[Mikey405]

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.