[TenTec] 1254 Q-Multiplier... Yes. Add a signal strenght meter too and replace those 1N4148s in the 2nd Mixer!

[Jerry Volpe]

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