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

Wed Dec 28 10:22:11 EST 2005

Yes, the 1254 provides a great foundation for modification or 
experimentation.

I put an RCA jack on the rear panel for muting the receiver, but I have 
not yet tried it with a transmitter. I might modify it to just cut the 
gain down, so it can be used as a sidetone monitor.

I'm pursuing low-cost filter options because many of the filters on the 
market cost half as much as the whole receiver. The Kiwa filters look 
like a good choice. I also saw a 6 kHz filter on the surplussales web 
site. The stock 4 kHz filter is a little narrow for pleasant shortwave 
broadcast listening (such as it it is). I'm thinking of putting in a 6 
kHz filter for broadcasts and something narrower for SSB and CW. The 
narrowest low-cost filters I have seen are about 3 kHz. Maybe I could 
make a good cw filter with a couple of those little plastic 455 kHz 
ceramic resonators. I did find a schematic for a solid-state 
Q-multiplier in the 1970 Handbook, but I'm  a little skeptical about the 
design. I think the design in the 1964 Handbook was better, but I don't 
have my 1964 Handbook anymore! Right now, I have no plans to hook up the 
Heath QF-1 to the Ten Tec, though it would be pretty funny and might 
work well.

I think that the 1254 is one of Ten-Tec's better kept secrets. Sure, its 
more expensive than the average shortwave portable, and it lacks the 
bells and whistles, but its a very solid design, in my opinion.

				73,

				Bob WB2VUF

On the S-meter, I was thinking of adding an outboard meter, but your LED 
idea is better. One could also use one of those LED bargraph driver 
chips. They can be set for a linear or log response.

There are more 1254 mods on the Morse Express web site 
(http://www.morsex.com/articles.htm), like shielding to reduce display 
hash, display shutoff, hiss filter, etc.

Jerry Volpe wrote:
> 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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