Revised Tale of My NorCal 40

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From: Jason Penn (jason@persoft.persoft.com)
Date: Fri Jul 08 1994 - 11:23:34 EDT


 
(This is a revised version of my posting from yesterday. As many of on list
probably knew before I did, I was wrong about some rather important aspects
regarding my NorCal 40's alignment. I am reminded of the quote, "I thought
I was wrong once, but I was mistaken".
 
Greetings to the QRP mailing list.
 
I just finished the assembly of my NorCal 40 Partial Kit. I did the bulk of the
soldering and inductor winding over the July 4 weekend. The only snag I ran
across was a dead transmitter section. After some signal tracing and head
scratching I found a tiny solder bridge shorting the VFO to ground near
the input of the xmit mixer. It puts out 3 watts according to my 'scope or
2.5 according to my questionable old Swan watt meter.
 
I did a few things different than specified, but not by choice. Since the
MVAM108 varactors have not come in I went to the local electronic
"odds-and-ends" dealer (mostly "odds" :-)).
He dug up a box labeled "varactors" and told me they
were something like 42 pF at 4.1 volts. He gave me one. In addition, Mouser
was out of stock on the 39 pF mica cap in the VFO, so I used a 33 pF mica
cap. The VFO was about 200kHz low. I took two turns off L9 for a total of
57 turns. C50 varied the VFO over a range of 260 kHz (wow! the manual says
it should vary about 75 kHz) and the tuning pot covers a range of 68 kHz
(which would seem about right). I set it up to tune from 7.082 Mhz to 7.150
Mhz. Oh yeah, I used 4.9152 MHz crystals because that's what I found cheap
at Dayton. I figured a 200 Hz change in the IF wouldn't matter.
 
It turns out that the above tuning ranges are hogwash. I (initially) used
my HF rig to "listen" for the tx/rx signals. I accidentally fell upon
what must have been the third harmonic of the VFO. The third harmonic
explains the ridiculously large apparent tuning range. A quick rewind of
L9 with 62 turns got it down to 7.108 MHz with a high end of 7.131 for a
range of 23 kHz. I verified these frequencies with the big HF rig *and*
a frequency counter. By the way, an oscilloscope probe makes a nice
frequency counter probe. Just be careful that the signals you connect to
the counter won't fry it. My counter isn't very sensitive and will take
up to a few volts. Your mileage may definitely vary.
 
I welcome anyone's comments on the use of the 33 pF cap and the unknown
varactor. The varactor looks like a glass diode with several funny bumps,
not a 2 leaded plastic transistor pack. I think the box they came from were
labeled with a part number, but I didn't write it down. If it is deemed
"acceptable" by Wayne, et. al., I might offer to send units to any takers
on this list building NC40 partial kits. I say "might" because the local
"odds-and-ends" dealer was going out of business the day I stopped in for
the varactor. It is unknown if I can still get them from him or if he can
still find them after his move. He is switching to strictly mail order, no
retail store front and moving to a smaller place that will only hold about
a third of his (then) inventory of "stuff". The MVAM108 is the hot setup,
but substituting another varactor can get you started until the real thing
arrives.
 
Now to (still) make that first NorCal 40 contact.....
 
73
 

-- 

Jason F. Penn N9RPT | Persoft, Inc. | jason@persoft.com Whenever I want to find something, it's always in the last place I look.


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