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

Fri Jan 9 17:46:56 EST 2015

Hi Les,

I built a 40m converter as front end to a BC-453 some time
ago. The converter was a beyond-restoration BC-455 originally.
The converter and Q5'er fit nicely into a heavily-prehacked
2-unit mount.

The converter uses the original RF amplifier feeding a crystal
controller converter, feeding the Q5'er. The BC-455 IF stages
are not present. The rest of the BC-455 real estate is used for
a small power supply (with VR-105/0C3 regulator) and an
audio output stage/speaker transformer.

The combination works nicely. It looks terrible, but at least
some of the badly hacked bits and pieces got a second life :)

73, ian K3IMW

On Fri, Jan 9, 2015 at 2:20 PM, Leslie Smith <vk2bcu at operamail.com> wrote:

>
>
>                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.
>
> --
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