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 Post subject: Radio Shack 200-629 with strange SW symptoms
PostPosted: Jan Wed 31, 2018 12:01 pm 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 797
I have a Radio Shack version of the Sangean ATS-505, model number 200629 (generally known as the "Radio Shack Synthesized World Receiver"). It's a typical Sangean digital SW radio with analog IF chip, FM front end chip -- and it's microprocessor controlled, with digital tuning, etc. On FM and MW the circuitry is similar to that of your average boombox or other AM-FM radio, but the SW circuits are added on, with extra stages of gain and filtering, mostly through FET's.

I've had it for about three years. I hadn't used it much on SW, and had it in the closet for maybe a year. I took it out of the closet, re-batteried it, and tuned the SW with it.

Weird things started happening. First, I noticed birdies here and there -- but they didn't seem to be too numerous. Then I noticed that when I unplug the headphones, the radio acts like a Theramin -- on SW only. On MW and FM it acts completely normal. Actually, it's a good performer on MW and FM.

This morning I tuned around on the radio and using headphones I was able to pick up quite a few SW stations, with very little QRM that seemed to be generated from inside the radio.

However, as soon as I unplugged the headphones, I noticed a whistle and then a popping noise. Then when I moved my hand close to the radio the whistle became theramin-like. Very odd.

I'm not sure I can fix this radio, as it is all surface mount. I have other SW radios to SWL with. But I'm curious as to what could cause this sort of thing? Is it a shield that maybe came loose? I've seen photos of the inside of this 200629/ATS-505 radio, and there are two metal shields, but they each have more than one solder point to ground, so I wouldn't think a cold solder joint could cause this issue.

And why would the headphones being plugged in make a difference? They don't seem to be connected to the ground, according to the schematic I found of the ATS-505 -- they are more or less directly connected to the two audio IC chips.

I've attached a pic of the schematic -- the majority of it. The whole thing wouldn't fit within the size limit, and if I resized it to fit the size limit it might be unreadable....

Any ideas?


Attachments:
Radio Shack 200629 schematic -- most of it 75pct.jpg
Radio Shack 200629 schematic -- most of it 75pct.jpg [ 184.35 KiB | Viewed 248 times ]
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 Post subject: Re: Radio Shack 200-629 with strange SW symptoms
PostPosted: Feb Thu 01, 2018 5:27 am 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 797
I forgot to mention that the theremin-like symptoms dissipated when I switched the DX/Local switch to Local. Still flummoxed over this bizarre 'defect'....


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 Post subject: Re: Radio Shack 200-629 with strange SW symptoms
PostPosted: Feb Thu 01, 2018 12:24 pm 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 797
It has something to do with the SW External Antenna jack, which looks (on the schematic) like it is stereo. I have a mono mini plug on my indoor antenna.

This a.m. when I plugged the antenna in all the way, the feedback/theremin stuff happened. When I backed the antenna jack out to where it was just engaged, 'poof!' -- everything seems normal. No feedback, no theremin.

Perhaps the mono plug in the stereo jack is causing some sort of feedback. For the life of me, I don't really understand the schematic enough to figure out why it is happening, but I'm relieved that (at least so far) the radio is normal, and the defect is just plugging a mono plug into the stereo SW EXT jack all the way.....


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 Post subject: Re: Radio Shack 200-629 with strange SW symptoms
PostPosted: Feb Fri 02, 2018 1:37 am 
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Joined: Mar Tue 27, 2012 1:24 pm
Posts: 797
This afternoon I used a mono-to-stereo adapter while tuning the higher SW bands (amazingly, there were several stations with good reception here). Problem solved. NO feedback, even with headphones unplugged. The radio was working well and completely normal.

I still don't understand why the feedback occurs when a mono plug is pushed into the stereo SW EXT antenna jack, but obviously the ATS-505 (or at least the newer ones, of which the 200629 is one of them) needs a stereo plug for EXT SW antenna input.

I tried a factory made external antenna (for my Panasonic RFB45 -- another SW digital tuned portable) and it has a mono plug. I plugged it in all the way. Feedback and theremin like symptoms. So it's not the way I wired my antenna's plug. It's definitely the SW EXT jack on the radio -- it is stereo, and needs a stereo plug in there to keep the radio from feeding back.

I have other Sangeans with stereo SW EXT jacks, but haven't had this issue before. Perhaps they are wired a bit differently.

I realise that no one has responded here... I've been adding to the thread just in case another Radio Shack Synthesised World Receiver (200629) owner, or newer ATS-505 owner, runs into this feedback/theremin problem and runs a search engine to find out about the issue. ARF threads do show up in Google and Bing's search engines.

I just wish the owner's manual explained the necessity for a stereo plug on an external SW antenna.


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