[ARC5] Converter/front end for Q5-er Sets

Fri Jan 9 19:37:53 EST 2015

LeslieWhen i was a boy someone took pity on a poor novice and gave me a R23 with an attached xtal controlled one tube 40 m converter.
It worked great?  With the IF transformer coupling rods pulled all the way out the selectivity was quite good, Almost as good as the HW-16 I eventually was able to afford.

If it was my project I would make the converter fixed tuned and tune the ARC 5 receiver instead.
I would use a two or three pole pre-selector type filter in front of the converter mixer or maybe even multiple varactor tuned resonant circuits.
I was unable to get access to the schematic you referred to .  If you would email the file to me I'd be happy to comment.
To get several fixed tuned bands you could build a simple phase locked loop to lock in - say 100 or 200 kHz intervals  This way the ARC 5 receiver dial calibration is still useful
Or you could look through the crystals available from Digikey, Mouser, Farnel or whoever is available in NZ land, choose the off ball crystal frequencies closest to the desired decimal integer LO frequencies and then deal with the dial calibration in another way such as supplemental pencil lines on the ARC 5 receiver spaced to the left and or the right of the actual dial indicator line.

      From: Leslie Smith <vk2bcu at operamail.com>
 To: ARC-5 List <arc5 at mailman.qth.net>; Ron Scifleet <ron.scifleet at sanitarium.com.au> 
 Sent: Friday, January 9, 2015 5:20 PM
 Subject: [ARC5] Converter/front end for Q5-er Sets

              CNV-412, MF/HF converter for Q5-ers

For some time I wanted to build a general purpose converter to use in
front of a 'command' 'navigation' receiver (e.g. BC-453/946 or R-23/4). 
 The circuit of a prototype, still under development, is shown in the
ARC-5Radio files, in a folder called Exptl_(LNS).

Find the circuit here:

File Name:  "CNV-412, R12 (on 5-0110).png" 
Folder: Exptl_(LNS)
Yahoo Group: ARC-5Radio
Web Reference: www.groups.yahoo.com

The circuit is quite conventional, although the local oscillator uses
the not-so-common source coupled oscillator.  Some mistakenly call this
a Franklin oscillator, and there are similarities, but the two are
somewhat different.  

Operational objective.  
To build a generic 'front end' to use with the Q5-er sets that will
operate on the 160/80/40m band and also the AM broadcast band.  The
circuit will be a solid state implementation of the popular converter
presented by Don Stone, but have the capability to use both a xtal
controlled and variable local oscillator.  

Construction.  I built the prototype circuit on strip-board, using the
more-modern 20 way edge connectors used in digital circuits.  Using a
plug-in type of construction shortens the develop time.  The mixer and
oscillator tank circuits are connected (at the moment) using alligator
clips to the LC tank components.  The AM broadcast band has a good
mixture of strong local and weak distant stations, and the oscillator
will be more stable at the lower frequency, so initial design focused on
520 - 1650kHz.  

It takes little imagination to see how quickly this could be moved to
160m and after that to 80m.

Present functionality.
The Q5-er used for this experiment is a not-so-badly hacked BC-453.  The
outer box and shield over the tuning gang were both gone; the 'common'
mods to the utility box on the front of the set were apparent.  The BFO
circuit had been re-wired, but speaking generally the set was quite
function, I thought.  After routine troubleshooting I could hear the
local 'nav' broadcast on 362kHz using a few meters of wire on the
aerial.

I run my 'command' sets on a 60V B+ supply; in this particular set I
replaced the 'flower-pot' caps with 100V mylar types, so the B+ absolute
limit is 90V DC.  I haven't used a 90V supply enough to say it would
give better performance than a 60V supply, but it may.  

To drive a loudspeaker I bought a surplus 110-11V power transformers,
and put this 'in series' with the existing 600 ohm transformer.  In
other words, I feed the 600 ohm output to the 110V primary, and connect
my speaker to the secondary.  Using this with the 60V B+ supply my set
give useful volume in a quiet room.  (More volume might be got by
reducing the value of the cathode resistor on the 12A6 output 'tube',
but I haven't done that yet.  I avoid modifying the set as much as
possible.)

Using the converter, I hear broadcast stations.  My idea of using the
Q5-er at a fixed 
frequency went out the window, because the tuning was 'touchy'.  That
surprised me, as a 'lash-up' oscillator I built previously (but using a
negative resitance oscillator) would stay zero-beat on the local ABC
radio station for long periods of time (I mean hours at a time).  The
next stage of development will be around the tank circuits - fitting
them to a metal chassis, with shielding.

I suspect the I.F. coupling transformer (at the 'top end' of the mixer)
lacks impedance.  In other words, I think I need more turns.  I'm going
to plot the response of this transformer, and rewind it if it is found
to lack performance.  

The signal from the local oscillator changes (in amplitude) a good deal
over the frequency range.  It's too low, even in mid-band.  It should
provide 5V p-p to the 'top' gate in the mixer, allowing class "C"
operation.  An earlier lash-up gave an output signal that varied no more
than 10% mid-band to the low and high frequency limits.  Clearly I must
do more work to the local oscillator circuit.  

I built the circuit so the L.O. signal came "off" the card, thru a link
and "onto" the card to the mixer gate circuit.  This allowed some
flexibility - if I wanted to substitute a different local oscillator,
all I had to do was open the link and insert the new L.O. signal.  This
idea works so well I'm going to use RCA connectors to switch the signal
path in the link circuit.  

Problems, deficiencies, bugs, work needed and so on.
First, the stability and tuning rate of the L.O is unsatisfactory.  To
tune SSB I need a tuning rate of about 50kHz per revolution of the
tuning knob.  I have no idea about a good tuning rate for AM, but
probably only 1/4 that needed for SSB.  Perhaps someone here can tell
me?  

The mixer tank tuning is hopelessly 'wrong'.  It doesn't peak
adequately; it's broad, and I suspect I should reduce the tank inductor
from the present 370uH to about 260 .. 300uH.  However, the problem is
more than simply wrong values.  When I peak the tank circuit I hear a
noise that (I suspect) comes from instability.  Yes .. I think my superb
design is oscillating.  I suspect I could replace the cascode mixer with
a straight single source coupled JFET mixer.  I don't need the gain the
cascode gives, at least not at the low end of the H.F. spectrum.  It may
be useful on 6m, but that wild-eyed idea (using a BC-455) is still a
distant flicker of hope.  

So - I need to debug or modify the mixer.  Ideas about how this might be
done are welcome.  

I think I might get the circuit working to my satisfaction after two or
three more months of work.  At the moment my prototype suggests the idea
is sound, but Don Stoner proved that decades ago, so that doesn't
surprise me.  Still, it's nice to start with a handful of components and
a circuit and slowly, slowly see the 'thing' come to life.

In the end I want a front-end 'box' that can sit proudly alongside a
'command' radio, knowing it is designed and built as well as the
original set.  The prototype might be strip-board and chewing gum, but
the finished rig-owner (me, in this case) should be proud of the final
article.

I want to say a few things about the drawing.  First, it's clearly
incomplete.  I hope it's sufficiently accurate to not confuse anyone
reading it.  Next, I drew this using the 'freeware' program TinyCAD,
running under Windows XP.  The program isn't perfect, but I like to use
'freeware' developed by an amateur coder.  This is 'neat' software.  If
enough people use this software it will survive, and wouldn't that be
nice?  The TinyCAD package allows users (I'm a 'user' not a 'loser') to
create libraries & symbols.  I created a library (I called it 'common')
and drew most of the symbols in the circuit you see.  Useful, eh?  As
for the European symbol for the resistor you see - if you don't like it
- it's easily changed, and the the more common 'zig-zag' resistor symbol
IS in the library too.  

TinyCAD from SourceForge.  Get yours today.  Don't leave home without
it!  Notice the size of the *.png file produced when I 'export' the
drawing.  27kB.  Neat eh?

Observations, suggestions and even hateful criticisms (or flames) about
my converter (the fabulous and exotic - at times quixotic - CNV-412) are
welcome!

Les
formerly VK2BCU.

-- 
http://www.fastmail.com - Send your email first class

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