UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 24th Sep 2018, 9:21 am   #1
G3VKM_Roger
Heptode
 
G3VKM_Roger's Avatar
 
Join Date: Jun 2012
Location: Southeast Norfolk, UK.
Posts: 514
Default Transformer safety?

Hi All,

I'm a little out of my comfort zone here, as I'm usually involved with amateur and military radio. Domestic sets are not something I'm too familiar with so I'm seeking some safety advice!

I have here a Murphy A362 AM/FM set which has been in my family from new and has been in dry storage for the past 20 years or so. I have a copy of the Trader's sheet and I see from the cct diagram that the mains transformer is what I'd call an "autotransformer" in other words a single, tapped, winding.

Would anyone care to comment on whether there are any safety issues around such a set, that wouldn't be an issue if the set used a conventional transformer with separate windings? My daughter has expressed a desire for a "Bakelite Radio" for her flat and whilst I'm competent to change out capacitors, etc. on elderly radios, the mil/ham sets I work on are for my own use and I wouldn't want to put her or others at risk.

I am aware of equipment like the DAC90 with live chassis, etc. and I know that doesn't apply in the case of the A362 but any advice would be welcome. It's very unlikely that an external aerial would be needed with the set as it will be used in central London.

Cheers

Roger
G3VKM_Roger is offline  
Old 24th Sep 2018, 9:41 am   #2
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,409
Default Re: Transformer safety?

If the set has an autotransformer the same safety issues apply as with the likes of the DAC90(A). You don't get the hot heater dropper to contend with nor the wasted power though.

Up and running with all its covers on, knobs fitted etc. it's safe enough for use by Joe Public; for working on with covers off take the same precautions as for any live mains equipment. Repair wise replace any mains connected capacitors with X or Y types as required, otherwise just use anything suitably rated.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline  
Old 24th Sep 2018, 9:41 am   #3
Studio263
Octode
 
Studio263's Avatar
 
Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,500
Default Re: Transformer safety?

Sets with an autotransformer are just like those which use the conventional AC/DC technique, e.g. the neutral side of the mains is connected to the chassis. The autotransformer takes the place of the dropper resistor, but being inductive means that less energy is lost as heat.

AC/DC sets (and therefore autotransformer ones as well) are in my view much safer than those with double wound transformers. If the lead is wired correctly, you KNOW that the chassis can only be a few volts away from ground potential. With a double wound transformer, leakage could occur at any point in the winding, meaning that the chassis potential could be anything up to full mains. AC/DC (etc) sets normally have protective measures (capacitor isolated sockets, non-accessible screws) to guard against accidental wrong connections, AC-only sets rarely do. Remember the only insulation in a transformer of this type is 50+ year old paper and wax, not something to rely upon.
Studio263 is offline  
Old 24th Sep 2018, 9:45 am   #4
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,898
Default Re: Transformer safety?

Auto transformers will not work on DC.

With an isolating transformer you can earth the chassis, seldom does this stress the insulation too much in my experience.
Boater Sam is offline  
Old 24th Sep 2018, 9:45 am   #5
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 7,045
Default Re: Transformer safety?

The set has an 3 core mains lead which is unusual for a set like this but it is not connected directly to the chassis.

As the other posters have stated it's a live chassis type of set and requires normal live chassis techniques for servicing and putting back into working order.
__________________
Frank
Nuvistor is online now  
Old 24th Sep 2018, 9:46 am   #6
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 14,311
Default Re: Transformer safety?

Hi Roger,

At least you spotted that - a lot of people would just see the mains transformer and assume it were an isolating tx.

In essence, treat this like you would a DAC90, as after all, it has neutral (hopefully!) connected straight to chassis.

e.g. make sure the knobs are present and correct, and securely attached; that there are no chassis screws accesible to be touched; that the back cover is undamaged and secure; and that no external LS or aerial etc. are connected. I would also ensure it's fed from an RCD-protected supply, and I'd also keep it out of the kitchen.

Good luck with it,
Nick.

P.S. If you want a nice-sounding, really well made, Bakelite set with VHF and a proper isolating transformer, look out for a Cossor 524. Fragile knobs aside, they're wonderful IMHO.

Sorry, crossed with the others, apologies for duplication. And interesting to hear about the advantages of this type of design, Tim, thanks!
Nickthedentist is offline  
Old 24th Sep 2018, 9:48 am   #7
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 8,127
Default Re: Transformer safety?

According to a schematic I'm looking at it's a live chassis design with a mains earth connection, the mains earth is connected to the chassis via a 0.01uF capacitor.

Mains neutral should be connected to chassis via the on/off switch and the auto transformer's heat fuse.

Lawrence.
ms660 is offline  
Old 24th Sep 2018, 9:49 am   #8
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 7,045
Default Re: Transformer safety?

Not sure what type of control knobs are fitted but if they use grub screws remember to fill the holes with wax so that the brass screw cannot be touched.
__________________
Frank
Nuvistor is online now  
Old 24th Sep 2018, 10:07 am   #9
M0FYA Andy
Octode
 
Join Date: May 2007
Location: Preston, Lancashire, UK.
Posts: 1,863
Default Re: Transformer safety?

What is the reason for the mains earth connected to chassis via a 0.01uF capacitor?
Obviously not for safety reasons, so I'm guessing an 'RF earth' connection.

Andy
M0FYA Andy is offline  
Old 24th Sep 2018, 10:15 am   #10
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 4,186
Default Re: Transformer safety?

Quote:
Originally Posted by Boater Sam View Post
With an isolating transformer you can earth the chassis, seldom does this stress the insulation too much in my experience.
Agreed, if you want 'better' safety then fit an external isolating transformer and earth the chassis but there will still be hazards from the HT and fire risk. When my daughter asked the same I gave her a vintage transistor radio.
PJL is online now  
Old 24th Sep 2018, 10:29 am   #11
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 7,045
Default Re: Transformer safety?

Quote:
Originally Posted by M0FYA Andy View Post
What is the reason for the mains earth connected to chassis via a 0.01uF capacitor?
Obviously not for safety reasons, so I'm guessing an 'RF earth' connection.

Andy
I can only presume that was the reason.
Interesting comment in the trader sheet, early sets had the LO on the high side of the signal, this was changed to the low side, this was to prevent the second harmonic possibly interfering with Band 3 TV.
__________________
Frank
Nuvistor is online now  
Old 24th Sep 2018, 10:32 am   #12
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 7,045
Default Re: Transformer safety?

Must admit if my family wanted an old radio it would be a battery transistor one, we know the possible, if small, dangers of these old sets, others dont and expect to be able to use them unattended as they do with modern equipment.
__________________
Frank
Nuvistor is online now  
Old 24th Sep 2018, 11:20 am   #13
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 14,311
Default Re: Transformer safety?

I agree. We have a Hacker VHF Herald in the kitchen. It's often left on all day, even when nobody's at home, is operated with wet hands, fiddled with by the kids and so on, and I wouldn't feel happy with a valve set in that role. OTOH my wife and her side of the family keep telling me I'm "too risk averse"
Nickthedentist is offline  
Old 24th Sep 2018, 12:37 pm   #14
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,409
Default Re: Transformer safety?

Is it run on batteries or from a modern wall wart?

Thinking about the "left on all day" battery costs"!

(Mine runs on an ex modem 16Vac wall wart transformer feeding a homebrew 15V (7815) regulated PSU card tucked in alongside the audio amplifier on the bottom om the cabinet.)

Back to the original set and its operating risks- suitably repaired it will be safe enough (memories of a tatty old Pilot Little Maestro running 16/7/365 in the kitchen as a lad) but for peace of mind place it somewhere reasonably fire resistant and try not to use it unattended.

It would be easy enough to add a one-shot thermal cutout in the top of the box if you're seriously paranoid!
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O

Last edited by Herald1360; 24th Sep 2018 at 12:50 pm.
Herald1360 is offline  
Old 24th Sep 2018, 12:45 pm   #15
Nickthedentist
Dekatron
 
Nickthedentist's Avatar
 
Join Date: Jan 2005
Location: Oxford, UK.
Posts: 14,311
Default Re: Transformer safety?

Same as yours Chris, but using a 7818.

Before that I ran it on 12 AA NiMH cells which lasted 3-6 months of heavy use between charges.

N.
Nickthedentist is offline  
Old 24th Sep 2018, 2:47 pm   #16
Skywave
Dekatron
 
Skywave's Avatar
 
Join Date: Jun 2006
Location: Chard, south Somerset, UK.
Posts: 6,405
Arrow Re: Transformer safety?

Quote:
Originally Posted by Studio263 View Post
AC/DC sets (and therefore auto-transformer ones as well) are, in my view, much safer than those with double-wound transformers.
Full quote shortened for brevity.
Obviously, you are entitled to your viewpoint on this - and I understand your reasoning - but I suspect that many here would not take that view.

1. Double-wound transformer: in the event of a pri. to sec'y. insulation failure, I would expect a fuse to fail or an RCD to trip. (Obviously depends on degree of insulation failure and where, in the transformer, that failure occurs. Also assumes that the chassis is connected to supply earth, as it should be).
2. Moreover, with some AC / DC radios, the mains connection to the set enables the connection to be reversed - thus making the chassis 'live'. (DAC90 springs to mind). And there is always the possibility of the mains plug being mis-wired, so that 'L' and 'N' are reversed.
3. Then there is the possibility, in AC / DC sets, of the failure of an isolating capacitor - such as the aerial isolation capacitor.

To me, the chance of any of those scenarios occurring is far greater than a pri. to sec'y. insulation failure in a double-wound transformer.

Al.
__________________
Never act on your first idea. Like solving a crossword clue, a better idea arrives tomorrow.
Skywave is online now  
Old 24th Sep 2018, 3:36 pm   #17
Nuvistor
Dekatron
 
Nuvistor's Avatar
 
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 7,045
Default Re: Transformer safety?

I have a problem with trusting old mains transformers, very often no fuses apart from the one in the plug.

I wouldn’t give or sell these old radios to anyone unless they knew the problems.
__________________
Frank
Nuvistor is online now  
Old 24th Sep 2018, 5:02 pm   #18
Trifocaltrev
Pentode
 
Trifocaltrev's Avatar
 
Join Date: Nov 2013
Location: Lancing, West Sussex, UK.
Posts: 154
Default Re: Transformer safety?

I find that using a 1 amp BS1362 fuse in the plug is normally sufficient protection to prevent any problems with unfused transformers.
Trifocaltrev is offline  
Old 24th Sep 2018, 5:40 pm   #19
G3VKM_Roger
Heptode
 
G3VKM_Roger's Avatar
 
Join Date: Jun 2012
Location: Southeast Norfolk, UK.
Posts: 514
Default Re: Transformer safety?

Thanks to everyone who responded, so quickly!

I think, on balance, that the Nays have it and so I will not let the Murphy go to my daughter and tell her to stick to the Internet! The original purchaser (my 92 year-old father-in-law) was keen to see it working again, which it does, but on a Variac and for a short period.

I think it will go back in the loft until Dad is no longer with us.

Again, thanks for the help, much appreciated.


Cheers

Roger
G3VKM_Roger is offline  
Old 24th Sep 2018, 5:50 pm   #20
Studio263
Octode
 
Studio263's Avatar
 
Join Date: Apr 2007
Location: Hampshire, UK.
Posts: 1,500
Default Re: Transformer safety?

Quote:
Originally Posted by Skywave View Post
1. Double-wound transformer: in the event of a pri. to sec'y. insulation failure, I would expect a fuse to fail or an RCD to trip. (Obviously depends on degree of insulation failure and where, in the transformer, that failure occurs. Also assumes that the chassis is connected to supply earth, as it should be)...
...but almost never is, as most radios of this era have two core mains leads. Fuses don't fail until a substantial current is flowing and not every building has RCDs (our house doesn't for example).

That's why the AC/DC method is safer, if you think about it sensibly and in practical terms. Its a lot easier to fit a few new 'Y' rated capacitors to the aerial socket (etc) than to make a 50+ year old mains transformer really safe.
Studio263 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:12 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2019, vBulletin Solutions, Inc.
Copyright ©2002 - 2019, Paul Stenning.