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Sherwood SE-3 Sync Detector mod for ham use




 
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Author Topic: Sherwood SE-3 Sync Detector mod for ham use  (Read 7074 times)
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w3jn
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« on: April 12, 2008, 09:09:09 AM »

I bought a used Sherwood SE-3 sync detector from Chuck, K1KW, and it's the second best investment I've made in my station (after going from coax to W7FG open wire antenna line).  The audio quality is truly outstanding, and it offers many useful features such as a product detector for SSB, amazing lock ability, etc.

The only bad thing is that the lock range is fairly limited - about +/- 150 hz.  So if you're in a roundtable, or breakin, with many stations slightly off frequency, the detector won't lock - or it takes a second or two to lock in during which you miss the first couple words of the station.  Truly unsat for typical AM ham use unless you're talking to only one station.

I got motivated to fix this problem after I figured KB3AHE was gonna come to my QTH and bean me with a plate transformer when I kept bugging him about his drifting VFO.  So, here's the easy cure:

You need one small SPST switch, that will fit into one of the existing holes in the back of the cabinet, and one 100K/1/4W resistor.

Remove the cabinet top, and find the "OFFSET/LOCK" switch - it's obviously the one in the upper left side as viewed from the front of the cabinet.  Run a wire from the top terminal of that switch to one terminal of your new switch.  Connect the 100K resistor in series from the other terminal of your new switch to the MIDDLE terminal of the OFFSET/LOCK switch.  That's it!

What this does is widen the bandwidth of the phase-locked loop.  You might notice that, before the mod, if you hold the OFFSET/LOCK to OFFSET while tuning, the PLL will hold lock over  a very wide range at the expense of audio distortion.  Too, this switch is momentary and has to be held in place.   This switch bypasses a 100K/.047/100K R-C-R network with either a 47K or 27K resistor to markedly decrease the PLL time constant.  So we want to do the same thing with our new switch but not quite so much - hence the 100K resistor.  What you're doing by turning on your new switch is essentially bridging the OFFSET/LOCK but by adding a bit of extra resistance, the audio isn't as distorted.  

With the modified SE-3, turn your newly mounted switch ON to widen the capture range of the PLL.  It'll now hold lock on all stations in a QSO unless someone is REALLY off frequency.  Even if they are, the lock speed is almost instantaneous.  

Turn the switch off for very lo-distortion audio, or for SSB.
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