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Repurpose of old radio
Repurpose of old radio
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Old 31st January 2019, 11:51 PM   #21
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Rasped View Post
I am genuinely scared by what you're saying.
In this case, fear is a good, sane response. These things were a terrible idea from the start. As you say, a death-trap waiting to happen.

Even in a radio, they were far from safe. All it takes is for one of the plastic knobs to fall off, and now the metal potentiometer shaft is exposed. Touch it to turn up the volume or tune in the radio station, and you have a 50% chance of finding your finger directly connected to AC mains voltage.

Other radios of this era seemed to use a sheet of cardboard or similar looking pasteboard to insulate the back of the housing. In humid environments, that stuff seems to conduct enough electricity to give a shock to someone who reaches back there.

There is a similar danger with the external wire antenna you could connect to those old radios. Typically the external antenna went to a coil inside the radio, and the other end of the coil went to chassis ground. If the chassis ever became "hot" because the AC plug was inserted the wrong way around, then the external antenna wire was also live, another deathtrap waiting for an unwary touch.

My family history includes a story in which my mother was nearly electrocuted by a miswired refrigerator. Somehow the entire metal body of the 'fridge was live, and when she got up in the middle of the night to get milk to feed me (I was a baby at the time), she was shocked so badly she couldn't let go of the fridge. Luckily my father heard some sort of inarticulate moan from her and managed to pull her free of the fridge.

Fast-forward to the present. I hate having to cut those awkward 6-sided holes for IEC receptacles in my DIY electronics projects, but I am very grateful that IEC inlets and 3-wire power cords and proper grounded 3-hole electrical outlets are now the norm. They're well worth the additional hassle.


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Old 1st February 2019, 01:53 AM   #22
Printer2 is offline Printer2  Canada
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We had a stove where if you touched the chrome and the fridge you would get a good zap. As far as playing through these amps that were built in simpler times without isolation, there was not much for electrical in a kid's bedroom, maybe a light and that was it. No grounded plugs and no real way to connect to ground completing the circuit. Mind you damp basement floors were known to wake up the young player. But playing in a room with wooden floors and plaster walls there was not too much of a hazard. And if we lost a kid or two, well that was the baby boom and we had lots. Today, we value our remaining days more. Also with 120V not too bad, 220-240V is a different ballgame.
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Old 1st February 2019, 08:50 AM   #23
Kay Pirinha is offline Kay Pirinha  Germany
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That's why I strongly recommended to install an isolation transformer properly.
Btw, I had to rethink my idea of feeding the heater string directly from the tranny's secondary, as in an high gain amplifier this is prone to induce massive hum. It is much better to have DC heating, with the input valve's heater close to ground. Use another silicon bridge for this, a filter 'lytic, and two capacitors in series with the secondary and the bridge's ~~ inputs.
Best regards!
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Old 1st February 2019, 10:42 AM   #24
Rasped is offline Rasped  Denmark
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I will install isolation transformers in two of the small radios I have that are in good condition.
I'll completely disassemble the B&O and make a new design from scratch. Just too many unidentifiable components and circuit parts... it's really cluttered.
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Old 1st February 2019, 11:19 AM   #25
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by Gnobuddy View Post
In this case, fear is a good, sane response. These things were a terrible idea from the start. As you say, a death-trap waiting to happen.

Even in a radio, they were far from safe. All it takes is for one of the plastic knobs to fall off, and now the metal potentiometer shaft is exposed. Touch it to turn up the volume or tune in the radio station, and you have a 50% chance of finding your finger directly connected to AC mains voltage.

What other choice did they have back then when there were either DC and AC power grids? Should they have said »Sorry, no radio!« to those who only had DC power?
At least in Germany TV sets using the same PSU principle were common for decades, even in times when DC grids completely had vanished. The TV manufacturers seem to have reinvented the power transformer just a few years ago, until those became obsolete due to SMPS's.

I believe your scenarios were rare exceptions, if ever.

Best regards!
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Old 1st February 2019, 11:22 PM   #26
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Kay Pirinha View Post
What other choice did they have back then when there were either DC and AC power grids?
Mechanical inverters (called "vibrators" were used to solve this problem in automotive valve radios. They could have been used in AC/DC "farm radios" too.

Quote:
Originally Posted by Kay Pirinha View Post
Should they have said »Sorry, no radio!« to those who only had DC power?
As you said, this comes down to how precious a human life is considered to be. Would you rather have no radio, or have a spouse or child electrocuted by an unsafe radio?

We all know that if you took a poll today (substitute "TV" or "fridge" for radio), the overwhelming majority of people would vote to keep their family members safe. I doubt that this would have been different among the civilian population even in 1930. But nobody told them these AC/DC radios were deathtraps.

Quote:
Originally Posted by Kay Pirinha View Post
I believe your scenarios were rare exceptions, if ever.
There is rare like winning the lottery, and there is rare like hitting your thumb with a hammer by mistake. I don't know anyone who won the lottery, but I have known many people (including myself) who smacked their own finger with a hammer at some time or the other.

I grew up in a time and place with 2-wire AC power and no safety ground, and I think just about every person I asked had experienced at least one electric shock. If it wasn't the 'fridge, it was the radio, or clothes-iron, or a wire that had pulled loose from the back of an appliance (no IEC outlets, just wire run straight into the appliance through a rubber grommet.)

Obviously those people who answered my question didn't die, but every now and then, someone did.

Was that a problem worth fixing? Most countries have answered that question with very different safety standards in place now, compared to, say, 1950.

I know there is no point to just blindly criticizing the past. It's already come and gone. But perhaps we can study it, and avoid making some of the same mistakes again.

IMO, recent experiments with putting members of the general public into self-driving cars engineered by Tesla, Uber, and lots of other manufacturers, is a classic example of repeating the same mistake as selling unsafe AC/DC radios. The mistake is unleashing half-baked, untested, and lethally unsafe technology onto unknowing consumers who do not fully realize the risks they are taking.


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Old 1st February 2019, 11:43 PM   #27
stephen_keller is offline stephen_keller  United States
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One must consider the times. Un-re-enforced masonry buildings were allowed back then, so were open sewers. We don't allow these sorts of things any more, but times have changed.
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Old 2nd February 2019, 01:51 AM   #28
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by stephen_keller View Post
We don't allow these sorts of things any more, but times have changed.
Indeed! My informal poll results are that very few millennials or members of generation X and Y have experienced an electric shock from a faulty AC-operated product.

I did know an electrical technology instructor in California who had a very low opinion of the two-wire, "double insulated" corded power tools that were legally sold during the last twenty-odd years. He could cite cases where both layers of insulation had failed, usually when the tool itself failed, endangering users because there was no safety ground.

But it's probably fair to say that the shock hazard from double-insulated, two-wire power tools is far, far lower than the corresponding hazard of typical AC appliance from a few decades ago.


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Old 2nd February 2019, 10:57 AM   #29
DF96 is offline DF96  England
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A few decades ago people expected life to be risky. Children were brought up to understand that actions had consequences. If you tripped over in the street you didn't sue the local authorities for having uneven pavements; instead your Mum told you off for not looking where you were going.
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Old 3rd February 2019, 05:22 AM   #30
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by DF96 View Post
...uneven pavements...
We are not equating the risk of tripping on uneven pavement with the risk of using a guitar amp that has a 50% chance of shocking the bejeezus out of you, depending on which way you insert the two-pin plug, are we?

Life was risky. Many babies never made it to adulthood, victims of deadly childhood diseases like diphtheria, polio, tetanus, tuberculosis, measles, whooping cough. We couldn't do anything about that prior to the discovery of effective vaccines and antibiotics. Children still die of those diseases today, in some poorer parts of the world.

But guitar amps operating on AC mains without power transformers? That was a design decision, not an overpowering reality of nature. We could indeed have done something about that.


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