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

Wed Dec 28 11:13:38 EST 2005

Bwana Bob -

Sorry about mentioning my 455kc/200Hz filter.  Seems you are a fone
man.  I work CW 99/44 100ths per cent of my time so narrow filters are
a must for me.  Sorry to bother you.... just realized it is much too
narrow for your application.

73 - Don

----- Original Message ----- 
From: "Bwana Bob" <wb2vuf at qsl.net>
To: <kg6tt at arrl.net>; "Discussion of Ten-Tec Equipment" 
<tentec at contesting.com>
Sent: Wednesday, December 28, 2005 11:22 AM
Subject: Re: [TenTec] 1254 Q-Multiplier... Yes. Add a signal strenght 
metertoo and replace those 1N4148s in the 2nd Mixer!

> 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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>
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