Let’s continue the transmit chain by adding the parts for the transmit mixer and the amplifiers (up to but not including the power amplifier, or PA). I originally planned to do only the TX mixer but when I was done there wasn’t enough output to drive the digital dial output, so I added the two transistor amplifiers that drive the power amplifier in order to boost the output. There’s really not a whole lot to analyze here, and because I’m on a deadline for finishing this radio in time for QRP To the Field (QRPTTF) in less than two weeks I’m not going to spend much time trying to explain this part of the circuit.

U1 is the other SA612 chip (U2 is the other one) that, on transmit, mixes the 9 MHz signal from the crystal filter with the ~5 MHz signal coming from the VFO, so we call it the transmit mixer. The output of the mixer is actually a bunch of frequencies, made up of the sums and differences of the 9 MHz signal from the crystal filter and the 5 MHz signal from the VFO and their harmonics. That ends up being a bunch of different frequencies, but the strongest signals are going to be the sum of the fundamentals (14 MHz) and the difference (4 MHz).

The mixing product that we’re interested in is the difference between these two, putting us in the range of about 4 MHz. Increasing the frequency of the VFO (by turning the big tuning cap) will reduce the frequency of the output of the transmit mixer. The output from the mixer comes on pins 4 and 5, and that’s fed through a tuned circuit formed by T4 and its neighboring capacitors to filter out the unwanted mixer products. What’s left gets amplified first by Q8 and then by Q4. The output of Q4 is ultimately what drives Q16, the IRF510B power transistor.

So, to build this part of the circuit, install the following components:

U1 (and its socket–make sure to orient correctly)

C12, C17, C18, C20, C21, C22, C24, C25, C26, C38

D4

Q4, Q8

R7, R8, R9, R10, R12, R15, R16, R19, R20, R22, R24

RT1

T4

We’ll test this part of the circuit by powering up the board, putting the rig in tune mode, and checking for a frequency somewhere around 4 MHz at the output of Q4 (I used the bottom left pad of where T2 will be installed, but you could also test at the Q4 leg closest to T2) while transmitting. By this time you should know how to wire everything up to make that test. When I did it, I got a frequency around 3.85 MHz. Here’s a picture of my test setup:

Testing the output of the transmitter circuit
Testing the output of the transmitter circuit

You can see that the digital dial reads 3.84, so I’m at least in the ballpark. I’ll have to adjust the windings on T5 at some point to get the frequency range I want.

Next: The Receive Mixer

4 thoughts on “Building the Survivor: TX Mixer and Amplifiers

  1. Hi Dave:
    Enjoyed your series tremendously. Wondering if you will have a segment on the power amp section. Max voltage I get on gate of q16 is 0.5 V and no indication of any output.

    Also wondering how you resolved your hi vol at speaker after building the rest of the circuits. I’m hardly getting any volume. Just barely
    Just bearly hear the tune beeps when activating ptt. Yet my scope readings at your various test/check points seem to agree with yours. The output at pin 5 of U8 is a clean ~1 V PP, but after c63 the wave looks distorted. -VE half larger than +ve and filled/dark not clear as +VE.

    Input Amp draw is ~45 ma quiescent on receive and 170ma when v3 up all the way, yet with barely hearing tune beeps.
    At 170 ma feed, u8 very hot at touch.

    On receive I spotted two very faint stations at 3.75 and 4 MHz at nighttime ~ 11pm est at max vol.

    Any thoughts appreciated

    Paul

    Reply
    • Hi Paul,

      I’ve finished the rig but haven’t written up the last section yet–I needed to get it done so I could take it with me on a backpacking trip. I can’t think of any obvious reasons for the problems you’re describing, but make sure to check for misplaced parts and missed solder joints and whatnot. You might try posting to the Hendricks_SSB Yahoo group to see if someone there (like maybe KD1JV himself) can give you some clues.

      73,

      Dave

      Reply
      • Hi,
        Very much looking forward for the completed write up and first usage iimpression. Any clarification you would be able to provide on how to change the tuning range with t4 in parallel I do not get it.
        Do you think anything speaks against integrating a SOTA mini tuner into the’ housing of the survivir so all is in one box?

        Thank you for sharing

        Reply
        • Klaus,

          If you’re asking about how to reduce the tuning range, You can do that by not jumpering the two outer tabs of the tuning capacitor (just connect one to the board). If you do that, you may need to adjust T5 to set the tuning range in your desired section of the band (you may even have to add or remove a turn). Once you have all that set, then you need to adjust T4 so that you get the maximum power out somewhere near the center of your tuning range.

          The only issue I can see with the tuner would be whether there’s enough space in the Survivor case. It seems a little on the tight side to me, but if you have one of your own, you can probably judge that for yourself. I’m not sure 75 meter SSB would be very practical for SOTA activations, if that’s what you have in mind. I’d think most of the activations would be during the day, better suited to higher bands, and the smaller antennas required by the higher bands would be more practical. Just my two-cents’ worth.

          73, Dave NK0E

          Reply

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