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Niechcial,Steve · 1st Dec 2021, 3:04 pm

I have now come back to this for some detailed fault finding. Just to summarise, scan, EHT regulation, convergence and sync faults are sorted, and electros blanket tested and replaced as necessary throughout.

I next tried setting up the grey scale and found that despite having the tube show near perfect on the B and K tester, the grey scale would not track over the usual brightness range. The problem was no clamping pulse on the three PCL84 colour difference amplifiers. Diode 5D12 on the convergence board, which feeds line pulses in, had gone o/c. Replacing that resulted in a very good definition black and white picture with correct grey scale

I am now left with three faults:
1) Bizarrely wrong colours on the colour bars.
2) Smearing/overshoot on the left hand horizontal lines of the test card into the grey area of the circle beyond.
3) Green tinted broad horizontal band over the middle (black and white part) of a test card.
Taking these in reverse order, shorting out the U and V inputs to the colour output valves confirms that the green tint is coming in mainly on the V (R-Y) channel from the decoder.
I am not sure whether the smearing/overshoot is coming is on the luminance output stage (which is largely rebuilt), or coming in from the IF panel. I don’t suspect alignment, as the picture is otherwise good definition . I will feed in a video signal from another source into the output stage to test.
The wrong colours are the most interesting fault. For the colour killer to be switching the colour in (which it is), not only does the chroma derived bias have to be present but so do bistable pulses. The ident which syncs the bistable is there looking exactly as it should on the scope, but I guess there is the possibility of some strange phase error in there somewhere. I will start by viewing the individual blue and red channels on the screen to see if both are contributing to wrong colours. If so, I will start by checking out the sync of the reference oscillator and take it from there. I suspect and hope that the green tint will vanish once I have sorted out the more major decoder fault.

1st Dec 2021, 3:04 pm	#23
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: Bush Dual Standard Colour Restoration I have now come back to this for some detailed fault finding. Just to summarise, scan, EHT regulation, convergence and sync faults are sorted, and electros blanket tested and replaced as necessary throughout. I next tried setting up the grey scale and found that despite having the tube show near perfect on the B and K tester, the grey scale would not track over the usual brightness range. The problem was no clamping pulse on the three PCL84 colour difference amplifiers. Diode 5D12 on the convergence board, which feeds line pulses in, had gone o/c. Replacing that resulted in a very good definition black and white picture with correct grey scale I am now left with three faults: 1) Bizarrely wrong colours on the colour bars. 2) Smearing/overshoot on the left hand horizontal lines of the test card into the grey area of the circle beyond. 3) Green tinted broad horizontal band over the middle (black and white part) of a test card. Taking these in reverse order, shorting out the U and V inputs to the colour output valves confirms that the green tint is coming in mainly on the V (R-Y) channel from the decoder. I am not sure whether the smearing/overshoot is coming is on the luminance output stage (which is largely rebuilt), or coming in from the IF panel. I don’t suspect alignment, as the picture is otherwise good definition . I will feed in a video signal from another source into the output stage to test. The wrong colours are the most interesting fault. For the colour killer to be switching the colour in (which it is), not only does the chroma derived bias have to be present but so do bistable pulses. The ident which syncs the bistable is there looking exactly as it should on the scope, but I guess there is the possibility of some strange phase error in there somewhere. I will start by viewing the individual blue and red channels on the screen to see if both are contributing to wrong colours. If so, I will start by checking out the sync of the reference oscillator and take it from there. I suspect and hope that the green tint will vanish once I have sorted out the more major decoder fault.