I seem compelled to return to the Corsair II. Its
appearance, simplicity, and receiver performance
always draw me back. In the mid-1990’s, I bought
and sold several CSII’s attempting to find the
‘perfect one’…which I did...and then foolishly
sold it later.
Last fall I decided to see if another Corsair II
might be in my future. While they are for sale
from time to time, most are getting pretty long in
the tooth. Many are mechanically worn out.I was
very lucky to acquire an excellent CSII from a W6.
For me, the CSII, with its fine receiver design
along with its pure analog sound, creates an RX
audio quality that is unmatched in most digital RX
designs of today. It’s a warm, unprocessed sound
that is very pleasing to listen to for long
periods without fatigue. Unlike many, I also
really enjoy the ‘hang’ AGC on SSB. I have always
replaced both stock SSB filters with INRAD 2.8 KHz
SSB filters for improved SSB audio while a TT 500
Hz filter is my choice for CW. Many believe that
the CSII is primarily a CW rig. True, but this is
the only rig from which I get unsolicited
compliments on my SSB audio using only an
unequalized electret microphone. The radio remains
a pretty darn good performer for a one designed in
an era pre-dating DSP and today’s sophisticated
microprocessors.
Much of the CSII’s receiver performance can be
attributed to its front-end design coupled with
the use of an electrically quiet, mechanical
PTO.But the PTO is prone to problems over time due
to the lubrication hardening in its drive bearings
along with considerable thermal drift in some
units. The PTO also has a stiffer feel than with
modern VFO encoders.
Both G3TXQ-Steve and N4YG-Joe have each designed
excellent DDS VFO replacements for the CSII PTO.
Both are different. One is a ‘drop-in replacement’
design and the other is a more significant
modification to the basic implementation of the
features set of the CSII. Both have received
excellent user reviews.
I went back and forth on which to choose. I
ultimately chose the design of G3TXQ-Steve Hunt,
(http://www.karinya.net/g3txq/corsair_pto/) as I
wanted to keep the CSII as true to its original
design, appearance and functionality as I could,
while reserving the (unlikely) option to easily
bring the CSII back to ‘stock’ condition. I also
wanted to undertake a building challenge without
step-by-step instructions.
The electronic fabrication (short of programming
the PIC and winding two toroids) is fairly simple.
I needed to acquire a custom PCB board from
Express PCB along with the requisite electronic
components including two toroids, a PIC, a quality
encoder and the usual resistors, caps, transistors
and Molex connectors etc. to build a board
containing the PIC controller, RF buffer, LPF and
5V regulator. All parts/components (with the
exception the PIC and toroids) are available from
either Mouser or Newark. Info on the PCB board’s
design/layout and the PIC code is available from
Steve’s web site above.A pre-assembled DDS
daughter board comprising an AD9851 chip and 30MHz
reference oscillator (the core component of this
DDS VFO) plugs into the board described
above.Readily available from eBay, its cost is +/-
$25.00.
The most difficult part of this project for me was
the metal fabrication involved in constructing the
enclosure box and a dial shaft adapter (so that
the original CSII tuning knob and logging scale
skirt can still be used). The VFO box is actually
a section of 2 X 3 X 1/8 inch rectangular aluminum
tubing. It must be drilled and tapped (#4-40) to
match the mounting holes of the original PTO box
to the CSII chassis. The new VFO encoder shaft is
too short and too large in diameter to accommodate
the original CSII knob assembly which I wanted to
preserve intact. It needs a custom shaft adapter
to be fabricated. All of this metal work,
especially the shaft adapter, was the real
challenge for me. In the end I did it all with a
hacksaw, file and hand drill.A couple of early
tries were a disaster, but finally it all came
together after five attempts.
The results have been fantastic including silky
smooth tuning with an incremental tuning speed
feature for rapid band excursions, excellent
frequency stability with little thermal drift from
start-up, a “drop-in” replacement design and no
more PTO rebuilds. I just completed conversion of
my CSII external VFO with identical results. I
suspect this DDS VFO replacement would easily work
in most other Ten Tec rigs using PTOs.
My thanks to Steve Hunt-G3TXQ for sharing his
design and answering my many questions and to
Jerry Haigwood-W5JH for his mentoring and hands-on
assistance during this project. Jerry has been a
frequent resource and advisor to me through all
phases of this project. Thank you Steve and Jerry.
Next Project….G3TXQ’s PIC replacement for the CSII
MPU display/keyer board.
With apologies to PBS, there is new life for….
“This Old ...Rig”.
73 de N1LQ-Dave
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