I think I would pursue the Q-multiplier concept for many of the reasons
you mentioned. Circuits are not all that common.... or at least harder
to find these days. I am looking at one of my favorite older
communication receivers.... an Allied SX-190... which is solid state and
has a 455 KHz IF and a Q-multiplier circuit. Except for the transformer
itself the components are pretty common. This receiver was custom made
for Allied by Trio back around 1971. Anyway, you could use its circuit
as a good starting point.
Now How about adding a Signal Indicator Mod.... NO HOLES!
I just completed my 1254 a few days ago and I am very impressed. But as
I have a hard time with shortwave receivers that don't have some sort of
physical indicator waving in sync with the fading signals I just had to
add one to my 1254. I am very pleased with the outcome.
There is room just above the 1254's digital display for a sequence of
eight large rectangular shaped LEDs laying on their side. I used five
green, one amber (signifying S-9) and two red LEDs. They are driven by a
simple circuit comprised of two LM339 Quad comparators, an eight
resistor voltage divider, a calibrating variable resistor connecting
that divider to the 1254's 10 vdc, eight 1k dropping resistors and the
eight LEDs. Just for fun I added a .01 ceramic cap across the vcc of
each chip, a 100 MFD electrolytic on the add-on cards vcc, a 5.1 volt
zener to provide lower common voltage to the LEDs (so that their
intensity would be significantly less than the digital display). The
eight reference voltage taps from the resistive voltage divider go in
turn to each of the + inputs on the eight comparators. The comparator
negative inputs are all connected together and have the 1254's AGC
applied (see below). The output of each comparator goes through a 1k
dropping resistor to the cathode of its LED... the anodes of the LEDs
are all tied to a single 5.1 vdc zener with an appropriate voltage
dropping resistor to the 1254's 10 VDC. Remember, your LEDs of choice
may have different dropping resistor requirements... however if you use
the 5.1 vdc zener the current is minimal and the LEDs run pretty dim
which is nice in this application.
The entire circuit is built on a portion of a small Radio Shack DIP
style IC proto board. This board is suspended about 1 inch below the
speaker bracket using spacers. The LED leads are left intentionally long
and bent in an upside down 'L' shape to allow them to lay physically on
the display LEDs with their leads hooked over the the top of the display
circuit board. I ran a small strip of one-sided sticky foam on top of
the LEDs so that the pressure of the upper lip of the front panel would
gently push down on the LEDs providing a nice visual alignment.
10 VDC is obtained from the main circuit board and the driving AGC
voltage comes from the AGC contol line for the 1254's front-end pin
diodes. I provide needed isolation between 1254 pin diode circuit and
the S-meter circuit by applying this AGC voltage through a 33k and 1k
ohm divider network (the 1k resistor goes to ground. Initially, the
eight resistor voltage dividers were all 100 ohm resistor and I found
this worked out pretty well although I did see some compression on the
AGC voltage causing overly liberal readings. As a consequence I re-did
the divider using smaller resistors for the high end and larger values
for the weak signal readings.... about a three to one ratio from bottom
to top. My values are 85, 95.5, 113, 137, 165, 215, 255, and 324 ohms.
The resulting calibration follows closely several of my more traditional
receivers when all fed from a common multicoupler. Not accurate I am
sure but what S-meters are these days? Anyway, if I still had access to
a calibrated signal generator I would put in calibrated signals at 14
MHz and then graph the resultant AGC voltage. Then I could use good old
OHMs law to determine the appropriate resistors for the voltage divider.
This is a fun, one evening project that probably costs about $20 in
parts. No holes in the circuit boards or front panel. No circuit
modifications whatsoever other than tapping onto existing solder
connections in three places. To calibrate you tune down to 0.000.0 and
adjust the calibrating resistor till all eight LEDs just come on. The
resulting mode is fun, functional, looks great. In fact it fits so well
you have to wonder if Ten-Tec hadn't thought of doing something like
this themselves at some point.
the 1254 is a simple but well designed receiver for casual shortwave and
ham reception.
And why not a slightly better 2nd Mixer....
Oh, I also suggest you consider replacing the four 1N4148 diodes used to
make up the second mixer circuit with four Schotkey diodes for better
linearity and far less mixer distortion. I used 1N5711 but you could
also use 1N6263 Schotkey Hot-Carrier diodes too.
My next mod will be to insert a ceramic filter board which will switch
form the voltages provided to the AM and SSB LEDs. I'll keep the
existing ceramic filter for SSB and add a second much wider filter for
better shortware listening audio characteristics. Once again a simple
mod that does not require front panel holes... although it would require
removing and moving the existing ceramic filter. Kiwa Electonics has
reed relay switched filter boards which I believe would work nicely.
The 1254 will never be and RX-340 but it is no toy either.
Have fun and be a ham!
73,
Jerry, KG6TT
Fairfield, CA
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