Now we’re to the point where we can use the audio oscillator as a signal source for the transmitter, so we can start to build up the transmit circuitry. Let’s start with the balanced modulator and crystal filter. On transmit, U2 (an SA-612 mixer) receives two inputs. At pin 2 it receives the audio from the mic or the the audio oscillator used for tuning and CW. At pin 6 it receives the output of the BFO, which is 9 MHz. The action of U2 is to amplitude modulate the 9 MHz signal with the audio input. The output of U2 is a double-sideband signal (9 MHz + the audio and 9 MHz – the audio). The crystal filter made up of X1-X5 and C32, C33, C35, and C36 removes the upper sideband so that only the lower sideband makes it to the transmit mixer U1.
As a reminder, recall that U1 and U2 are also used in the receive circuit. What we want to remember here is that U5 (the microcontroller) switches the VFO and BFO between U1 and U2 depending on whether we’re transmitting or receiving. On receive, the VFO frequency (~5MHz) is fed to pin 6 of U2, but on transmit it instead receives the BFO frequency (9 MHz).
One other note: the Survivor manual’s Theory of Operation section mentions that Q9 is an emitter follower circuit that buffers the output of U2 to provide the crystal filter with a resistive load, helping to reduce ripples in the output of the filter.
For this section, install the following components:
U2 (in its socket–make sure to observe the correct orientation)
C29, C32, C33, C35, C36, C37, C39
Q9 (make sure to install it in the correct orientation)
X1-X5 (make sure to read the instructions in the manual for installing the crystals–they need to be installed flush to the board and have their cases soldered to the square pads they overlay)
Once all the parts are installed, power up the board. An interesting first test is to verify that the BFO and VFO switch between the two mixers between TX and RX. You can use the digital dial to measure the frequency at pin 6 of U1 and U2 in TX and in RX and see how they change. Just for fun, I hooked up the o-scope and took a look:
A good place to see the output from the crystal filter is at the pad for R24 that’s closest to C29. If you look for a signal there while in RX mode you shouldn’t see anything (because we don’t have any input coming in from the antenna). If you put the rig in tune mode and then transmit by shorting PTT to ground, you should see an output like this one:
Using the digital dial you should measure about 9 MHz.
Here’s a shot at the board so far, connected for testing: