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Ron;
You are correct. I took the jack and the switch loose and could see the slightly raised edge of the chassis metal around the openings they were fit through. They were obviously drilled later on. While this may have been a nice feature to the customer, they are not original. Factory made punched holes have no raised ridge around the openings and no rough edges like these.
One issue with adding a phonograph input to an existing radio is that the circuitry is not equalized to RIAA specifications, so the audio response curve will not be accurate. Many people would not notice the errors in the sound though.
I have no intentions of using a phonograph with this radio, so I may remove the circuitry and put it back like it was originally. I can find some snap-in chassis hole plugs to cover the openings.
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If the phono cartridge is a ceramic the intrinsic characteristics actually equalize the RIAA curve very well. A magnetic cartridge will require equalization but since the output is in the 10's of millivolts range it will also need about 40db of amplification to take it to the ceramic output levels. So you would need a preamp to use a magnetic cartridge with an old radio which most likely will have built in RIAA equalization.
Steve D
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Since it is already there and looks period to the radio, you might consider keeping the jack. A extra audio input really comes in handy if you ever want to connect a higher quality audio source like a CD player, etc.
Typically the jack just connects to the top of the volume control, with the switch selecting either the detector output or the external input.
You would be surprised how good the audio quality of these radios is when fed from a decent source rather than an AM station.
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The existing phono jack installation is a 1/4" jack rather than the now more common RCA phono jack. If I decide to keep this feature, I may change it to an RCA type. Ceramic phono cartridges typically do not provide for very low tracking force, and I could always add a magnetic phono preamp circuit to boost the signal level and also provide proper RIAA equalization. A CD player connected would have plenty of signal level and no equalization issues to deal with. One way to get around the lack of good AM radio stations is to get one of the low output AM transmitters and connect a phono or CD player to that and then tune it in on the radio.
Joe
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While an AM transmitter is an effective way to listen to a CD player or other source on a vintage radio, there is still a significant loss of fidelity. Keep in mind that the IF transformer selectivity curve effectively cuts the higher audio frequency sidebands and limits high freq response to less than 5 KHz (Unless you have one of the wideband "high fidelity" models!)
With a direct connection, frequency response of the audio circuitry extends to 10 KHz or more and there is a definite improvement in quality when you don't modulate and then demodulate the audio signal.
Having listened to the same audio signal from a CD direct into the audio stage and also AM modulated, there is no comparison in quality. This is why I suggested keeping the audio input so you can appreciate how well these radios audio sections can perform independent of the limitations of AM technology.
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Mondial;
Yes that is true, but I like the idea of exercising all the tubes in the set. It would be nice though to still have the option, so I may leave it in the set.
Joe
