Hi All,
I'm sure this really is my last update before Malaysia.
Quote:
Originally Posted by line sync
Very neatly drawn circuit diagram of the vision RF stages andy , must have tock you a while to do that.
Robin
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Thanks Robin.
Drawing circuits is part of my day job, though there are no valves used at work! When I started drawing out the sound circuit I started off using rectangular resistor symbols which looked out of place, so I changed them for the zig-zag style symbol. I also made a mistake in the vision circuit in that the sensitivity control should actually be labled contrast.
Anyway. I unsoldered the contrast control (VR1) and found a dry joint at one end, so now I had end to end continuity, but nothing on the wiper. I took the control apart by squeezing its metal body slightly with pipe grips so that the metal end cover came off. I did bend the wipers in the process, so I couldn't say what was causing the problem. After bending the wipers to shape and greasing the wirewound track it seemed to work ok.
I replaced R14 (10K) with a vintage type. R8 (5K1) I had to replace with a vintage 4K7 type. Interestingly if I heated up R8 very hot with a soldering iron it would read ok until it cooled and flicked back to around 50K, very odd.
I fitted EF91 pentodes and an EB91. I connected the vision reciever to the HT of the radio and connected the heaters to the DC bench supply set to 6.3V. I hooked up the scope to vision output 1 and fed in test card C from the Aurora and hey presto it worked
So the photos show the contrast control, R8 before and after replacement, the RF unit connected to the radio chassis and the output on the scope.
Cheers
Andy