General Electric T-142A mp3 Conversion

View previous topic View next topic Go down

General Electric T-142A mp3 Conversion

Post by Gcollins on Tue Aug 29, 2017 8:35 pm

I have a 1940 rca 45x17 that needs some attention. new power cord etc but I want to add mp3 to it. But that'll be in another topic. I Have a 1961 General Electric radio that I'd like to practice on and use. It does sound good but its only am broadcast. I'm not too familiar with this sort of thing but I'm getting the hang of it. I have a video here that I got the idea from. My main question is what wire to tie into on the volume pot to add the switch and audio jack. And if its a hot chases. I doubt it because its actually a mother board type thing kind of bizarre. If any one has any suggestions let me know

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Bill Cahill on Tue Aug 29, 2017 10:17 pm

Welcome to TRF. Other guys could guide you better. But, being a hot chassis sounds dangerous. You might want to get an isolation transformer.
Bill Cahill

_________________
Please also join us here;  http://videokarma.org/index.php
avatar
Bill Cahill
Admin
Admin

Number of posts : 4548
Age : 66
Registration date : 2008-03-12

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Tue Aug 29, 2017 11:19 pm

Thank you! Whats a Isolation Transformer? Also is there a way to tell if it has a hot chassis with out starting it up?

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Bill Cahill on Tue Aug 29, 2017 11:35 pm

An isolation transformer is one with two separate windings. One for primary, other, secondary.

_________________
Please also join us here;  http://videokarma.org/index.php
avatar
Bill Cahill
Admin
Admin

Number of posts : 4548
Age : 66
Registration date : 2008-03-12

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Wed Aug 30, 2017 9:37 am

Hmm okay. Ill have to add one just to be safe for my sake and for my phones sake

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by jsweinrich on Wed Aug 30, 2017 10:20 am

I agree with Bill, you'll need an isolation transformer assuming you have a hot chassis. Transformers come in many types and designs. The transformer will need to be an audio type if want a decent frequency response. The other thing important here is one with an isolation rating. Most audio transformers are not rated for isolation so you might need to wing it. If you could find one with a split bobbin that would be best since the secondary to primary capacitance would be inherently low, reducing the capacitive coupling. I too will be doing this to a tube car radio. Another design issue is impedance difference between the MP3 player and the tube circuitry. My go to vendor for general electronic components is Digikey.com, they have a great search engine with all the parameters. My plan is to connect the MP3 jack to the source signal side of the volume pot and just off tune to a quiet spot on the tuning dial. The reason for this is that you can use the jack to play you MP3 player through the radio and use the radio as a source to drive an amp/speaker system if one chooses. The trick is to not load the radio circuit down with the added transformer/circuit but provide adequate coupling for the MP3 player. Most headsets for MP3 players I think are around 40 to 200 ohms and if you look at the volume pot resistance, that will tell you that impedance. So, if you volume pot is say 300k ohms then you know you can attach a 300k ohm load and only loose 3 dBm (half volume) of signal loss. The dilemma is proper matching. The two signal voltage levels are probably not that different but the impedances are very different. To match impedences you'll have a transformer with a to radical turns ratio so you'll end up with some kind of resistive pad in addition to the transformer so that the transformer does not load the radio circuit down. My engineering tendency would be to add a switch so that the added circuit would not load the radio circuit when the MP3 jack is not in use. If you don't mind the switch that would be the best way to go. What is you volume control resistance?

jsweinrich
Member
Member

Number of posts : 42
Registration date : 2017-06-22

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Wed Aug 30, 2017 6:43 pm

I had initially planned to put a switch of some kind on there. I looked all over the volume pot to see if there was anything on the resistance. It said cts 125 volt 1 amp. Here's  couple pictures. one is the board and the other is the volume pot. How do I check the resistance on the volume pot?



Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by CaptainClock on Wed Aug 30, 2017 10:20 pm

Sir, you have a hot chassis radio, and that switch you have is a combination Power switch and volume control switch, of which one side of the power switch is wired to the hot side of the power cord which you do not want to mess with especially if you're going to try and wire up an external MP3 jack because that jack will be always hot because of this being a hot chassis radio, which means that you can risk seriously electrocuting yourself or damaging your MP3 player if you don't do things right. If I were you I would try and find an old radio with a line-level phono jack already wired into it and just use that instead because at least with that you have at least an isolated jack and you won't be risking your life or your MP3 device/Cellular device. Zenith, RCA, Motorola and a few other companies made radios with built-in Line-level Phono jacks that could be switched on or off, and they have a much better build quality than this old GE.

CaptainClock
Member
Member

Number of posts : 81
Age : 29
Registration date : 2017-07-13

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Wed Aug 30, 2017 11:07 pm

CaptainClock wrote:Sir, you have a hot chassis radio, and that switch you have is a combination Power switch and volume control switch, of which one side of the power switch is wired to the hot side of the power cord which you do not want to mess with especially if you're going to try and wire up an external MP3 jack because that jack will be always hot because of this being a hot chassis radio, which means that you can risk seriously electrocuting yourself or damaging your MP3 player if you don't do things right. If I were you I would try and find an old radio with a line-level phono jack already wired into it and just use that instead because at least with that you have at least an isolated jack and you won't be risking your life or your MP3 device/Cellular device. Zenith, RCA, Motorola and a few other companies made radios with built-in Line-level Phono jacks that could be switched on or off, and they have a much better build quality than this old GE.

So would a isolation transformer help with that? or is it that much voltage coming through the pot?

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by jsweinrich on Thu Aug 31, 2017 2:47 am

The problem is not high audio signal voltage but the power line voltage. In this type of receiver one side of the power line entering the radio is connected to chassis or signal ground. In many of these radios there was no provision to prevent the hot side of power being connected to chassis/signal ground (no polarized plug) they instead and in some designs relied on isolating the entire circuit from the user using a plastic enclosure and plastic knobs.

An isolation transformer would provide some isolation as long as the transformer had a very low primary to secondary capacitance. This type of transformer would usually be a dual bobbin type because it does not have a primary winding over a secondary winding (or the reverse) that would add a good bit of capacitance between them. In using an isolation transformer this way one should connect one lead of the isolated side winding to earth or safety ground to prevent any voltage (mostly static in nature) from developing on the isolated side.

Another option would be to use a bluetooth headset to provide the connection to the circuit by severing the ear pieces and connecting the cables to the radio. This way the MP3 player would not have any electrical connection to the radio. Of course one would have to charge the headset battery and fiddle with the controls to make the Bluetooth connection...

John

jsweinrich
Member
Member

Number of posts : 42
Registration date : 2017-06-22

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by CaptainClock on Thu Aug 31, 2017 4:14 pm

As Mr. Weinrich said They built these Radios with plastic cases and knobs for a reason, because the chassis and the control knobs have 120 volts AC on them at all times which means that trying to wire up a 3.5mm jack to the cabinet and having it wired to the chassis would mean that you would be subject to 120V AC whenever you plugged your MP3 player into the radio because there'ss nothing isolating the jack from 120V Line Current, and thus a hot jack which could potentially injure you and or your MP3 player. Which is why I suggested you try and find a radio that had a switched line-level phono jack built in to the Radio, that way the jack is already wired up in a way that its isolated from the AC line Current.

CaptainClock
Member
Member

Number of posts : 81
Age : 29
Registration date : 2017-07-13

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Thu Aug 31, 2017 6:16 pm

I like the Bluetooth idea. I've got another project radio and Gibson guitar amp if incase this one doesn't work out. I figured Id learn off this one before I go to the more valuable ones. That'd be in another post. This all makes sense on why it has plastic knobs and plastic case. But when I first acquired the radio it was missing the volume knob. and Id grab the metal prong to use it till I got a knob for it. if I remember right it hummed really loud when you touched it.

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by CaptainClock on Thu Aug 31, 2017 8:06 pm

The reason why it hummed really loud when you touched it was because you were more or less completing the circuit with your body, had you had one part of your body touching a source of ground you would of gotten a serious jolt from the radio, but since it seems you weren't touching a source of ground you didn't complete the circuit and it only sent audible feedback to the radios amp stage in the form of a hum.

CaptainClock
Member
Member

Number of posts : 81
Age : 29
Registration date : 2017-07-13

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Chas on Thu Aug 31, 2017 10:06 pm

It may be possible to get passable reproduction using a line to voice coil transformer. The type used for public address or background music. The smallest one available that is somewhat adjustable, (taps). O

On the line side couple the low to the radio circuit return at the volume control and couple the high of the line side via a .047/630 volt capacitor to the top of the volume control. The cap will further increase the impedance. On the voice coil side, select the windings for the audio in that give the best reproduction. Do not connect either winding of the voice coil or speaker side to any circuits in the radio.

You may have to try different ratios in line to voice coil transformers so before permanently mounting tack solder the connections and test.

GL

Chas
avatar
Chas
Member
Member

Number of posts : 68
Age : 70
Registration date : 2008-07-24

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Thu Aug 31, 2017 11:37 pm

Wow! I really could've got a shock from that little radio. Where did you guys learn all this stuff? I was in middle school at the time and it was my first tubed radio I could find that worked. I have truly learned alot of new things here! And that a radio missing a knob could be a hazard. A voice coil transformer? So what do you all think my first approach should be? I appreciate all the info guys

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Chas on Fri Sep 01, 2017 12:13 am

Shock, yes...

Where did I learn? From getting shocks, and surviving Shocked It is called a line to voice coil transformer. That should provide the required impedance change and isolation. Another option is a microphone transformer. That will be a more expensive option.

Finally, there is the choice to use an AM radio broadcaster. It is a mini transmitter for AM that uses a typical medium impedance input signal to modulate a low power, FCC part 15 oscillator. The radio would not be modified at all. The AM broadcaster would be electrically connected to the mp3 source and turned on. Then the radio tuned to the frequency of the broadcast. Such a device would work with any AM radio. Some of these broadcasters are better than others. I have an SSTran and a Talking house.

First? Get the knob so you don't get a shock...

Chas
avatar
Chas
Member
Member

Number of posts : 68
Age : 70
Registration date : 2008-07-24

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Gcollins on Fri Sep 01, 2017 9:36 am

I already got one. Its not a original but it looks like it belongs on it. Its insulated too. I do like the idea of a radio transmitter for them. I could put music on a loop and I don't have to modify anything.

Gcollins
New Member
New Member

Number of posts : 10
Registration date : 2017-08-29

Back to top Go down

Re: General Electric T-142A mp3 Conversion

Post by Sponsored content


Sponsored content


Back to top Go down

View previous topic View next topic Back to top

- Similar topics

 
Permissions in this forum:
You cannot reply to topics in this forum