[R-390] MY EXPERIENCES WITH INTERNAL PRODUCT DETECTORS

[Bob kb8tq]

Hi

The problem with an internal product detector is that it is simply the first step in a 
very long series of steps. You hack in the detector and get it feed back into the audio
system. Now the AGC really isn’t right so you start hacking it. SSB AGC isn’t as easy
as it seems so it’s a bit of work. Next you decide you want SSB noise limiting. That
makes the AGC stuff seem like a cake walk. 

At this point we’re done and it’s time for beer and your favorite sausage on the grill
right? Not so fast. There’s the pesky issue of selectivity and you really wanted at 
least one notch filter didn’t you? 

It just goes on and on ….

Bob


> On Oct 16, 2017, at 5:18 AM, David Johnson via R-390 <r-390 at mailman.qth.net> wrote:
> 
>   MY PERSONAL PREFERENCE IS FOR AN INTERNALLY MOUNTED DEVICE. IF YOU ARE HAPPY WITH HAVING A SEPARATE BOX THAT IS EXTERNAL TO THE RECEIVER THEN YOU HAVE LOTS OF OPTIONS, STARTING WITH A CV-157 OR A CV-591A, WHICH IS OF COURSE THE OPPOSITE OF AN INTERNALLY MOUNTED DEVICE.
> MY FIRST EXPERIENCE WITH AN INTERNAL MODIFICATION WAS THE LEE PRODUCT DETECTOR FROM CQ MAGAZINE IN 1961. THIS PRODUCT DETECTOR WORKED WELL. HOWEVER, IT DID NOT ADDRESS THE AGC ISSUE, SO WHEN I WAS DONE, OVERALL PERFORMANCE WAS POOR.  THE LEE MODIFICATION IS STRAIGHTFORWARD, BUT ITS NOT THE SORT OF THING YOU CAN PERFORM IN AN HOUR.
> MY SECOND ATTEMPT AT A PRODUCT DETECTOR APPEARS AT PP. 42-45 OF THE AUGUST, 1974 ISSUE OF 73 MAGAZINE.  IT IS MERELY A REWIRING OF THE EXISTING DETECTOR AND LIMITER TUBE SOCKETS AND ASSOCIATED COMPONENTS.  THIS MODIFICATION WAS OK ON CW AND SSB, BUT NOT GOOD ON AM AND THE LIMITER WAS NO LONGER USEFUL, WHICH I KEENLY MISSED ON 80M CW.
> THE LAST DEVICE I TRIED WAS THE JAN SKIRROW DETECTOR, WHICH IS A FINE PERFORMER, BUT SUFFERED FROM SEVERAL SHORTCOMINGS, SOME OF WHICH ARE JUST MY OWN PERSONAL PREFERENCES. FIRST, WHILE THE BOARD IS SMALL ENOUGH TO FIT WITHIN THE R-390A CABINET, IT IS NOT SMALL ENOUGH TO FIT WITHIN THE INTERMEDIATE FREQUENCY MODULE ITSELF.  SECOND, THE USER MUST MAKE SEVERAL CRITICAL LEVEL ADJUSTMENTS VIA ON BOARD POTENTIOMETERS.  THIRD, THE AGC CIRCUIT IS UNMODIFIED AND THE RECOMMENDED OPERATING PROCEDURE IS TO BYPASS THE AGC CIRCUIT AND USE THE RF GAIN CONTROL AS A VOLUME CONTROL. I WANT TO EMPHASIZE THAT IT NONETHELESS WORKS WELL WHEN PROPERLY ADJUSTED.
> THE DEVICE I RECEIVED FROM THE 1U2U.NET SITE INCLUDED THE COMPONENTS AND INSTRUCTIONS FOR ADDING A SIMPLIFIED (AND IMPROVED) LANKFORD AGC CIRCUIT, AS WELL AS THE PRODUCT DETECTOR BOARD ITSELF, SO THAT WAS A DEFINITE PLUS IF YOU ARE STARTING WITH A STOCK RECEIVER. THE BOARD FITS ENTIRELY WITHIN THE INTERMEDIATE FREQUENCY MODULE.  IT SAVED A LOT OF TIME AND IT RESULTED IN A COMPLETE SOLUTION (PRODUCT DETECTOR/AGC/BFO).
>   DAVE WB4JTT
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