Question about measuring and setting horizontal frequency

[Penthode]

Also, when adjusting the RCA synchroguide transformer, it is best to short out the ringing coil I believe terminals C and D first. After putting the the oscillator on frequency, remove the short then use the scope and follow the service notes to achieve the proper waveform. If you do not first short out the ringing coil when first adjusting frequency, it may cause spurious triggering and instability. Leaky paper capacitors and drifted carbon composition resistors plague this circuit so if after following the above you cannot achieve the correct frequency, check the components and the tube (especially if the resistors and capacitors have not been checked already!)