The Ten-Tec 1254 is a great radio for the money. It is small, simple to operate and usable for ham radio or shortwave listening.

As a kit, it is easy to build and works well with very little fuss.

I've had my 1254 for over 4 years. This is a neat little radio. I've modded it to the point it is not the same radio at all.

About my mods....

1) I replaced the little RCA antenna jack on the back panel with a BNC connector.

2) The 1N4148 diodes in the mixer are less than ideal. The 48' is intended to be used as a switching diode or rectifier. It's capacitance, leakage and forward voltage specifications are not really very good for use in a diode mixer. I replaced them with some good schottky RF mixer diodes. This makes better use of the local oscillator drive and reduces intermod a bit. The end result is that you get better sensitivity and better strong signal performance. Don't use germainum diodes here , they are far too noisy. Get some good RF mixer diodes. I used Agilent 5082-2835 schotky mixer diodes. They can be difficult to find, and cost under $1 each.

3) I overhauled the 455 KHz IF and ripped out the AGC (the pin diode attenuator was too lossy). This set of mods is *extreme* and you might want to think it over before trying them. (This mod works really well, but disables the AGC). This mod is discussed in detail below. It is a ton of work , and really alters the way the radio performs, so be sure you understand what you are doing before you attmpt it.

4) The 455 KHz BFO oscillator uses a ceramic resonator. The original one had a lot of phase noise and you could faintly hear unwanted responses from it when tuning weak signals. I replaced it with another 455 KHz resonator and things improved a lot. It may be that using a proper 455 KHz quartz crystal in the BFO and tweaking the feedback capacitors on the product detector might reduce the spurious responses in the product detector a bit more. (This is untried).

5) One ought to be able to mod the existing AGC to provide positive going DC voltage to the source of the new JFET IF amplifier transistors. This will decrease linearity a bit, but ought to workable., and will provide those who must have AGC with a certain amount of happiness. (This is untried).

6) I've always thought a better encoder for the main tuning would be an improvement. The feel of the stock encoder is a little sloppy. (This is untried)

7) Some extra decoupling caps and shielding on the display board might reduce birdies radiated by the display. Overall it is not too bad, but it could be better. (This is untried and probably not worth the effort)

8) Pouring wax into the PLL VCO's pretty much eliminates microphonics. Don't get wax in the coil slugs. (maybe it is better to pour wax in *after* getting things aligned) I used a soldering iron to melt a chunk of wax and let it dribble neatly into the VCO enclosures before soldering the top shield covers on.

9) It would be really nice if there was a way to reprogram the PIC microcontroller to acheve 1 KHz tuning (or even 1.25 KHz). This would allow the radio to be easily useable as a communications grade reciever for CW/SSB. (this mod is untried).

10) The 1254' has a really good AF power amp. However, the built in speaker is not of sufficient quality to take advantage of the AF power amp performance. Uee a good external speaker and the radio sounds really nice.

The problem...

The problem is, TenTec packed 100 dB gain in two stages (the IC IF amplifiers). Doing so allowed the designer to reduce the parts count of the radio by only having one AF stage. It also reduced the size and cost of the radio, but causes it to have a really terrible noise floor , and leaves the IF with a tendency to oscillate when properly aligned. They tried to overcome the instability by lowering the Q if the IF transformers, with parallel resistors used to swamp the LC circuits. The results were adequate, but it resulted in a slight performance compromise.

You can demo this by unplugging an antenna while listening on a quiet frequency and you will notice that the noise output of the radio does not decrease much (or at all) when the antenna is disconnected. This means you can't hear the natural radio noise made by the earth. It also means you can't hear weak signals on HF because the radio is making too much noise internally.

My mods improve the noise floor, at a slight reduction in overall gain. The sensitivity and usable dynamic range is greatly improved. An unhappy consequence is that the AGC will be permanently disabled.

The fix...

Replace the IC IF amps with JFET transistors and add a high gain/low noise audio preamplifier/active filter. We are trying to lower the noise floor fo the radio and still have a reasonable amount of gain. We do this by replacing the noisy , high-gain IF amplifiers with low noise, moderate gain JFET amplifiers and adding some extra AF gain stages. The addition of a gentle first order active filter gives the audio a nice tailored response that is very good for SSB and AM voice communications, while being usable for CW and AM broadcast modes.

Click here to see the schematic of the improved IF circuit.

click here to see the schematic of the AF preamplifier.

Here is how...

Remove U8 and U9 in the 455 KHz IF. These will be replaced with MPF-102 JFET transistors later

Remove R61, 62, 63. (this disconnects the AGC circuitry)

Remove Q9, Q10, Q 11 , D14 and L12 . Replace D13 with a wire jumper to disable the AGC and eliminate the high insertion loss of the pin diode front end attenuator.

The IF mods...

Install MPF-102 transistors in the holes where U8 and U9 were as described below;

At U8 install the GATE lead of the MPF-102 at the hole for pin 4. Install the SOURCE at pin 5, install the DRAIN at pin 8.

On the underside of the board install a jumper between the holes at pins 6 and pin 7.

Install a 270 ohm 1/4 watt resistor between pin 3 and pin 5

At U9, things are a little different...

Remove R52.

Connect the gate of a JFET to the hole at pin 6 of U9, connect the SOURCE at pin 4, connect the DRAIN at pin 8

On the underside of the board connect a 4700 ohm resistor between pin 3 and pin 6.

On the underside of the board connect a 270 ohm resistor between pin 4 and pin 3 of U9

You can test the radio and realign the 455 KHz IF. The IF amplifiers hould not oscillate and the coils can now be peaked at resonance.

Now, you have a low noise IF. But you now don't have enough overall gain, so you need to build an AF preamplifier.

 If the IF amplifiers tend to oscillate, check to be sure everything is installed correctly. If all is otherwise okay, you may need to add a resistor from the gate of the JFET at U8 to ground to decrease the Q of the IF tuned circuit, I would suggest 100K as a good starting value for this resistor. The idea is to use the largest resistor value possible that will keep the IF amp from oscillating. Should the resistor value be too small, the Q of the input tuned circuit for the JFET at U8 will be too low and result in excessively low gain. If the resistor is too large, the amp will be noisy and may tend to "squeg" (oscillate) when the tuning slug of the input IF transformer is adjusted.

The 4700 ohm resistor at the gate of the JFET at U9 should not be changed. This resistor provides impedance matching for the IF crystal filter. Departing from this value significantly will cause the response if the IF crystal filter to be badly skewed.

The AF mods...

I used a home brew audio preamplifier with an audio active filter made with an MPF-102, and an LM741.

Build an inverting op-amp actuive filter with a gain of 10, and a cutoff suitable for voice audio response.

Then put a single transistor JFET preamplifier ahead of it. This provides a gain of about 1000 overall without adding tons of noise.

Building theAF preamplifier...

Build it on a 3/4 inch wide bit of perfboard. "ground" is the supply ground.

Pay close attention to layout and good decoupling from the power. This preamp has a lot of gain and you don't want it to be noisy.

Cut the foil between the IN pin of the audio amp (U6) and C65 (the output of the product detectors).

Connect the source of the JFET to ground.

Connect a 100K resistor from the gate of the JFET to ground

Connect a 4.7 uF cap from C65 to the gate of the JFET

Connect 1200 ohm resistor from the drain of the JFET to the +10V on the preamp board.

Bypass the power side of the 1200 ohm resistor to ground with a 10 uF capacitor and a 0.01 uF capacitor. keep the leads short and to the point.

Be sure you tap the 10V supply near the voltage regulator (U10). If you tap 12V near the AF power amp, the new AF preamp might oscillate (it is really sensitive).

Connect one lead of a 4.7 uF capacitor to the drain of the JFET

Connect the other end of the cap to a 10K 1/4 watt resistor

Connect the other end of the 10K resistor to the inverting input of the op-amp

Connect a 100K 1/4 watt resitor from the output of the op-amp to the inverting input

Connect a 0.0022 uF ceramic cap from the output of the op-amp to the inverting input (across the 100K resustor).

Connect a 10K resistor from the non-inverting op-amp input to ground.

Connect a second 10K resistor from the non-inverting input to the +10V supply on the preamp board.

Now, connect a 4.7 uF capacitor from the output of the op-amp to the IN pin of the audio amplifier IC ad U6.

Check everything over and test the radio , then realign the IF (if needed). There should be no smoke, no squeals, the radio will be very quiet sounding.

If you did everything right, you ought to have a pretty good radio.

I'm getting about 0.1 uV sensitivity for a 10 dB S+N/N. Adding a 20 dB preamp brings the sensitivity on low HF to insane levels, but it will overload easily.

Addimg preamp(s) ahead of the radio will add usable gain if their noise figure is good. But adding more gain will compress the usable dynamic range of the radio. With this in mind, keep the gain stages ahead of the radio to the usable minimum. Hard core users might want to add a high gain RF preamplifier with a preselector, followed by a 0-30 dB step attenuator between the radio and the preselector.

The radio will now be decptively quiet until you plug in an antenna, indicating it has a very low noise floor anf good gain. With an antenna disconnected you should hear a soft hiss soming from the radio with the volume turned all the way up. Turn the volume down to about 1/4 volume and plug the antenna in. and you should hear a big jump in noise output as soon as the antenna is conncted. Now you can hear the natural environemntal radio noises, which means you can hear the weaker signals on the HF bands. 

click HERE to return to hy home page