Gang,
I have gotten some inquiries asking for a "review" of the scope mixer
described in the Jan 73 Magazine. Well, I am not sure one can review a
homebrewed project which is built from a magazine description. But for what
it's worth:
I built the Scope Mixer project from the Jan 1997 73 Magazine together with
its separate SWR/Range unit over the past couple of weeks. Actually, it is
in the shack for about a week. The system consists of four units: The
Scope/mixer; the SWR/Range unit; a signal generator (MFJ 207) and an old WW2
vintage scope. First of all, for the less scope-sophisticated, note that the
main purpose of this project is to be able to see the waveforms of HF signals
that are normally far beyond the range of these ancient scopes. The highest
frequency my old scope can see is about 1.6 mHz, which is below even the 160
amateur band. So, the idea is to mix my transmitter with another signal that
is about 1 mHz distant from the transmitter output, resulting in a difference
output from the mixer of 1mHz--which the scope can see. This is the idea
behind the project. One box contains a mixer. It uses a cheap 2N222A and a
somewhat hard to find old fashioned ferrite loopstick. I had to buy one (7
bucks with shipping and handling!). The second unit uses a pair of
transformers based on 100uH chokes from the corner Radio Shack. Both units
were easily assembled "ugly" style by soldering the grounded parts to
unetched circuit board and hanging the rest in mid-air. I used RS metal
project boxes and everything connects up with BNC connectors.
You need to get a local oscillator signal for the mixer from somewhere. The
article suggests laying your hands on an MFJ 207 or one of its cousins. The
'207 sells for 80 bucks new, but the author suggests you can get one for
under $40. He is right. I paid $35 at the last flea market without much
effort. And the thing is obviously very useful for a million other things as
well.
The SWR/Range unit is both an attenuator (must be used in conjunction with a
dummy load--see below.) and a bridge for SWR readings.
In practice, I found that the bridge wasn't agreeing with my OHR or MFJ SWR
meters. This led to my first discovery: The old Heath cantenna under the
corner desk was shot. The oil had leaked out (non-PCB thank goodness) and
was now working its way through my basement foundation. Not particularly
good for the environment, but at least it was drug store mineral oil. This
led, as usual with my projects, to the spending of more money. I bought the
Ten-Tec 300 watt dummy load kit for 24 dollars (dry, no oil -- learned that
lesson!) So, the bridge wasn't working because one of the branches had the
wrong resistance!
Continued in part II
Preston WJ2V
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