01-08-2012, 06:29 PM
(This post was last modified: 01-08-2012, 06:36 PM by Robert Darwent.)
Hi John,
First of all, the dial scan;
The inside of the dial glass was very dusty and I was unable to remove everything whilst cleaning for fear of also removing the legend. So some of the dust inevitably ended up being prominent on the scan. However, I think I've removed the worst of it digitally.
Second, the point where the aerial coupling capacitor is soldered to;
I hope its clear enough in the image (please excuse the dust as I have yet to clean the chassis properly) but the capacitor is soldered to the first tag on the switch wafer below the long mounting rod.
Regarding your capacitive dropper calculation made today, for a 110V supply and a 70V heater at 0.3A with a 10V surge limiter, I get 12.65 uF with 33 ohms, 3.0W for the limiter. You've somehow arrived at 10.68 uF. In any case wouldn't it be more practical for you to design the dropper for a 220V supply (which I believe is the mains in Switzerland) rather than 110V?
For my set, I am intending to use a 240V to 80V capacitive dropper for the heater chain/dial bulbs. With a seperate 240V to 120V diode/resistor dropper to provide the HT, I calculate a 330 ohm resistor at 7.5W will suffice in series with a 1N4004 diode. It's something of a compromise as regards heat, but physically small sized metal clad 330 ohm resistors at 25 or 50W are available relatively cheaply and being vastly over-rated for this application should run quite cool.
Regards
First of all, the dial scan;
The inside of the dial glass was very dusty and I was unable to remove everything whilst cleaning for fear of also removing the legend. So some of the dust inevitably ended up being prominent on the scan. However, I think I've removed the worst of it digitally.
Second, the point where the aerial coupling capacitor is soldered to;
I hope its clear enough in the image (please excuse the dust as I have yet to clean the chassis properly) but the capacitor is soldered to the first tag on the switch wafer below the long mounting rod.
Regarding your capacitive dropper calculation made today, for a 110V supply and a 70V heater at 0.3A with a 10V surge limiter, I get 12.65 uF with 33 ohms, 3.0W for the limiter. You've somehow arrived at 10.68 uF. In any case wouldn't it be more practical for you to design the dropper for a 220V supply (which I believe is the mains in Switzerland) rather than 110V?
For my set, I am intending to use a 240V to 80V capacitive dropper for the heater chain/dial bulbs. With a seperate 240V to 120V diode/resistor dropper to provide the HT, I calculate a 330 ohm resistor at 7.5W will suffice in series with a 1N4004 diode. It's something of a compromise as regards heat, but physically small sized metal clad 330 ohm resistors at 25 or 50W are available relatively cheaply and being vastly over-rated for this application should run quite cool.
Regards
