UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Baird T18

TNC · 14th Feb 2007, 4:54 pm

Well I think I it's sorted.

The focus coil's indicated resistance is 1180 ohm, voltage in was 340 with a 50 volt drop across.
Shunting this coil made the focus even worse.
I then chcked the focus VR (VR3) - this is 50k and to get the best focus (which was awful) it was set to offer zero resistance. No point in shunting!

It seemed to me that the coil needed more umph so I decided to shunt R12 which is nominally 20k but in this case reads about 13.7k. Using a 2w 6k shunt resistance the focus finally resolved with the adjustable V3 set at 9.7k.
The shunt now makes R12 read 4.17k so with the variable resistance at 9.7k there is a total of 13.24k between the coil and the earth rail. Without the shunt, the circuit's lowest resistance was 13.72k.
The shunt runs warm but I make the current to be about 14ma although I may not have worked that out correctly...I am sure someone will correct me!

Anyway the Brimar Cintel C12B is now happily displaying for the first time in many years (despite only at 5kv instead of its preferred10kv)...and I will be have to drop the previous owner a line to let him know the good news! (Incidentally, yes - he did watch the Coronation on it!)

Thanks to all for great inoput/assistance. I will post a couple of .jpgs when I get the chance.
atb
Trevor.

14th Feb 2007, 4:54 pm	#7
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 385	Re: Baird T18 - focus problem. Well I think I it's sorted. The focus coil's indicated resistance is 1180 ohm, voltage in was 340 with a 50 volt drop across. Shunting this coil made the focus even worse. I then chcked the focus VR (VR3) - this is 50k and to get the best focus (which was awful) it was set to offer zero resistance. No point in shunting! It seemed to me that the coil needed more umph so I decided to shunt R12 which is nominally 20k but in this case reads about 13.7k. Using a 2w 6k shunt resistance the focus finally resolved with the adjustable V3 set at 9.7k. The shunt now makes R12 read 4.17k so with the variable resistance at 9.7k there is a total of 13.24k between the coil and the earth rail. Without the shunt, the circuit's lowest resistance was 13.72k. The shunt runs warm but I make the current to be about 14ma although I may not have worked that out correctly...I am sure someone will correct me! Anyway the Brimar Cintel C12B is now happily displaying for the first time in many years (despite only at 5kv instead of its preferred10kv)...and I will be have to drop the previous owner a line to let him know the good news! (Incidentally, yes - he did watch the Coronation on it!) Thanks to all for great inoput/assistance. I will post a couple of .jpgs when I get the chance. atb Trevor.