History is a marvelous teacher which a large segment of our society recently ignored, and we have the chaos we have today. As my mom used to say: "a leopard doesn't change his spots". So, it is with ham radio as historical events serve us well as we try to troubleshoot our rigs.
Early SSB transmitters did not use crystal filters and our amazing SDR rigs today are direct descendants of those efforts using phasing techniques. The W2EWL converted ARC-5 SSB mobile rig and the SSB Jr. from the GE Ham News are the history.
The SSB Jr. used three tubes less the RF drive coming from either a Crystal Oscillator or VFO. Before you hit the send button yes, the "IMP" from Joe, W4IMP was also a three-tube circuit and later converted to solid state.
Basically 1/2 of a 12AU7 is a Microphone amp which passes audio through an RC Phase Shift Network to create an I and Q signal streams and then amplified by two sections of the 12AT7. The 2nd half of the 12AU7 (the other half of the Mic Amp) amplifies the RF Drive operating at frequency, like 7.202 MHz.
The I/Q audio plus RF is magically mixed through the actions of T2, T3 and L1 and L2 and the Diode Mixer (G1-G4). The Pots R16 and R17 enable balancing the inputs to the diode mixer so that the final product is a SSB signal. The result is amplified by the 6AG7 and thereby on the air. Pots R5 and R12 also are involved in providing AF Balance. R5 assures that audio stream to the RC Phase Shift Network is balanced and R12 assures the I and Q streams from the 12AT7 are balanced.
Yes, immediately there are 4 pots that require frequent fiddling. That is one of the reasons why the shift from Phasing to Filter topology -- too much fiddling. But the advent of Digital Electronics, embedded computers and improved component stability has now reversed the shift back to phasing and digital signal processing. So, if you really want to diddle with the receive and transmitted signals then SDR is the path.
Now the history part and a little insignificant item known as a DPDT switch which you can see right below T3. This is how you select Upper or Lower Sideband. The switch mechanically reverses the I and Q signal streams which results in a Sideband inversion.
In my early work using the ADE-1's I found that often you had to invert the Sideband in the QUISK software so that when you selected LSB in fact you would be receiving LSB. Since there was no DPDT switch, it was merely a case of how the circuit was originally wired. Then I discovered that while I fixed the receive part there was a sideband inversion on transmit.
My fix to this problem was a DPDT relay that would reverse the sideband only on transmit. So now with the relay-- LSB on Receive and LSB on Transmit. When I read the manual closely, I realized that QUISK also had a button to have the software switch the sidebands on Transmit and so no relay needed. RTFM!
Yesterday I had a head scratcher as I noted that I had USB receive when the QUISK button was on LSB. What in the hell is going on here? Sure, enough the Receive sideband selection in the QUISK software had not changed but the real signal did.
After about 10 Minutes I realized that the answer was right under my nose. If we go back to the schematic, we have a case of raise the bridge or lower the water. In the SSB Jr. at the time it was developed, about 75 years ago, it was far easier to reverse the audio stream than the RF stream. But changing either one would switch the sidebands.
The Phased RF signals coming from the Si5351 CLK0 and CLK1, are terminated in SMA connectors and a 6-inch length of Shielded cable interconnects to SMA connectors on the main board. In the process of doing some work on the main board I had inadvertently reversed the cables from the Si5351 to the main board. It was staring me in the face and regrettably the last thing I checked. Switching the cables back to the original configuration fixed the problem.
I sure wished that a simple cable switch could fix other issues that face all of us today.
Them that know can make it go.
73's
Pete N6QW