Model 1254 Signal Indicator Mod

From the Ten-Tec Reflector December 27, 2005

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!

Jerry, KG6TT

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