I put the scope probes on the two ends of the transformer at the output of the balanced modulator, set up the the display to sum the two signals, then tuned it to minimum level while in LSB and USB modes.
The balanced modulator is designed to block the carrier frequency, by driving equal and opposite voltages to the transformer. Any voice will unbalance the modulator, causing a frequency shift that produces a signal above or below, depending on sideband selected, and passes through because it is unbalanced by the degree of audio input signal.
In CW or Tune modes, the modulator is unbalanced by shorting on side of the transformer, producing the carrier oscillator signal on the output of the transformer. When balanced, the output of the transformer is only those frequencies that shift off the balance point, i.e. speech.
I will debug the voice input circuit to be sure I am getting the voice signal into the balanced modulator and that it produces the shifted frequency I expect. Nothing, so moved upstream to mic amplifier. Nothing. Moved on to the Astatic mic itself which appeared to have the right connector on it.
The mic stand is mis-wired! The microphone head plugs into the stand with a three pin connector. Pin 3 is ground, hooked to the case and it should be hooked to the shield of the mic cable and thus to the radio ground. Pins 1 and 2 should be wired to two of the four pins in the mic connector at the HW-100, but in fact pins 2 and 3 at the mic stand are shorted together deliberately.
Time to rewire this as it should be, validating each of the four pins on the HW-100/mic connector and through connectivity to the microphone head itself. With that done, I get voice on the audio amplifier output. Further, I get imbalance on the balanced modulator when voice is present.
I then retuned the zero balance on the balanced modulator to absolutely minimize RF output in LSB or USB with no input from the microphone amplifier. In CW and Tune, I get solid RF. I moved on to the transformer T1 that couples the modulator with the isolation amplifier V2 and confirmed the signal gets to the cathode of that tube. I peaked coil T1 to produce the highest amplitude and nicest looking version of the fixed RF signal.
I can't absolutely confirm that the balanced modulator produces sum and difference waveforms between the carrier oscillator and the voice frequencies because my scope gets confused in its frequency identification logic. Thus, I can't verify the frequency it displays but I do see varying RF signals as I speak.
Next up, I watched the signal at the output of the isolation amplifier V2, before it enters the crystal filter. The crystal filter has a very narrow passband centered at 3395 Khz, thus allowing only the upper or lower sideband signal to pass, or the CW signal if in CW or Tune mode. The output of the filter will go to the 1st IF amplifier and further.
I don't see any signal coming out of the crystal filter, but I might be loading it down with the scope probe I hooked up a jumper to use as a morse key substitute, triggering the amplifier when I hit the key. I can here a tone being produced. Also, the ALC goes up and down with the CW gain control.
Moving on to the first IF amplifier V3, I see the frequencies present at the output, which would be coupled to the first transmitter mixer V5A where it has the VFO mixed with it to produce signals in the range of of 8395 to 8895 KHz as the sum of the VFO (5000 to 5500) and the carrier oscillator (3395 CW/Tune, with additional modulation from voice if in LSB or in LSB).
The second transmitter mix will combine the 8395-8895 signal with the output of the heterodyne oscillator - the oscillator which is switched to different frequencies by the band switch - to yield a sum or difference that is passed through bandpass filters selected by the band switch. The sum or difference that was selected then is coupled to the driver V7 and ultimately to the final RF amplifiers V8 and V9.
What appears odd, since the ALC level goes up or down with mic or CW mode transmission, based on the voice level or the CW gain control, is that the relative power reading is always zero. I will look at this next because it is pretty essential to tuning and alignment. It also serves to tell me if the meter circuit is bad or if the transmitter is not operating.
The way the relative power is measured is a very simple circuit that rectifies and smooths signals from the final tank circuit, consisting of two resistors, a diode and a capacitor. I am pretty sure I am not energizing the transmitter with much power, since the dummy load would get hot if I were. Also, cathode current to the final tubes doesn't change noticeably.
I have a dinner party to attend tonight, thus have to stop work right now. I also have a pretty busy day tomorrow, which means I won't make much more progress on this over the weekend.
HEATHKIT C-3 CONDENSOR CHECKER RESTORATION
My replacement capacitors arrived late last night, ready for installation today. Fired up the unit and found it working plausibly, having tested an 8uf 450V electrolytic on the bridge and leakage functions. My 20K test resistor gave a slightly high answer on the Rx100 scale and a odd reading on Rx1.
No interest in using it for resistance measurement, will not investigate further. The magic cye is a bit dim, but still usable. Back in case and ready to use testing/reforming electrolytics in the future.
ALTO DISK TOOL
We have a session planned for Friday where I will attempt to archive as many of the cartridges on hand as I can. After that, we will test the tool on the Alto's Diablo drive and also debug the booting problem we are facing with cartridges I wrote.
The session will begin early because we have a long-ish interruption at midday, where we are panelists at a special lunch at Xerox Parc, talking with the staff about our restoration experiences. Afterwards we hope to get a few more hours of work in.