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Old 07-13-2017, 04:31 PM
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wa2ise wa2ise is offline
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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, 50 views)
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Old 07-15-2017, 09:41 PM
dieseljeep dieseljeep is offline
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Is that 30 pin chip, the only one in the Sony radio, or is there a separate audio output stage?
The Sony looks like the old design, tuning capacitor frequency selector, instead of the newer voltage controlled synthesizer type tuning, used in the newer designs.
On a side note, I'm amazed how well the first four transistor AM table radios performed. I'm referring to the ones built in the late 60's, that had the high voltage output transistors. They performed and sounded as well as the AA5's of the day.
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Old 07-16-2017, 12:14 AM
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Findm-Keepm Findm-Keepm is offline
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Quote:
Originally Posted by wa2ise View Post
...It was about as sensitive as a toilet seat, and it had a boring plastic cabinet anyway. So...
Your mods are still the best! Your website full of AA5 mods was among the first I bookmarked back in '95, never imagining 22 years later you'd still be banging out improvements, what-the-hecks, and "What if I...." sets.

Keep it up, wise one!
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Old 07-16-2017, 02:25 AM
old_coot88 old_coot88 is offline
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Quote:
Originally Posted by Findm-Keepm View Post
Your mods are still the best! Your website full of AA5 mods was among the first I bookmarked back in '95, never imagining 22 years later you'd still be banging out improvements, what-the-hecks, and "What if I...." sets.

Keep it up, wise one!
Yep. His homepage is among my very oldest faves, from about '97 when I first got internet access. He's hands down the Grand Master of AA5 modding/hotrodding and a lot of related neat stuff too.
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Old 07-16-2017, 06:44 PM
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Yep. His homepage is among my very oldest faves, from about '97 when I first got internet access. He's hands down the Grand Master of AA5 modding/hotrodding and a lot of related neat stuff too.
Bob Casey is in a "mode" I wish I could forever be in - playing with electronics. I had 6 months of "free time" back in 1989-1990, at sea on the Forrestal. I was selected to go to the cal lab as a repair tech - they had a backlog of both calibration items and repair items, and I was given a chance to prove myself, so I did, working myself out of a job, and right into "play mode" - building noisemakers, fixing discarded/off-inventory Fluke meters (13 of them...) and I even got to fix an arcade machine for the officer's wardroom. Then it was on to VCR repair for crewmembers (all idlers and belt kits..). My last big project was an old Wavetek function generator - all analog wave shaping, and the dang thing went to microHertz. In pulse mode, I had to wait 10,000 seconds to see an output, so off to chow while it transitioned. I never went back to that mode again - all work in other specialties, with an ever-increasing management role.

Play on Bob....
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Old 07-17-2017, 11:58 AM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by old_coot88 View Post
Yep. His homepage is among my very oldest faves, from about '97 when I first got internet access. He's hands down the Grand Master of AA5 modding/hotrodding and a lot of related neat stuff too.
His entries inspired me on a few of my little projects. I also used his formula for my capacitance-reactance voltage drop calculations, to replace bad resistance line cords and ballast tubes.
BTW, Bob calculated the C/R voltage drop for 125 volts, instead of 120 volts, for the Admiral, which is a good policy.
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Old 07-18-2017, 03:32 PM
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Tim R. Tim R. is offline
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Quote:
Originally Posted by old_coot88 View Post
Yep. His homepage is among my very oldest faves, from about '97 when I first got internet access. He's hands down the Grand Master of AA5 modding/hotrodding and a lot of related neat stuff too.
That was one of the first websites I ever visited - back when I was still in elementary school! It helped cement what turned out to be a lifelong interest in electronics.
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Old 07-18-2017, 04:19 PM
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a very cool Maude....er....mod
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Old 07-18-2017, 05:07 PM
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wa2ise wa2ise is offline
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Thanks all.

The Sony chip does have an internal audio amp, which I used to feed the AA4 volume control. I set its gain to get a reasonable audio level to feed that pot, with clean undistorted audio.
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Old 07-24-2017, 06:24 PM
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benman94 benman94 is offline
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I think every radio I own that originally spec'd a 6SA7 now has a 6SB7Y, and every 6SK7 RF preamp has been switched out to a 6SG7, thanks to you Bob. You're website is the bible for AA5 modding and hot rodding. Thanks so much.
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