[TenTec] This Old …Rig” – Tonight’s Project: Replacing the Corsair II PTO with a DDS VFO

[David and Dianne on Comcast]

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