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Powering A Preamp From A TSI 3733



Don't ask me why I wanted to do this, as most people, including K5GNA, will tell you this is not worthwhile since this downconverter already has a low NF.  But, I did it anyway...  The main reason was I wanted to be able to easily swap downconverters to compare performance (and you must have a backup for everything, right?).  

Anyway, to make a long story longer, I first attempted to power my DEM 13ULNA preamp from my Drake 2880 directly from the 7810 regulator over to the RF-input, but this experiment failed--as the effect on the Drake was significantly reduced performance (don't try this at home):

http://groups.yahoo.com/group/AmsatTech/files/wire_20.JPG

David, G0MRF, looked at it and determined stray RF from the LO may have been the problem.  He developed a simple, working solution he documented here:

http://www.g0mrf.freeserve.co.uk/drake2.htm

David obviously has a more up-to-date junkbox and better eyes than I do.  I really wanted to do this, but I just could not face using an SMA inductor and trying to solder an SM cap on it's side, so I just used some "regular" components I had in my junkbox.  I found a molded inductor about the size of a 1/4 W resistor and a small ceramic cap (33 pF) and installed those in the same location as his example.  By stacking them I was able to fit them in the "hole" in the Drake's internal cover.  It seems to work just fine, even though it does come very close to the RF amp.  I put tape around the solder connection above the RF amp to make sure I did not short to the FET:

http://home.swbell.net/k5oe/power_50.jpg

Once that was done, and tested on UO-11, I thought, "why not do the same for the 3733?"  Bob's (K5GNA) objection is the stock 3733 has a grounded RF input.  This makes the device pretty much safe from static buildup charges, stray voltages, etc.  This mod isolates the input before the first filter, but the FET is still at ground potential.  So (finally), here's the beef, complete with instructions:

http://home.swbell.net/k5oe/3733_12Vdc.jpg

73,
Jerry, K5OE

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