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Old 07-13-2017, 03:31 PM
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wa2ise wa2ise is offline
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Join Date: Dec 2002
Location: USA
Posts: 3,147
Yet another mod: Modified boring AA4 with SS radio chip

Had an Admiral AA4 radio, the kind that used an autodyne 12AU6 converter, no IF stage, and the usual 12AV6 50C5 and 35W4 circuits. I was about as sensitive as a toilet seat, and it had a boring plastic cabinet anyway. So...

At a garage sale picked up a Sony AM-FM "transistor" radio that had a beat up cabinet. I figured I could use this radio's circuit board to replace the RF sections of the above AA4.

In step stages, I removed the FM specific parts from the SS board, making it AM only. Tested it to see if I broke something. Removed its tuning cap and measured it with a cap measurer. The antenna was 150uF max, and the osc 100uF max. The AA4's original tuning cap is antenna 400uF max and osc 100 uF max. This means I can use the SS board's osc coil, but tring to match the antenna tuning cap to the SS antenna needs work. Never did get satisfactory results, but I then decided I could make use of the AA4's loop antenna, and install a tap to feed the SS chip's AM antenna input. Looked at the data sheet of the chip, a CXA1019S and it had a spec for the AM antenna listing the number of turns for the LC circuit and turns for a secondary to feed the chip. Which gave me a turns ratio that I can then pick a point on the loop antenna to install a tap, here at 20%. And this allowed me to keep the original loop and tuning cap to preserve the original antenna LC circuit. The osc tuning cap was very close, and the SS osc coil is adjustable, so mating the original tuning cap to the SS osc coil was no problem. Tested it at this point, to see if any mistakes happened, and to tune things up.

With this success, now it's time to merge this above chip to the audio driver, output and rectifier tube circuits. Need a 4V power supply for the chip, which I can get from the 50C5 cathode circuit. This cathode runs around 6V, so I used a resistor voltage divider to get the 4V. And this divider lets me use a cap to filter the 4V to remove cathode follower induced audio. But still letting me have some cathode follower feedback for the 50C5. The chip's speaker output now feeds the AA4's volume control. The "transistor" radio's volume control I set to give me a reasonable level of audio to feed the AA4's pot, selecting a undistorted audio level, aka "line level" that you'd get from the old tube AM detector circuit.

Used an AC voltage dropping cap to drop the missing 24 volts of now missing heater. This means the radio must operate off AC powerlines, but have you ever encountered DC powerlines in the past 50 years?

Packed it up, and externally the radio looks the same as before, but it really pulls the stations in. And we still have AA5 tube sound

Note that the tuning cap and the loop antenna is now a low impedance path to the powerline, thru a 1.25V regulator circuit inside the chip. So be sure you can't touch it when the radio fully assembled, else you could take a shocj and probably fry the chip.
Attached Images
File Type: jpg adm-ss-mod-AAsch.jpg (86.1 KB, 36 views)
File Type: jpg adm-mod-AAsch.jpg (45.6 KB, 42 views)
File Type: jpg adm-ss-mod-AA4.jpg (45.1 KB, 51 views)
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