Marconi T11DA Capacitor Dropper - UK Vintage Radio Repair and Restoration Discussion Forum

Top Cap · 4th Oct 2008, 1:24 pm

I was asked to get going a Marconi T11DA that arrived complete with rotten and crumbling line dropper cord

.
It was the first time I have came up against one of these since I was an apprentice all those many years ago when it was a simple task of going to the store room for either .15 or .3 dropper cord and cutting off the correct length. Those days of course have now gone and I have yet to find any line cord for sale on the internet, so I looked up the T11DA on the Forum and found this excellent thread :-
https://www.vintage-radio.net/forum/...ighlight=t11da
So I set about converting the radio to capacitor dropper and mounted the required components on a similar piece of tag board as shown in the photo of that thread provided by Barrie. My interpretation is shown in the attached line sketch.
Two holes were drilled into the chassis to place the tag board just to the left of an existing hole which I fitted with a suitable grommet.
I must confess to being very apprehensive at switch on and glad I had a Variac, just in case. At 230V I was rewarded with excellent reception, in fact one of the best performing radios on medium wave here in my tower block, stations all over the dial instead of just the usual three. Perhaps it is the frame aerial, all my other radios use ferrite rods.
The 117V heater rail was slightly high by a couple of volts which I reckoned would be all right, evened out over the chain of five valves.
So my first capacitor dropper installation had been done and I was rather impressed with the way it all worked. The attached photo shows the final view after a safety cover had been fitted using threaded pillars to secure both the board and cover to the chassis, finally being topped off with two nylon screws. I just hope this thread encourages others to have a go when a similar problem arises with them.
Les

Sideband · 4th Oct 2008, 8:49 pm

Nothing wrong with a capacitor dropper...the idea is by no means new (as anyone who remembers a certain Thorn black and white chassis will know).

Some people are uneasy about what will happen if the capacitor(s) go short. I think this is very unlikely if you use good capacitors rated for mains voltage and motor-start types will be if you need larger values. Then fit parallel X2 types to adjust the value for the exact voltage.

Good work and very neat.

Rich.

julie_m · 5th Oct 2008, 6:08 pm

The thermostat / ignition controller boards fitted to modern gas boilers often use capacitive voltage droppers, as (1) they don't consume much power anyway and (2) flame detection requires a common connection with mains earth or neutral. If you use the neutral as common, then you get an added benefit of polarity-sensitive operation.

What they do there to mitigate the risk of short-circuit capacitor failure is to fit a metal-film resistor in series with the capacitor. If the capacitor ever fails short-circuit, the resistor promptly fails open-circuit.

4th Oct 2008, 1:24 pm	#1
Top Cap Octode Join Date: Mar 2006 Location: Watford, Hertfordshire, UK. Posts: 1,270	Marconi T11DA Capacitor Dropper I was asked to get going a Marconi T11DA that arrived complete with rotten and crumbling line dropper cord . It was the first time I have came up against one of these since I was an apprentice all those many years ago when it was a simple task of going to the store room for either .15 or .3 dropper cord and cutting off the correct length. Those days of course have now gone and I have yet to find any line cord for sale on the internet, so I looked up the T11DA on the Forum and found this excellent thread :- https://www.vintage-radio.net/forum/...ighlight=t11da So I set about converting the radio to capacitor dropper and mounted the required components on a similar piece of tag board as shown in the photo of that thread provided by Barrie. My interpretation is shown in the attached line sketch. Two holes were drilled into the chassis to place the tag board just to the left of an existing hole which I fitted with a suitable grommet. I must confess to being very apprehensive at switch on and glad I had a Variac, just in case. At 230V I was rewarded with excellent reception, in fact one of the best performing radios on medium wave here in my tower block, stations all over the dial instead of just the usual three. Perhaps it is the frame aerial, all my other radios use ferrite rods. The 117V heater rail was slightly high by a couple of volts which I reckoned would be all right, evened out over the chain of five valves. So my first capacitor dropper installation had been done and I was rather impressed with the way it all worked. The attached photo shows the final view after a safety cover had been fitted using threaded pillars to secure both the board and cover to the chassis, finally being topped off with two nylon screws. I just hope this thread encourages others to have a go when a similar problem arises with them. Les Attached Thumbnails __________________ Whether the Top Cap is Grid or Anode - touching it will give you a buzz either way!

4th Oct 2008, 8:49 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,580	Re: Marconi T11DA Capacitor Dropper Nothing wrong with a capacitor dropper...the idea is by no means new (as anyone who remembers a certain Thorn black and white chassis will know). Some people are uneasy about what will happen if the capacitor(s) go short. I think this is very unlikely if you use good capacitors rated for mains voltage and motor-start types will be if you need larger values. Then fit parallel X2 types to adjust the value for the exact voltage. Good work and very neat. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

5th Oct 2008, 6:08 pm	#3
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: Marconi T11DA Capacitor Dropper The thermostat / ignition controller boards fitted to modern gas boilers often use capacitive voltage droppers, as (1) they don't consume much power anyway and (2) flame detection requires a common connection with mains earth or neutral. If you use the neutral as common, then you get an added benefit of polarity-sensitive operation. What they do there to mitigate the risk of short-circuit capacitor failure is to fit a metal-film resistor in series with the capacitor. If the capacitor ever fails short-circuit, the resistor promptly fails open-circuit. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.