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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 15th Mar 2021, 8:07 pm   #1
Malcolm G6ANZ
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Default BBC colour coder and more

I'm not sure where to put this essay. It could be under vintage television or home construction or success stories. Way back I played around with amateur television transmitting, in those days you could transmit 625 line TV all over the 70cm band. Time moved on and my TV kit was put into store.
I recently became interested in CCTV again and hauled out the vision rack I had made up. Part of this rack was a BBC white module colour coder. The backplane had been hacked around quite a lot and I needed the full set of diagrams to sort out the mess. A wanted post on here resulted in a complete manual for the coder. Many thanks to Nimrod 121 (Guy). With this to hand I sorted out the backplane.
When I came to setting up the coder it seemed very sensitive to adjustments and was not really 'right' although it did produce colour bars from RGB input. The other strange problem was that the output only worked on one of my monitors, the other one refused to display anything. The monitor was checked with a Philips pattern generator and it produced a good picture.
My thoughts turned to the SPG I was using so I measured the crystal frequency from which all the timings were derived. The SPG was part of an EMI CT scanner that was scrapped form work in the 70's. The crystal frequency was a bit slow so I changed the trimmer capacitor and by careful adjustment got the frequency to be exact. It was so close that the scope trace of the pattern generator and the SPG were almost the same frequency. OK so the line speed was correct. I next checked the colour subcarrier from my home made generator. This was in spec as well. The pulses from the SPG card went through a homemade pulse distribution amplifier this is for Line drive, field drive, mixed sync, mixed blanking, burst gate and PAL switch. While looking at the timings I noticed that the pulses from the amplifier appeared to be narrower than those from the SPG. So much so that the blanking pulse was the same width as the sync pulse. After much head scratching I added coupling capacitors to the inputs of the amplifiers. This cured the narrow pulse problem and I could now set the colour burst in the correct place in the blanking area. With this done the monitor that refused to display a picture started working correctly. Please note that the colour subcarrier is free running and not locked to line frequency as a professional one would be. Also that the burst gate is set by a pair of monostables, one to set the position in the back porch and other to set the width.
I then followed the setup sequence for the coder. Again it was a bit 'iffy' so I decided to replace all the electrolytics. The unit was build the late 60's so they were probably past their best. With the caps replaced I went through the setup procedure again. This time the unit responded quite well and set up OK.
I could now produce colour bars from separate RGB inputs. I really wanted to do something extra. I found in a old BATC handbook a circuit for a colour synthesizer. In essence this takes a black and white picture and splits it into three levels, white, grey and black. Each level can then be assigned a different colour made up from RGB. On completion of the circuit there was a problem with the luminance in the coder. It looked as though the blanking pulse was inverted, so during the blanking period when the video should be suppressed it was being enabled. This depressed the real video giving a black screen. Chrominance was unaffected. I finally cured the problem by not using the final set of buffer gates in the logic section. This put the blanking pulse the correct way up and I had a proper false colour image. I think that the circuit must have been designed and not actually built or else this fault would have been discovered.
Apologies for the long screed but feel as though I have achieved a good result and learnt a lot about the obsolete PAL system. Pity it’s too late.

Malcolm
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Old 15th Mar 2021, 9:01 pm   #2
Nymrod121
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Default Re: BBC colour coder and more

Fascinating stuff ... glad the T.I.'s came in handy
All the best
Guy
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Old 15th Mar 2021, 9:44 pm   #3
beamcurrent
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Default Re: BBC colour coder and more

Impressive work, I remember those coders from 40 years ago, in my ATV setup. They were iffy then so you have done well. Mine drifted a lot, I had three of them!, I changed them for the link version, still 3U high and finally for the Cox 1U coder. I still use those.

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Old 16th Mar 2021, 12:16 am   #4
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Default Re: BBC colour coder and more

4.43361875MHz +/- 0.5Hz !

You've just given me a nostalgic trip. My project for the final year of my BSc was to design an IC-based colour synthesiser. The college lab was used over the summers for courses for IBA people, so there was cash around and a lot of TV-related test and production equipment. Choosing a TV related project was a way to dodge the shortage of general lab equipment. I had a lab to myself.

Looking back, the MC1496 i used were OK, but using a diode ring would have been better IF the signals and LO had been conveyed as currents rather than voltages... meaning current source drive for the signal, and a square wave current source (balanced) for the subcarrier. Filter afterwards to remove subcarrier harmonics.

And after a hard day on the project (!) walk down across Leeds centre and stick my nose in at M&B's to see what they'd got in recently.... them were the days.

David
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