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 2 radios on 1 isolation xformer?

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Federated_FP



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PostSubject: 2 radios on 1 isolation xformer?   Sun Oct 17, 2010 11:07 pm

Hi,
I keep several working tube AM sets in living room. 2 or 3 are AC/DC. Is there any safety lost from running 2 AM AC/DC sets from one islolation transformer, as compared to the safety of running 2 AM sets each with its own iso xformer?
Question
Thanks for any advice.
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pixellany



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PostSubject: Re: 2 radios on 1 isolation xformer?   Sun Oct 17, 2010 11:12 pm

There is no safety difference----the only thing to be concerned about is to make sure the isolation transformer can handle the total current for everything that you connect to it.
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Federated_FP



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 12:09 am

Thank you. That gives me the green light to buy another set (without another transfomer).

I presume 250 VA rating xformer is the same as 250 watts rating when we are talking old tube table radios of the AD/DC type?

Thanks again.
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radiotechnician



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 12:18 am

2 ACDC radios on one isolation transformer is not safe.

Think of these conditions. Both radio have hot or compromised exposed metal hazards.
Perhaps this is the reason for the transformer. Both radio have non polarized plugs.

Even odds you will end up 2 hot chassis, one connected to opposite terminals of the isolation transformer. Someone touches both sets, one hand on each. Zap 120 volts. Worse if in Europe.

Radios dont even have to be on.

de
VE7ASO
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Federated_FP



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 12:38 am

I sort of thought there might be a problem something like you described.

Now let me ask another question.

Condition1: Lets say I plug two AC/DC sets each into their own isolation transfomer. The 2 radios are placed on opposite sides of a large room where nobody is going to touch both at same time.

Condition2: Now I move one radio and set it side by side next to the other radio. They both still have their own separate isolation transfomer.

Is condition 1 any safer than condition 2?

Thanks.
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Radiosmoker

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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 1:52 am

Use a light bulb and connect to the output sockets in this manner. Transformer 1 hot lead to transformer 2 hot lead, then Transformer 1 ground lead to transformer 2 ground lead.
Then go from either transformer hot lead to other transformer ground lead.
If any of those combinations light the bulb you have a problem.

The reason for a isolation transformer is to isolate from earth ground. Not from Electrical ground.

I think the problem will surface if the phasing on one transformer isn't phased same lead for lead on the two transformers. Suspect

The same would be on two radios on one transformer. Especially if you had the chassis exposed next to, or wired together such as antenna ground. Wink
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Federated_FP



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 4:21 am

Thanks.

Using that method I can produce both a bulb lights arrangement affraid , and the correct setup and then hardwire it in the safe configuration.



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Dave Doughty



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 5:24 am

I really don't see a need for an isolation transformer unless the chassis is operated out of it's cabinet for troubleshooting and alignment purposes. There should be no safety issue for a restored chassis properly installed in it's cabinet. The exceptions are those depression era radios which have their hot chassis mounting screws exposed on the bottom and/or no backs.

When testing an AC/DC radio out of it's cabinet, it should be the only item connected to the isolation transformer. Think of the isolation transformer as if it were the radio's power transformer.

Dave
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pixellany



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 5:46 am

radiotechnician wrote:

Think of these conditions. Both radio have hot or compromised exposed metal hazards.
Perhaps this is the reason for the transformer. Both radio have non polarized plugs.

Even odds you will end up 2 hot chassis, one connected to opposite terminals of the isolation transformer. Someone touches both sets, one hand on each. Zap 120 volts. Worse if in Europe.

Good point!! Although someone touching both chasses might not be likely, this is indeed a reason not to share a transformer.

What I would do---even with one radio / one transformer---is to run a 3-wire power cord, with the ground going to the chassis of each set. With multiple radios, you'd also need fuses to keep the same scenario (above) from frying your iso transformer. (The 3-wire cord and the fuses are cheap compared to the cost of iso transformers)
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Reece

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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 6:49 am

Many AC/DC sets had warning labels not to connect a ground of any kind to them. Running a ground like the green wire of a power cord to a hot chassis set, even one which is "not so hot" that has a cap and resistor from common bus to chassis could involve fireworks. Think that these sets were used for decades even in places like kitchen counters and sometimes people got zapped but so did they if they stuck a fork in the toaster, and "hot" toasters are still with us! (don't tell Washington.) An AC/DC set in the living room is not going to be a problem. What you can do with some of the more scary ones is to replace the bottom chassis screws with nylon ones, and make a new back to keep fingers out while properly ventilating.

Reece
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radiotechnician



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 8:38 am


Federated_FP wrote "I presume 250 VA rating xformer is the same as 250 watts rating when we are talking old tube table radios of the AD/DC type?

One circumstance of interest is when radios, which previously had line cord or other filament voltage dropping resistors replaced with a capacitor.

The amount of current drawn is the same as it was before the modification, but they consume
less wattage. The isolation transformer VA rating would apply. Multiplying the line current by the voltage will not indicate the wattage. It will indicate VA or volt amps. A good gadget to have is a Kill-A-Watt plug in watt meter. The cost about $25.

It is cheaper to operate capacitor-dropped radios, since the power company bills you for actual watts, not the volt-amps, however there are reasons why this 'poor power factor' is unwise.
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Federated_FP



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PostSubject: Re: 2 radios on 1 isolation xformer?   Mon Oct 18, 2010 4:48 pm

True story. During the middle of the night I kept a small glass of red wine to act as a sleeping pill if needed. On the night stand. Alongside the home security control box.

One moring I woke up with a wet burgular alarm controller. It hasn't worked since, unrepairable.

Nowdays there is a 1949 "golden throat" radio with a 1949 45 rpm changer plugged into it where the alarm box used to sit.

I do have an isolation transfomer on the radio.
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