PAL Receiver Shows a Black-and-White Picture
A 29-inch PAL-D/K colour CRT television produces a sharp black-and-white picture and good sound, but no colour on any channel. The customer says colour worked intermittently for a few weeks before failing completely. On the bench, the video detector output has a normal composite signal. At the chroma input to the delay line, the oscilloscope shows a burst of about 280 mVpp and the chroma signal is present. The 4.43 MHz crystal oscillator test point shows no oscillation. When a signal generator injects 4.43 MHz into the oscillator stage, colour appears, but the hues are wrong and the picture has horizontal colour bars that drift. The color killer control is at its factory mark. The set uses a PAL decoder with a glass delay line and a 4.43 MHz crystal. The workshop has a frequency counter, an oscilloscope, and a PAL pattern generator.
1. Explain why the picture is black and white and why colour returns with an external oscillator but with wrong hues.
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The burst and chroma signal are present at the delay-line input, so the tuner, video detector, and chroma amplifier are delivering colour information. The local 4.43 MHz subcarrier oscillator is not running, however. In a PAL decoder, the two synchronous demodulators need a locally regenerated subcarrier whose phase is locked to the transmitted burst. Without it, the R−Y and B−Y demodulators cannot recover the colour difference signals, so the set displays only luminance. The color killer may also mute the chroma channel when the oscillator or burst phase is missing.
When an external 4.43 MHz signal is injected, the demodulators produce colour, but the generator is not phase-locked to the station burst. The demodulation phase therefore changes relative to the transmitted chroma phase, giving incorrect hues. Because the external oscillator is free-running and not tied to the burst, its phase drifts, producing the slowly moving horizontal colour bars. This confirms that the decoder and CRT are capable of colour; the fault is in the local oscillator or its phase-lock/burst phase loop.
2. List the tests you would perform to isolate the oscillator fault and rule out the color killer.
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- Check the DC supply and bias to the 4.43 MHz oscillator transistor; an open resistor or dried capacitor can stop oscillation.
- Measure the crystal with a known-good test oscillator or substitute a known 4.43361875 MHz crystal. A cracked crystal or drifted trimmer is common after intermittent operation.
- Use the frequency counter at the oscillator test point; expect approximately 4.43361875 MHz when free-running and locked to burst.
- Inspect the oscillator coil and trimmer for contamination or a broken solder joint; tap the board gently to see whether oscillation returns.
- Check the burst phase and killer control voltages. Because burst is present, a correctly adjusted killer should not mute the channel. If the killer voltage is wrong, trace the killer detector and threshold preset after the oscillator is restored.
- Confirm that the delay line and PAL switch are not open; a missing delay-line signal can also remove colour, but here the oscillator test is decisive.
3. Explain the correct adjustment procedure after repair, including burst phase and hue.
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Replace the faulty crystal, oscillator transistor, or bias component, then set the oscillator to its free-running frequency with the trimmer. Use a PAL pattern generator and a vectorscope or colour-bar display. First adjust the oscillator trimmer for stable lock and correct 4.43361875 MHz. Then adjust the burst phase so the demodulation axes are correct; the red and blue bars should be pure and the yellow/green/cyan bars should not show heavy cross-colour. Check that the hue control gives a symmetrical range around the correct setting rather than compensating for a phase error. Verify the PAL switch and delay line by observing the Hanover bars or a known PAL test pattern; any alternating hue error indicates a PAL switch or delay-line problem. Finally, check colour killer threshold and colour-on/off behaviour on a weak signal, then run the set for 30 minutes to confirm that the oscillator does not drift as it warms.