12-09-2016, 07:53 AM
As Terry said, it was a great day. On the TV engineering side full credit must go to John Wakely and Peter Sanders who perform the Sisyphean task of keeping the TV display in good order. Due recognition for all the team. Eileen in overall charge, Dave Grant, Richard Stowe, Chris Rowe. Apologies to any who I have failed to mention.
I went to the final maintenance session on the Friday evening before the event. We finally tracked down the intermittent hum that was afflicting BBC1. This has been annoying us for several weeks, if not months. At first I thought the PSU for the Auroras was a bit iffy and changed it. Then I swapped out the BBC1 Aurora so I could look at it on my bench at home. It's a very early one which can suffer hum problems due to a peculiarity of the modulator chips. I changed the 2 relevant resistors and took it back with me on Friday. John Wakely said that he'd seen the hum with the repalcement Aurora which was known to be OK so back to square one.
I forget whether it was John or Peter who had the brainwave, observing that the hum appeared when one group of sets was switched on. We unplugged them one by one until we found the guilty set. A Pye LV20. I've attached a partial circuit of the almost identical B18T. John soon found a problem with the aerial input. This resulted in what was probably a neutral earth short and lots of 50Hz flowing round the co-axial outers of the aerial distribution. It was going to be intermittent because the neutral potential is always variable wrt earth. If the set's mains had been reversed the result would have been a blown fuse and hopefully no further damage.
I went to the final maintenance session on the Friday evening before the event. We finally tracked down the intermittent hum that was afflicting BBC1. This has been annoying us for several weeks, if not months. At first I thought the PSU for the Auroras was a bit iffy and changed it. Then I swapped out the BBC1 Aurora so I could look at it on my bench at home. It's a very early one which can suffer hum problems due to a peculiarity of the modulator chips. I changed the 2 relevant resistors and took it back with me on Friday. John Wakely said that he'd seen the hum with the repalcement Aurora which was known to be OK so back to square one.
I forget whether it was John or Peter who had the brainwave, observing that the hum appeared when one group of sets was switched on. We unplugged them one by one until we found the guilty set. A Pye LV20. I've attached a partial circuit of the almost identical B18T. John soon found a problem with the aerial input. This resulted in what was probably a neutral earth short and lots of 50Hz flowing round the co-axial outers of the aerial distribution. It was going to be intermittent because the neutral potential is always variable wrt earth. If the set's mains had been reversed the result would have been a blown fuse and hopefully no further damage.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
