Instruction manual

VLF1 • 16
SETUP AND TESTING
With all this done, it is now time to set up your VLF1 and apply power!
 The first thing to do is plug in the power adapter and see that nothing
burns out or gets hot. If nothing does, congratulations! At least you haven’t
made any catastrophic mistakes.
 The next step is to tune the quartz oscillator to exactly 4MHz. To do this,
connect your receiver’s input jack to the VLF1’s output jack J2. Tune your
receiver to 4MHz and set it to sideband mode. Now turn variable capacitor
C10 with the red plastic screwdriver until you can’t hear anything coming
out of your receiver. This is called “zero beating”, and is useful for tuning
circuits like this without the need for a scope. Now that the Quartz
oscillator is tuned, your kit needs an input! At this point you can plug in an
audio generator or an antenna to input jack J1. That’s right, an audio
generator! The VLF1 will give you a mixed output with less than 10dB loss
at 10KHz input! Note that the VLF1 has been tested using from 4MHz to
10MHz crystals, and this tuning method works for all.
 Your receiver has to be tuned to a frequency that is the sum of whatever
frequency you are interested in listening to plus 4MHz. If you want to see
if there is something going on at 300KHz, you tune your receiver to:
300KHz + 4MHz = 4.3MHz. If you are using an audio generator to test out
your kit at 10KHz, set its output level to something like –20dBm and set
your receiver to 4.01MHz. You should then hear your audio signal coming
out. If you can’t immediately hear something with your antenna, be
patient. In different areas you will have different stations available to you.
TROUBLESHOOTING GUIDE
 If your kit doesn’t work it’s most likely a problem with the transformer T1.
Be really sure that the primary and secondary windings are wired onto the
board correctly. Also, check to make sure that none of your coils are
shorted to the ferrite core. You can check this with an ohmmeter. Measure
across each of the wires and the core. If you get anything other than
infinity, there is a short. You will have to rewind the transformer. Don’t feel
bad, it’s hard to do. We’ve given you enough wire to mess it up pretty
good a couple of times and still end up with a working transformer.
 The next most likely boo-boo is that one or both ICs are in wrong.
 The third likeliest possibility is that one or more polarized components are
backwards. These would be the electrolytic caps, the diode, and
transistor.