Ultra R906: Power Supply Mods - UK Vintage Radio Repair and Restoration Discussion Forum

HowardB · 5th Jun 2020, 3:00 pm

I'm finally gearing up to work on my R906, the case is off and it's on the bench now. Just looking for advice regarding the power supply...

Unfortunately, mine didn't come with the original mains plug and someone has wired the mains lead directly to the socket, so if you were to put your hand round the back of the set and touch the pins - lethal!! Obviously I need to correct this, but it's got me thinking. If I'm never going to use batteries, I could wire it so that the Mains/Battery switch (S4) is bypassed.

Also, looking at the circuit diagram, I notice that the On/Off switch (S3) doesn't properly disconnect the set from the mains. When in the 'Off' position, the Neutral is disconnected, but not the Live. The Live feeds through the rectifier and dropper resistors to generate the HT at C20, even when the set is Off. Surely, that can't be right?

I'm therefore thinking of wiring it so that;
a) the Mains/Battery switch is bypassed
b) the On/Off switch properly disconnects the set - i.e. both Live and Neutral are switched before they reach any components.

If I'm correct, this relatively small change will improve the safety of the set, but on the other hand, it will affect its originality.

Does anybody have any thoughts on this?

HowardB

TonyDuell · 5th Jun 2020, 3:51 pm

With S3 turned off, there is no connection to one side of the mains (probably the neutral side), so no complete circuit to the mains. Therefore C20 will not be charged.

That said, a double pole mains switch would be safer. But unplugging the set from the mains when not in use is even safer (switches can break down, I've never seen a mains plug that leaps into a socket...)

Personally, I would keep the set 'as original' with the battery switch and connectors correctly wired up. And I'd probably run it off an isolating transformer if I wanted to listen to it.

HowardB · 5th Jun 2020, 6:54 pm

Thanks Tony, it's always good to get a second opinion. I understand what you are saying about C20 but I still wouldn't touch it if my feet were well-grounded.

TonyDuell · 5th Jun 2020, 6:57 pm

I wouldn't touch any internal part of that set when connected to the mains if I was well-grounded!

5th Jun 2020, 3:51 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Ultra R906: Power Supply Mods With S3 turned off, there is no connection to one side of the mains (probably the neutral side), so no complete circuit to the mains. Therefore C20 will not be charged. That said, a double pole mains switch would be safer. But unplugging the set from the mains when not in use is even safer (switches can break down, I've never seen a mains plug that leaps into a socket...) Personally, I would keep the set 'as original' with the battery switch and connectors correctly wired up. And I'd probably run it off an isolating transformer if I wanted to listen to it.

5th Jun 2020, 6:54 pm	#3
HowardB Pentode Join Date: Dec 2017 Location: Staines-Upon-Thames, Surrey, UK. Posts: 122	Re: Ultra R906: Power Supply Mods Thanks Tony, it's always good to get a second opinion. I understand what you are saying about C20 but I still wouldn't touch it if my feet were well-grounded.

5th Jun 2020, 6:57 pm	#4
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,208	Re: Ultra R906: Power Supply Mods I wouldn't touch any internal part of that set when connected to the mains if I was well-grounded!