[Mondial]

If the osc waveform only bounces slightly, but never drops out or changes drastically as you tune below 700, then the oscillator is probably not the problem. More likely the trouble is in the antenna tuning section of the cap, which is shorting out the signal at the low end of the band.

The AM sections of the tuning cap are the ones with many aluminum plates. There are three FM sections, but these have only a few plates each and are mounted between the AM sections. I am going from memory here, but I believe the AM ant section is the one closest to the front of the chassis and the osc is toward the back. You can confirm by touching each AM stator contact at the top of the tuning cap with your finger. The osc section will show a define reaction on the scope waveform when touched, the ant section little or nothing.

You can also try connecting an ohmmeter on high range to the top of the ant cap with the set off, as Terry mentioned previously. You don't have to disconnect any wiring. This cap section should measure several megohms to ground, as it is isolated by 4.7 megohm resistors to the AVC line. If the resistance changes suddenly as you tune below 700, then there is something touching between the plates and shorting out.