It looks like a new 12AT7 tube may be in order for this unit. Here’s what I found as I was tracing through it. You can refer to the schematic, there’s a link to that in the last post.
First I removed the 6C4 audio oscillator tube so I knew the audio modulation circuit was not going to affect the measurements.
Then I started looking at the RF modulator and what I found was that it was oscillating quite nicely, but the amplitude of the oscillation was so high that it was overdriving the second triode stage, thus causing the distortion. Here are the signals on the gate and plate of the right-hand triode in the circuit diagram (pins 7 and 6 respectively).
Both signals are at 50V/division and ground level is one line down from the center of the display.The gate signal is on the bottom and the voltage on the plate is the top waveform. The frequency is set to about 200 KHz.
Looking at the data sheet for the 12AT7, it’s clear that by the time the gate voltage gets to about -4 volts, the tube should be shut off completely and the plate voltage should be at maximum. But what we see here is that the gate voltage is reaching about -70 volts before the plate voltage reaches maximum. So it appears that the gain of this tube is way down from the specification. That’s not too surprising since the tube appears to be original – it has a “made for Knight” stamp on it.
I did then replace the 6C4 audio oscillator tube and checked that circuit, which seems to be working fine and provides a nice sine wave at around 600Hz to modulate the RF signal.
So now it’s time to order a 12AT7 tube and see if this diagnosis is correct.