2020, 12:46 am   #6
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

A topic about the Philips 570 was started in January 2018 by forum member Tas:

https://www.vintage-radio.net/forum/...d.php?t=142998

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Old 10th Oct 2020, 9:58 am   #7
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Is there colour? That might point somewhere, or then again it might not. Having a look under the bonnet of the TBA might reveal something unexpected.
On an old TV you'd be looking at the flywheel diodes, so something must be upsetting that part of the circuit. Bearing in mind the iconic IF strip, could the video signal be distorted in a very strange way? More straws, I'm afraid...
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Old 18th Oct 2020, 7:19 pm   #8
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Default Re: Poorly Philips A4 Chassis (AKA Pye 717)

Hello Mike, Glyn, Rich, and David.

Thanks for the replies. Unfortunately, I have been side tracked into looking at a load of my transistor radios that I have ben toying with the idea of getting shot of. The reason being that I had no room to even wok on this little tv so no updates at the moment. I'm determined to get to the bottom of the fault though so it's not forgotten!
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