Surprise today - UK Vintage Radio Repair and Restoration Discussion Forum

Pete_kaye · 5th Jun 2021, 11:25 am

I tried to use a power supply that had been unused for 3 years (2V, 90V, 120V) and it tripped the wall socket .My kitchen has been rewired recently to 'latest standards' , professionally. I found that the Neutral pin on the power supply is s/c to earth and case . Is this correct? It is a home made device which worked well last time. I can post photos and circuit later.

kellymarie · 5th Jun 2021, 11:29 am

Thats why it tripped neutral and earth must remain separate thruout your instalation the only place a joint is allowed is at the service head or elsewhere on the DNO side of the network

Radio Wrangler · 5th Jun 2021, 11:33 am

You normally wind up with a couple of volts on neutral with respect to earth due to ohmic drop in the neutral connections. Short neutral to earth and the earth will carry a bit of the current, plenty enough to trip an RCD.

So no-surprise. It doesn't always happen, but enough to be quite common.

David

Dave Moll · 5th Jun 2021, 11:35 am

I assume that there was no RCD in the circuit when it "worked well", so a s/c neutral to earth would have gone undetected. Any such connection will trip an RCD every time as there will be an imbalance between the current through the phase and that through the neutral - exactly what an RCD is designed to detect.

If you wish to use this power supply, it will be necessary to ensure isolation between the neutral and anything earthed.

Pete_kaye · 5th Jun 2021, 6:20 pm

Am I mistaken but was there a sticky thread on home built power supplies like mine? I can’t find it anywhere. I want to compare circuits.

Pete_kaye · 5th Jun 2021, 6:52 pm

Found it. Via vintage-radio.com projects to build

Sideband · 5th Jun 2021, 7:03 pm

Quote:

Originally Posted by Pete_kaye

I found that the Neutral pin on the power supply is s/c to earth and case .

The question is 'Why'?

ValvoStef · 6th Jun 2021, 11:40 am

Quote:

Originally Posted by Sideband

Quote:

Originally Posted by Pete_kaye

I found that the Neutral pin on the power supply is s/c to earth and case .

The question is 'Why'?

To eliminate a possible voltage between neutral and earth on old installations with insufficient earthing arrangements. Can’t think of any other reason.

Chrispy57 · 6th Jun 2021, 12:31 pm

Hi - I wondered if the quote "I found that the Neutral pin on the power supply is s/c to earth and case" means that a resistance check revealed that as a fault condition that has arisen, or inspecting the wiring showed an intentional direct wired connection? Two different scenarios that would require different remedial action.

Good luck with it
Chris

factory · 6th Jun 2021, 1:43 pm

Quote:

Originally Posted by Pete_kaye

Found it. Via vintage-radio.com projects to build

Was it built to the circuit shown here?
https://www.vintage-radio.com/projec...y-set-psu.html

I can't see why it would have a short from the case ground to neutral, unless there is a fault such as a trapped wire to the transformer or another fault.

Edit: maybe not that one as the first post mentions three outputs of 2V, 90V & 120V.

David

Maarten · 7th Jun 2021, 10:28 am

Quote:

Originally Posted by Sideband

Quote:

Originally Posted by Pete_kaye

I found that the Neutral pin on the power supply is s/c to earth and case .

The question is 'Why'?

I'd say a paper cap in the mains filter, if present.

Pete_kaye · 7th Jun 2021, 10:30 am

Sorted after an early breakfast. It turned out that a short was occurring due to worn insulation on the N pin to transformer which only happened when the back plate was screwed in. It gave good voltages except for the one labelled 1.4v reading 1.7 .

Sideband · 7th Jun 2021, 12:20 pm

Quote:

Originally Posted by Pete_kaye

It gave good voltages except for the one labelled 1.4v reading 1.7 .

That will need sorting as 1.4V valves don't like any sort of overload. Is it stabilised? (I think it should be if it isn't). Maybe it will read closer to 1.4V on load.

PJL · 7th Jun 2021, 1:07 pm

The circuit you mentioned is for later miniature valve battery sets and includes a stabilised 1.4V. The example you have has been modified for earlier radios that use 2V filaments and a grid bias battery. Most likely the extra voltages are created using simple potential dividers.

I built a similar one but with a higher AC volts from the transformer and no voltage doubling. In mine, the grid bias voltages are taken from the HT supply by 'stealing' some HT voltage with a zener.

Pete_kaye · 8th Jun 2021, 9:01 am

This is the circuit. It was built by the late Trevor Beamond(G3VLF) and given me by his sister recently.

PJL · 8th Jun 2021, 2:00 pm

Looks good, the Zener in the HT centre tap is a bit odd but I guess the HT volts was too high. Providing the transistor that supplies the 1.4V is still OK, I would expect the voltage to fall under load but I suggest you test it with a dummy load.

Herald1360 · 8th Jun 2021, 3:57 pm

Doesn't the HT- to centre tap zener provide the -ve bias supply voltage scaled by the resistor chain for the different bias voltages?

A variation on a resistor in the same place to develop bias voltages for a set with a known constant HT current load (such as one with single ended class A output).

There're a few neat "wrinkles" in the design to get the differing HT and LT voltages too!

Pete_kaye · 11th Jun 2021, 11:04 am

I connected a 39 ohm across the 1.4 v and it went down to 1.44v . I think is ok

vinrads · 11th Jun 2021, 12:08 pm

Hi Peter I think you got the numbers reversed, neat power supply could be tempted to build one , Mick.

Cobaltblue · 11th Jun 2021, 1:15 pm

Quote:

Originally Posted by vinrads

Hi Peter I think you got the numbers reversed, neat power supply could be tempted to build one , Mick.

The 1.4V was previously reading 1.7V in post #12

Cheers

Mike T

5th Jun 2021, 11:29 am	#2
kellymarie Pentode Join Date: Sep 2020 Location: Weymouth, Dorset, UK. Posts: 135	Re: Surprise today Thats why it tripped neutral and earth must remain separate thruout your instalation the only place a joint is allowed is at the service head or elsewhere on the DNO side of the network

5th Jun 2021, 11:33 am	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,876	Re: Surprise today You normally wind up with a couple of volts on neutral with respect to earth due to ohmic drop in the neutral connections. Short neutral to earth and the earth will carry a bit of the current, plenty enough to trip an RCD. So no-surprise. It doesn't always happen, but enough to be quite common. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

5th Jun 2021, 11:35 am	#4
Dave Moll Dekatron Join Date: Feb 2005 Location: West Cumbria (CA13), UK Posts: 6,127	Re: Surprise today I assume that there was no RCD in the circuit when it "worked well", so a s/c neutral to earth would have gone undetected. Any such connection will trip an RCD every time as there will be an imbalance between the current through the phase and that through the neutral - exactly what an RCD is designed to detect. If you wish to use this power supply, it will be necessary to ensure isolation between the neutral and anything earthed. __________________ Mending is better than Ending (cf Brave New World by Aldous Huxley)

5th Jun 2021, 6:20 pm	#5
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Surprise today Am I mistaken but was there a sticky thread on home built power supplies like mine? I can’t find it anywhere. I want to compare circuits.

5th Jun 2021, 6:52 pm	#6
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Surprise today Found it. Via vintage-radio.com projects to build

5th Jun 2021, 11:25 am	#1
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Surprise today I tried to use a power supply that had been unused for 3 years (2V, 90V, 120V) and it tripped the wall socket .My kitchen has been rewired recently to 'latest standards' , professionally. I found that the Neutral pin on the power supply is s/c to earth and case . Is this correct? It is a home made device which worked well last time. I can post photos and circuit later. Last edited by Pete_kaye; 5th Jun 2021 at 11:27 am. Reason: Am I in wrong section?

6th Jun 2021, 12:31 pm	#9
Chrispy57 Hexode Join Date: Mar 2020 Location: Rochdale, Greater Manchester, UK. Posts: 452	Re: Surprise today Hi - I wondered if the quote "I found that the Neutral pin on the power supply is s/c to earth and case" means that a resistance check revealed that as a fault condition that has arisen, or inspecting the wiring showed an intentional direct wired connection? Two different scenarios that would require different remedial action. Good luck with it Chris

7th Jun 2021, 10:30 am	#12
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Surprise today Sorted after an early breakfast. It turned out that a short was occurring due to worn insulation on the N pin to transformer which only happened when the back plate was screwed in. It gave good voltages except for the one labelled 1.4v reading 1.7 . Attached Thumbnails Last edited by Pete_kaye; 7th Jun 2021 at 10:58 am.

7th Jun 2021, 1:07 pm	#14
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Surprise today The circuit you mentioned is for later miniature valve battery sets and includes a stabilised 1.4V. The example you have has been modified for earlier radios that use 2V filaments and a grid bias battery. Most likely the extra voltages are created using simple potential dividers. I built a similar one but with a higher AC volts from the transformer and no voltage doubling. In mine, the grid bias voltages are taken from the HT supply by 'stealing' some HT voltage with a zener.

8th Jun 2021, 9:01 am	#15
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Surprise today This is the circuit. It was built by the late Trevor Beamond(G3VLF) and given me by his sister recently. Attached Thumbnails

8th Jun 2021, 2:00 pm	#16
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Surprise today Looks good, the Zener in the HT centre tap is a bit odd but I guess the HT volts was too high. Providing the transistor that supplies the 1.4V is still OK, I would expect the voltage to fall under load but I suggest you test it with a dummy load.

8th Jun 2021, 3:57 pm	#17
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Surprise today Doesn't the HT- to centre tap zener provide the -ve bias supply voltage scaled by the resistor chain for the different bias voltages? A variation on a resistor in the same place to develop bias voltages for a set with a known constant HT current load (such as one with single ended class A output). There're a few neat "wrinkles" in the design to get the differing HT and LT voltages too! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

11th Jun 2021, 11:04 am	#18
Pete_kaye Octode Join Date: Jun 2005 Location: Sheffield, South Yorkshire, UK. Posts: 1,227	Re: Surprise today I connected a 39 ohm across the 1.4 v and it went down to 1.44v . I think is ok

11th Jun 2021, 12:08 pm	#19
vinrads Dekatron Join Date: Feb 2009 Location: Chesterfield, Derbyshire, UK. Posts: 3,762	Re: Surprise today Hi Peter I think you got the numbers reversed, neat power supply could be tempted to build one , Mick.