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Saturday, July 22, 2017

Intuitive Repair of a Sony Shortwave Portable

A few years ago my wife got me this nice little Sony ICF-SW7600GR receiver.   On the front it proclaims that it is "AM DUAL CONVERSION" and "PLL SYNTHESIZED."   It has a BFO, and a filter of suitable width for SSB.  It also has a synchronous detector -- it generates an internal carrier that matches the frequency and phase of the carrier being transmitted by the SW broadcast (or ham AM) transmitter.  This helps overcome the selective fading that often plagues AM signals.  Sony advises switching to USB or LSB once the synchronous generator locks onto the carrier.  Pretty cool. 

The BFO is the reason I wanted this receiver. And wouldn't you know, when I dropped it,  it landed EXACTLY on the little BFO fine tune control pot.  It was as if the Radio Gods disliked all the fancy digi PLL synchronous IC circuitry.   

I tried without success to find the value of the destroyed pot.  Finally, last week I just decided to have a look in there to see if I could just figure it out. 

On the board I could see that the pot only connected at two places, so I figured it would be a varactor circuit on the BFO with one end of the pot to DC and the wiper to the varactor diode.  I figured I'd try a 10K pot. 

This seems to have been some good radio intuition.  It works.  I went with a small trimmer because it is less obtrusive and because once I set the BFO in the right spot, I think the de facto channelization of the 40 meter ham band will keep most of the SSB sigs in tune. And the Sony only tunes in 1 kHz increments. If necessary I can move the BFO a bit with a small screwdriver.  I just glued the trimmer pot onto the back of the receiver -- two wires covered by heat shrink run back into the circuitry. 

10k might be a bit too small.  Maybe 100k would be better?  As it is, I can move the BFO above and below the "zero beat" point, and I don't need more range. Mouser has a small trimmer pot with a tuning wheel that looks like it might fit, so I may try for a proper repair. 

5 comments:

  1. Hi Bill, I have the ICF-7600G model and the potentiometer for ssb fine tune is listed as 50K. BTW great little radio.
    Armand

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  2. Bill, keep in mind that pots that are designed to be trimmers have a very limited rotational life - sometimes as little as a few hundred wipes. It won't take too many tweaks before before it becomes intermittent. If you're going to do a proper repair, use a pot rated as a control, not a trimmer.

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    Replies
    1. Good point Alan -- thanks. I may not have to move it too much because 40 has become so channelized in 1Khz increments.

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  3. Hi Bill: stopped by hoping for a new episode of SolderSmoke podcast just to find and certainly like your first-aid on that receiver: SONY´s ICF-SW7600GR schematic lists a 50K linear pot (RV201, Part Number 1-227-388-11; https://sony.encompass.com/) for the BFO control - aka “SSB fine tune”, steering a VCO deep inside the black box frontend.
    Best - Joh

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