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46-1209 using phono input for bluetooth? - Printable Version

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46-1209 using phono input for bluetooth? - mancave - 01-26-2024

I have completed electronic restoration on a Philco 46-1209. The radio portion works very well and sounds great. I'd like to work out how to install a bluetooth input. This radio has a built-in phonograph, and before connecting a bluetooth receiver I attempted to feed signal from my cell phone earphone jack through the phono pickup transformer, but no sound came through. I then tried bypassing the phono pickup transformer, but that did not work either. I had connected right and left channels together through a pair of 39 ohm resistors and tested the output with a spare set of headphones to verify output. 
The schematic of the radio/phonograph is here http://www.nostalgiaair.org/pagesbymodel/586/M0013586.pdf

RE: 46-1209 using phono input for bluetooth? - fenbach - 01-27-2024

Do you get a sound when you rub your finger on the needle? Maybe the phono/radio pushbutton switch isn't making contact?
Or maybe check the 7A7 phono amp tube? I think if you touch the tip of a soldering iron to the grid pin of that tube, you should hear a 60 cycle hum. With the iron off but plugged in.

RE: 46-1209 using phono input for bluetooth? - mancave - 01-28-2024

I solved the problem. The.phono step up transformer had an open secondary. I filed through the four rivets that held the top plate onto the transformer, then heated the cast iron housing enough to allow me to move some of the wires.. apparently the heat reflowed some solder and restored the secondary connection.

RE: 46-1209 using phono input for bluetooth? - fenbach - 01-29-2024

Curious. Any idea why you got no sound when you bypassed the transformer??

RE: 46-1209 using phono input for bluetooth? - mancave - 01-31-2024

fenbach, I've made a few more discoveries. The first is that the phono transformer still does not work. The second is that, if I bypass the transformer, feed the output from my phone directly into the line for the phono unput, and turn the radio volume all the way up, I can faintly hear the music playing. I took apart a wall wart and used the little transformer inside it as a stepup transformer and that made quite an improvement, although it is still not nearly enough. The volume of the radio has to be turned up to a very high level. I need to find a transformer with similar characteristics as the original. Any thoughts on that? The schematic is a little blurry so it's difficult to work out the original specs, but it looks like the primary measures .8 ohms while the secondary measures 5800 -- that's quite a difference!

RE: 46-1209 using phono input for bluetooth? - RodB - 01-31-2024

You might take a look at an output transformer from a transistor radio and use it backwards.

RE: 46-1209 using phono input for bluetooth? - fenbach - 02-01-2024

mancave,
If you have a signal generator, you might try injecting an audio signal into the grid of the 7A7, where the audio for the phono goes in [pin 5] to see if the signal gets lost in or after the phono pre-amp 7A7. Or if the switch isn't making good contact. Or, again, try the tip of a soldering iron on pin 5.
The Philco library has a better schematic and information about the radio itself, but I don't see the phono transformer on the schematic, just in the parts list.

https://philcoradio.com/library/download/service%20info/yearbooks/1946-1947/split/Part26-Philco%20Home%20Radio%20Yearbook%201946-1947.pdf

RE: 46-1209 using phono input for bluetooth? - MrFixr55 - 02-02-2024

Hi Gang,

Mancave, if the radio plays loud and proud, this proves out all of the amplifier section except the phono preamp. Try injecting your Bluetooth audio right at the phono-radio switch by either using jack with internal disconnect or adding another switch at the junction between the plate coupling capacitor of the phono preamp and the radio/phono switch. IMHO, this is the correct junction to add a "line level" or "aux" input anyway.

The reason for the extra stage of amplification for the phono is because this unit uses a magnetic pickup, whose output is much less than that of a ceramic pickup. The output of a magnetic pickup is somewhere between 5 and 20 mV except for some of the really early Victor horseshoe heads. The output of a ceramic or crystal cartridge is between 500mV and 3V. (Many portable phonos of the 40s and 50s employed a crystal cartridge with an output between 1 and 3V and only 1 tube, usually a 117L7 output tube / rectifier or a 25L6 output tube and a selenium rectifier. If your Philco used a crystal or ceramic pickup, the extra stage of amplification and the transformer would not exist. The advantage of a magnetic or the Philco "Beam of Light (BOL) pickup is much greater fidelity and lower distortion.

The output of a Bluetooth, MP3, etc. is usually over 1 volt, and this will "rail" a phono preamp designed for an input between 10 and 100mV. In addition, phono preamp stages usually provide a level of equalization that is not appropriate for another input such as TV, MP3, Bluetooth, CD Player, Computer, etc.

Hope all of this helps!

RE: 46-1209 using phono input for bluetooth? - mancave - 02-08-2024

touching the tip of a soldering iron to pin 5 of the 7AF7 does produce a hum

RE: 46-1209 using phono input for bluetooth? - Dadsoldradio - 02-16-2024

Measure the resistance that your phone or mp3 player expects from headphones as the device is looking for this lead to output there. Add a resistor on screen pin or wire direct to volume pot. I'm currently using the first two preset buttons on my 40-180 for what you are doing to imput guitar and aux/bt.