All the following is used with permission of the author. Thanks guys.
Increased Tuning Range
Two varactors are included with the SST with instructions to select the one that provides the tuning range you are most interested in. To use them both:
Install a small SPDT switch (RS 275-635) on the front panel.
Clipping the leads as short as possible, solder the hot side of each varactor to the outer terminals of the switch (hot lead of one varactor to one side of the switch, hot lead of the other varactor to the other side).
Using a short wire, connect the center lead of the switch to the hot side of the D4 mounting position on the circuit board. With the D4-side of the board facing you, this will be the left-side hole.
The two remaining leads of each varactor are now joined together and connected to the other D4 mounting hole.
Linearity can be a (minor) problem with varactor-tuned rigs. To significantly reduce this in the SST, solder an 18k ohm resistor across the tuning pot between the wiper lead and the +8 volt lead. Experiment with this value as it varies from band to band and from one type of varactor to another.
My SST 40m version also howled when the audio gain was at maximum. Looking at the application notes on the LM386 audio chip that is in the SST, I noticed that there is a comment that if the chip oscillates then an optional DC filter can be installed to prevent the oscillation. The filter is only a resistor and a by-pass capacitor placed in the lead supplying power to the chip. I have installed the modification and it seems to work. The changes can be done on the etched side of the pc board.
1. cut the trace going to pin 6 of U3
2. solder a 100 ohm 1/4 watt resistor from pin 6 of U3 to the contact next to pin 1 of U3 .
3. solder a .1mf disc cap from pin 6 of U3 to pin 4 of U3
There are two ways to reduce the sidetone volume in the SST – reduce the AF Gain or, as recommended by the rigs’ designer, Wayne N6KR:
Usually there is a good balance between the incoming signals and the sidetone in the SST. But you’re right–if you have to turn it way up, the sidetone gets loud.
The only sure-fire way to mute the sidetone to a lower level is to insert a low pinch-off JFET like a J201 into each leg of the audio connection from the product detector to the audio amp. This is how I do it in the NC40A. Actually you can probably find other JFETs that will work; J310s are mostly low pinch-off, too. MPF102s will work if you hand-select them for low pinch-off.
The source leads go to the ‘602 and the drains to the original capacitors that go to the ‘386. Tie the gates together and add a 1 to 10M pull-up resistor from the gates to one of the source leads. Next, connect a diode from the gates to the key input. Test the circuit thus far by keying the rig: you should hear ZERO sidetone at this point, because pulling the gates low cuts off the JFETS, making them look like an extremely high resistance. If you hear a click on keydown, put a resistor (start with 1K) in series with the gate diode. If you hear a click on key-up, add a capacitor from gates to ground; start with about 0.1uF and see if you can go smaller. (It will depend on the pull-up resistor; 10M and .047 work well in most cases.)
Once this much is working, you can add a resistor *across* one of the JFETs (source to drain) to allow some sidetone to sneak through–as much or As little as you want. It will take a large resistor, something like 1 to 15M in my experience.
20 Meter SST to 15 Meters
Dozens of requests on moving 20mtr SST to 15mtrs, so here is how I did it. If you make changes or improvements other than these, please let me know so I can use them as well.
These are standard values from any handbook:
L2, L3 = 11turns #26 on T37-6(yellow)
C34, C36 = 150pf, I used silver mica
C35 = 270pf ” ” ”
L1 = 18t #26 on T37-6(yellow)
C27 = 30pf, I used ceramic NPO VXO
X6 = 25mhz crystals. I used two, one on top, one under board.
RFC3 = 23t #26 on T37-2(red). This was a critical value for proper oscillation and range. You may have to adjust a turn plus or minus, or compress or spread turns to adjust to desired tuning range.
RFC1 to 2.7uh(or close to it). Improves match to Filter and recieve.
2.2uh molded choke from base of Q2 to ground.
As above changes were all that was neccessary to put it on 21.050 to 21.070 with the MVAM108. Output was 1 watt, so I put in MRF237(ECG341) for two watts out. Its also very easy to move to 17mtrs by using same approach.
I put two crystals in both of my SST’s to extend vxo range. One is 20mtrs and the other is one I moved to 15mtrs. With the MVAM108 varactors I was able to get about 20khz range. I was experimenting with different varactors I had on hand. Was not getting much difference. Then I tried a MV1404. WOW! The 20mtr version is covering from 14.013 to 14.064. The 15mtr version is covering from 21.039 to 21.073. Scope shows solid waveforms
across oscillator ranges. I don’t know where to get them. They don’t show up in my cross reference, so I don’t even know what they are rated as. Mine were in a envelope of a dozen, that was in one of Dans Small Parts
20lbs of parts for $10.00. They are not listed on his current page. If any one knows what they are rated or where to get them, they might inform the rest of the list.
The TiCK keyer in my SST is mounted on the rear panel above the key jack. The new jack (required for paddles) is mounted above the RF Gain control on the rear panel and the pushbutton switch is mounted above the antenna (BNC) connector. Using a Xerox machine, I reduced the the size of the TiCK’s program menu and laminated it to the top cover of my SST.
Also taped to the top panel is a tiny chart showing my operating frequency at 9 O’clock, 12 O’clock and 3 O’clock positions for both positions of the SPDT varactor switch.
To make tuning easier, a larger tuning knob is helpful. I use a RS 274-402.