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 Post subject: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 8:43 pm 
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Location: Summitville IN, 46070
I have this:
Attachment:
image.jpeg
image.jpeg [ 35.91 KiB | Viewed 327 times ]


Someone wants me to put it into this zenith to use it as a guitar amp.

https://www.radiomuseum.org/r/zenith_k_731k73.html

The am band does not work so I figured the am band is the new audio input band. This is a hot chassis radio, how would I go about doing this?


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 9:05 pm 
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The only really safe way to do it is to either use a power isolation transformer or an audio isolation transformer (which will be the cheaper option).

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 9:49 pm 
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You can also do this by using 2 ceramic .01 caps per side of the input. One goes to center volume tap, the other to the neg. side of the vol pot. This will lower impedance and result in current under 5 millivolts. If AM doesn't work you can disconnect the lead from the RF can to the other side of the vol pot. Its probably overkill but you can also add a polarized plug so that it ensures that the chassis won't ever be the hot side.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 9:51 pm 
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Okay, I will look into getting one. I have never put in an audio cord to any radio so I will need a total walk through of it.

Edit: did not see bob's post.


Last edited by Gwcoty on Feb Tue 06, 2018 9:56 pm, edited 1 time in total.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 9:54 pm 
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Two questions: how do I know what side is the negative of the volume pot? And, so let me get this right, in series with each wire of the audio cable, I put in a ceramic .01 cap?


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 11:22 pm 
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Usually the negative side of the pot goes to ground and the other goes to the RF stage. You can usually tell by tracing where the leads go and the one that goes to one of the RF cans from the pot indicates that its the one. So you have the center tap for positive, One leg that connects to the RF and the last is the negative. And yes- you will want to have caps for BOTH pos and neg leads- no direct connection of the leads to the radio itself.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Tue 06, 2018 11:26 pm 
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Okay got it. Thank you!


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 10:48 am 
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Seems like an invitation to a personal injury lawsuit to me. Don't do it- counsel your client to purchase an inexpensive solid-state amp from Amazon or eBay. These operate off of a "wall wart" power pack and don't
involve lethal house power voltages.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 3:47 pm 
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All I get from the radio is kind of a humming sound with the volume turned up. I could not get any stations to come in through it. I attached my CD player to the two wires I hooked up to the volume pot. I can hear it with the volume turned down but as soon as I turn it up, it gets distorted and the humming comes back. What would cause this? Could the way I routed the wires cause this? I must admit, I have never done anything like this before.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 4:35 pm 
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lorenz200w wrote:
Seems like an invitation to a personal injury lawsuit to me. Don't do it- counsel your client to purchase an inexpensive solid-state amp from Amazon or eBay. These operate off of a "wall wart" power pack and don't
involve lethal house power voltages.



I agree, this is a recipe for disaster, you should never add an input that wasn't there from the factory to an AC/DC type radio.

Unless you permanently install an AC isolation transformer in the set (inside where no one can tamper with it) there is no way you can insure that every outlet or extension cord the owner plugs this into in the future is properly wired. When they get shocked or killed, the liability is 100% on you as the person who did the modification.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 5:15 pm 
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Quote:
Seems like an invitation to a personal injury lawsuit to me.


EIDTS MADE...

You could very well say that for any of the radios we restore- whether they are totally bone stock or fully restored. So let's say he doesn't do anything and restores the set, hands it back to the client fully recapped as it came from the factory. The user happily plugs it in and somehow touches the back while touching ground. Zap! 110 AC right there. Or maybe the client realizes the set is made out of toxic stuff- like the fact that its got a cadmium plated chassis or maybe it has asbestos insulation. Or maybe that the tubes get blazing hot and they burn themselves. So regardless of whatever anyone does when it comes to these radios there is ALWAYS the possibility of a lawsuit. If you're worried about being sued then you probably shouldn't be restoring any radios for anyone other than yourself.

Now as far as adding an input here it all comes down to the math really. The safety code for today's electronics is that said device must create no more than 5mA. So for example those GFCI outlets? Those trip at 5mA because that's the threshold. The aforementioned addition of 2 .01 caps on both the positive and negative leads of the input device will yield less than 5mV. So for example when I do this I actually have a GFCI outlet on the bench. I will then intentionally short out the input jack to hard ground on both sides of the plug if its not polarized. While you'll get a buzz from the speaker the GFCI won't trip- again because there is less than 5mV coming off the jack. Why- again because there are ceramic caps on both ends.

To be sure, yes- you can do this via an isolation transformer as well and it will perform a similar task, which is to isolate the user from high current hazards.

Quote:
you should never add an input that wasn't there from the factory to an AC/DC type radio.
Totally disagree because the sets that came from the factory with inputs simply ran the input directly to the audio section and the only thing isolating the user is usually something like a .047 or even in some cases a .1 or higher cap. That's many magnitudes off the charts when it comes to safety. In some cases the cap is only on one side of the input and so if the user were to have the plug reversed you could get a pretty nasty shock if you were to for example touch the metal parts of the record player. ANY set that has an audio input should have the correct modifications made to ensure that the impedance levels are lowered to under 5mA. Otherwise its just another hot chassis set from whatever era with electrical safety standards that are grossly outdated.


Last edited by bobwilson1977 on Feb Wed 07, 2018 6:17 pm, edited 1 time in total.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:10 pm 
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Oops, empty post... ignore.


Last edited by lorenz200w on Feb Wed 07, 2018 6:16 pm, edited 1 time in total.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:12 pm 
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Quote:
The safety code for today's electronics is that said device must create no more than 5mV. So for example those GFCI outlets? Those trip at 5mV because that's the threshold. The aforementioned addition of 2 .01 caps on both the positive and negative leads of the input device will yield less than 5mV. So for example when I do this I actually have a GFCI outlet on the bench. I will then intentionally short out the input jack to hard ground on both sides of the plug if its not polarized. While you'll get a buzz from the speaker the GFCI won't trip- again because there is less than 5mV coming off the jack. Why- again because there are ceramic caps on both ends.


Its my understanding the codes regarding leakage current state that leakage current shall not to exceed 4-6 mA.

https://www.ul.com/wp-content/uploads/2 ... evices.pdf

The specification is a current rating, not a voltage rating.

I think the distinction being made concerning liability is that of modifying an existing circuit in a way that could injure someone vs. just working on the appliance, not changing the circuit, and someone being injured through their own negligence.

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Last edited by processhead on Feb Wed 07, 2018 6:16 pm, edited 1 time in total.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:16 pm 
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Quote:
Its my understanding the codes regarding leakage current state that leakage current shall not to exceed 4-6 mA.


Correct and clearly its too early in the morning for me. Yes- the rating for a GFCI outlet to trip is at the 5mA mark. So yes- while you could still measure a full 120 volts AC coming off of the input jack, with the aforementioned caps you will measure less than 5mA, which means that will not even trip the GFCI outlet and be within today's code

Quote:
I think the distinction being made concerning liability is that of modifying an existing circuit in a way that could injure someone vs. just working on the appliance, not changing the circuit, and someone being injured through their own negligence.


Well, the tricky part is this: So let's say that we the typical restorer do what we typically do: we recap a AA5 and restore it to working order. By fact that we replaced all of the original capacitors with new ones means that we've now modified the set from its orginal factory condition. As such any problems that may occur with it could theoretically be attributed to the restoration. We had this very discussion at the museum a few years ago. That when a set is restored it is no longer as originally constructed.

Anyway we could really get into the weeds over this. We actually have people sign a 2 page document drawn up by our lawyer anytime a set is sold which in laymen's terms explains that no- these sets do not meet today's standards and that there are inherent risks associated with owning and operating a period electronics device of any kind.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:30 pm 
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Then I suggest you have your lawyer draw up a document that removes you from any liability for modifying the original design of a set. There is a HUGE difference between repairing, using modern components which are the electrical equivalents of the original parts, and changing the design.

I worked for decades for the largest service organization in the world for consumer products, and we were repeatedly told by the corporate attorneys that any modification to the design fell solely on the person who did it and the technician could be sued by the owner of that equipment should something happen related to our work if we did not follow the original design and use factory approved currently available replacement parts.

I'd hate to see something happen.

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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:45 pm 
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Quote:
Then I suggest you have your lawyer draw up a document that removes you from any liability for modifying the original design of a set.


I have discussed this with him a number of times and seeing as how I am a volunteer at a non profit this more or less removes said volunteers from liability.

Quote:
here is a HUGE difference between repairing, using modern components which are the electrical equivalents of the original parts, and changing the design.


... But that doesn't necessarily apply to a vintage piece of equipment that when new was inherently unsafe to start with. Like I said- we could really dig deep into then weeds here but the fact is that even a fully restored set made safer with more modern capacitors- even the addition of safety grounding caps is still going to fail today's safety standards and any potential injury from that device will carry a certain liability. The distinction between restoring and modification could be argued either way really: replacing ancient wax and paper caps with modern plastic ones could by some definition be considered a modification since the caps that were replaced were not the same construction.

All I can say to anyone working on these sets is that yes- there will be always the potential for liability. Do the best work you can, ensure that your sets are as safe as they can be but always keep in mind that the radios will never pass today's safety requirements.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:55 pm 
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ok-I have made up my mind to not do this. I never knew how controversial this could become.


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 6:59 pm 
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I suppose something I'd say to the person who asked you to do this is that the sound quality would probably be pretty awful for a guitar amp anyway...


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Wed 07, 2018 7:58 pm 
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Gwcoty wrote:
ok-I have made up my mind to not do this. I never knew how controversial this could become.

You have made a very wise and safe decision.

Dave


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 Post subject: Re: Attaching audio input cord to hot chassis radio
PostPosted: Feb Thu 08, 2018 8:58 pm 
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Old radios make lousy guitar amps.

There are adaptors anyone can purchase to convert the RCA input jack, or a patch cord could be made up to fit the standard RCA jack.

But let's not encourage the practice, especially on transformerless units.

There is plenty of room in that cabinet for an isolation transformer.

Finally, it's a Dumb idea.

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