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Old 16th Jan 2021, 1:37 pm   #1
Nightcruiser
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Default Advice wanted as to step down transformers

I’ve recently bought a rather smart looking 110/20v MAGNAVOX radiogram from the USA dated 1946, but of course i will need to buy a step down transformer powerful enough to operate it safely after it has been recapped and i do not know what ampage to seek

According to the SAMS photofact the main chassis/tuner (CR190A) draws 0.80 amp and seperately to that is the optional extra FM tuner (Chassis CR192A) which was available at the time rated at 0.43 amp and then of course the turntable which will not run at the correct speed until the pully is modified.

Can someone more knowledgable than myself advise please

John
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Old 16th Jan 2021, 1:55 pm   #2
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Default Re: Advice wanted as to step down transformers

Well 120 volts times 1.23 Amps is 147.6 Watts.
So get a transformer 300 to 500 Watts so it will run cool.
Sometimes bigger ones are as cheap as the small ones, just buy a good make not 'cheap' Chinese...

The mains transformers in the Magnavox was designed for 60Hz mains, not 50Hz mains, so will run hotter in the UK. Keep an eye on it when you first use it.
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Old 16th Jan 2021, 2:17 pm   #3
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Default Re: Advice wanted as to step down transformers

Thank you for answering me so quickly Snowman

I have heard the Magnavoxs do run hot anyhow when operated over here because it utilises 6V6 tubes so i shall keep a look out for that. What i was worried about was buying something not safe or powerful enough to operate it.
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Old 16th Jan 2021, 2:43 pm   #4
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Default Re: Advice wanted as to step down transformers

Will the motor in the gram be 60hz?
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Old 16th Jan 2021, 2:51 pm   #5
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Default Re: Advice wanted as to step down transformers

Buying a "site" transformer may be worth considering.

Greg.
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Old 16th Jan 2021, 4:57 pm   #6
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Default Re: Advice wanted as to step down transformers

A lot of these transformers are actually just 2:1 step-down. They may be marked 230V in, 115V out. But although we are 'harmonised' to the European 230V norm, in fact the UK mains is supplied at 240V (with quite wide variations). So a 2:1 transformer will typically give you 120V out. If it's over-specified, so it runs cool, it may actually give you slightly more than 120V. The combination of >120V and 50Hz may put quite a lot of stress on your 110V 60Hz Magnavox.

More sophisticated step-down transformers with multiple output voltages are available. But they're a good deal rarer than the 2:1 ones.

Cheers,

GJ
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Old 16th Jan 2021, 5:06 pm   #7
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Default Re: Advice wanted as to step down transformers

Once you get the set itself and can physically see it, you may find it has adjustable tappings for 100-110, 110-120 etc, so this may not be a problem in practise. Just choose the highest option and use it with a 2:1 transformer.
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Old 16th Jan 2021, 8:09 pm   #8
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Default Re: Advice wanted as to step down transformers

Hi John, if your magnavox has a double wound mains transformer you could use an autotransformer. These are physically smaller and often come with tappings so you can get a closer voltage match. I would still go for a 300/500W unit though

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Old 16th Jan 2021, 8:33 pm   #9
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Default Re: Advice wanted as to step down transformers

I would agree with Ed regarding an auto tranny, it's half the weight and gets the job done.
Another thing worth pointing out would be the smoothing caps, I don't know how many cans are in this particular set but it would be worth slightly uprating their/it's value to compensate for mains frequency being lower, the hum will be reduced.
The only thing I have that's American fitted with a tranny is a valve tester, the tranny doesn't seem to get hot on 50Hz.
As others have pointed out, it is worth monitoring the power tranny, incase it acts as a frying pan device!
I would still recommend uprating the smoothing section, as high as the rectifier will allow but not beyond, it'll throw a tantrum!
It will be interesting to find out how you get on with the restoration.
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Old 16th Jan 2021, 9:08 pm   #10
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Default Re: Advice wanted as to step down transformers

Quote:
Originally Posted by snowman_al View Post
Well 120 volts times 1.23 Amps is 147.6 Watts.
So get a transformer 300 to 500 Watts so it will run cool.
Sometimes bigger ones are as cheap as the small ones, just buy a good make not 'cheap' Chinese...

The mains transformers in the Magnavox was designed for 60Hz mains, not 50Hz mains, so will run hotter in the UK. Keep an eye on it when you first use it.
I looked up the schematic on Nostalgia Air. The parts list shows a mains transformer rated for 50-60 cycles, the same thing with the tuner.
Dave, USradcoll1
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Old 16th Jan 2021, 10:08 pm   #11
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Default Re: Advice wanted as to step down transformers

Quote:
Originally Posted by usradcoll1 View Post
Quote:
Originally Posted by snowman_al View Post
Well 120 volts times 1.23 Amps is 147.6 Watts.
So get a transformer 300 to 500 Watts so it will run cool.
Sometimes bigger ones are as cheap as the small ones, just buy a good make not 'cheap' Chinese...

The mains transformers in the Magnavox was designed for 60Hz mains, not 50Hz mains, so will run hotter in the UK. Keep an eye on it when you first use it.
I looked up the schematic on Nostalgia Air. The parts list shows a mains transformer rated for 50-60 cycles, the same thing with the tuner.
Dave, USradcoll1
Who's to argue with someone who lives there!
I have noticed that some American stuff is rated for 50 & 60Hz.
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Old 17th Jan 2021, 1:38 am   #12
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Default Re: Advice wanted as to step down transformers

Quote:
Originally Posted by Glowing Bits! View Post
Quote:
Originally Posted by usradcoll1 View Post
Quote:
Originally Posted by snowman_al View Post
Well 120 volts times 1.23 Amps is 147.6 Watts.
So get a transformer 300 to 500 Watts so it will run cool.
Sometimes bigger ones are as cheap as the small ones, just buy a good make not 'cheap' Chinese...

The mains transformers in the Magnavox was designed for 60Hz mains, not 50Hz mains, so will run hotter in the UK. Keep an eye on it when you first use it.
I looked up the schematic on Nostalgia Air. The parts list shows a mains transformer rated for 50-60 cycles, the same thing with the tuner.
Dave, USradcoll1
Who's to argue with someone who lives there!
I have noticed that some American stuff is rated for 50 & 60Hz.
There was some regions that had 50 cycles, but IIRC, they were connected to the main power grid a little later, especially the southwest states.
Dave, USradcoll1, always interested!
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Old 17th Jan 2021, 9:45 am   #13
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Default Re: Advice wanted as to step down transformers

I was using a wall wart type auto transformer to power my RCA Victor 45X11 but it burnt out, now uncharacteristically for me I threw it away without finding out why and bought another, I then had concerns about this type of transformer, if the 'top' winding between 120V and the 230V burnt out the set would just stop working but if the 'bottom' winding between 0V and 120V had gone it would act as a choke and probably pass sufficient voltage to burn the valves out so I then bought a double wound toroidal replacement.
I do appreciate that an auto transformer to the suit Nightcruisers power requirements would probably have a far superior construction than my 50W wall wart.

John
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Old 17th Jan 2021, 2:00 pm   #14
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Default Re: Advice wanted as to step down transformers

The original turntable is stuffed. it had been retro fitted with the Webster Chicago three speed back in 1950 but it was really the worse for wear anyhow so i shall fit it with a mid fifties Garrard or similar using a 240v output on the step down transformer. That way i will get around the pully problem.

i recently bought a 3000w toroidal transformer with a 120v and a 240v output to get around this.

i don’t know what you guys think of the toroidal transformers on the market?

Again, it would be good to know your views.
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Old 17th Jan 2021, 2:20 pm   #15
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Default Re: Advice wanted as to step down transformers

Quality is variable... Some designers saw grain oriented silicon steel with the grain all going in the right direction and started saving on the copper, reducing the turns per volt as far as they could. The result is transformers that make acoustic noise. Factor in the general degradation in mains waveform purity and you have frankenstein film sound effects running all the time in the background.

You have to be quite choosy to get something reasonably silent.

David
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Old 18th Jan 2021, 4:56 pm   #16
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Default Re: Advice wanted as to step down transformers

https://youtu.be/O3EeBrJFlbE

As they say you get what you pay for. I did a review of a step down transformer I thought was good value and I have been using for a couple of years now. This unit is more powerful than you need but I chose this giving me options in the future. Who know what I will buy next from the USA? Link shown above to my review and YT channel

Chris
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Old 18th Jan 2021, 7:46 pm   #17
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Default Re: Advice wanted as to step down transformers

Hmmm.... you shouldn't really use a fixing for something (in this case the handle) for a safety earth connection. A dedicated screw or stud together with suitable external tooth star washer should be used. Technically the safety earth points should also be labelled as such too, (Earth symbol in a circle.) but you're not placing the thing on the market, so.......
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Old 21st Jan 2021, 1:30 pm   #18
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Default Re: Advice wanted as to step down transformers

i have a large step down transformer here made by Hubble which rates as having a 15a output but it has lost its original service information plate from its body so i cannot give you guys any more detailed information.

Can the wattage be reverse engineered and worked out from the output ampage alone.?
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Old 21st Jan 2021, 1:35 pm   #19
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Default Re: Advice wanted as to step down transformers

Quote:
Originally Posted by Nightcruiser View Post
i have a large step down transformer here made by Hubble which rates as having a 15a output but it has lost its original service information plate from its body so i cannot give you guys any more detailed information.

Can the wattage be reverse engineered and worked out from the output ampage alone.?
Approximately by Ohms law.

Err on the modest side, so 110 volts x 15A = 1650 Watts.
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Old 21st Jan 2021, 2:25 pm   #20
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Default Re: Advice wanted as to step down transformers

Quote:
Err on the modest side, so 110 volts x 15A = 1650 Watts.
In other words, more than adequate for your needs!


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