Another 40-40! Don't be con-FUSED!

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From: Mike J Pulley (Mike_J_Pulley@ccm.ch.intel.com)
Date: Thu Dec 08 1994 - 19:43:05 EST


Text item: Text_1

     If you plan to build a 40-40 kit, read on...
     
     Yep, I got my own 40-40 up and running. After the normal cockpit
     errors, it went on the air without any smoke escaping from the
     components.
     
     THE PROBLEM
     Output power was low... only about 160mW. All the tuning
     capacitors peaked up just fine. However, driving the transmitter
     above 160mW caused wild oscillations and distortion on the output
     signal. (Yes, I did make an inaugural QSO or two at 160mW
     anyway! 1000m/W, here I come!)
     
     THE SOLUTION
     The fuse resistance was too high. Go figure! I used a 1/8 amp
     fuse to protect my handiwork during the smoke test phase. Heck,
     I build a holder into the chassis just because that's the
     conservative kinda guy I am. During transmit, the fuse reduced
     VCC down to 10V, preventing the transmitter from developing full
     power.
     
     A 1/2 amp or 1 amp fuse works just fine. Now I have a bone
     crushing milli-legal kilowatt!
     
     If you use a fuse for protection, make sure its big enough to
     support your operating voltage during transmit.
     
     
     Here's a freebie...
     
     SUPPLY VOLTAGE (VCC) CHARACTERIZATION TESTS
     How low can VCC sag before the transmit signal turns to hash?
     
     Properly tuned according to Dave Benson's instructions, I found
     the transmitter operates just fine with power supply voltages
     down to about 9V. I tested up to 14V, too, before I lost my
     nerve. The output power tracks VCC slightly, but never below 1W;
     the transmitter produces a squalling racket before the power dips
     lower than that.
     
     All tests were performed using a 50 ohm resistive load. YMMV.
     
     Why should you care? Now you know how low you can tolerate
     battery voltage before you're forced off the air during FD!
     
     Regards,
     
     -- Mike, WB4ZKA


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