AVO CT378AFM - how does the FM modulation work?

[Studio263]

Can anyone see how FM modulation is applied to the AVO CT378AFM HF signal generator? I have a CT378A (kindly donated by another forum member) and it would be useful to be able to add a basic FM facility to give the standard +/- 75kHz deviation at 10.7 MHz for testing FM receiver IF strips.

I've borrowed the circuits for both from elsewhere on the forum and attached them to this thread. My guess was that the reactance element would be a diode, but all the ones in the -AFM circuit seem to have direct equivalents in the -A circuit (AM only). V4 clearly has something to do with it, its anode is fed from the HT supply through a complicated network of coils and capacitors and its screen is connected to the stabilised HT across the neon regulator V2. There is an input to the control grid which disappears off into the switches and a sweep output which presumably goes off the 'x' input of a 'scope, but I can't see how all this lot influences the frequency of the main oscillator.

These circuits aren't that well drawn; for example the coupling that exists between some of the coils isn't made very clear and its hard to work out what the switches do in each position.

Any ideas?

[Nuvistor]

The input to V4 control grid seems to come from the audio amp V3b via the switches, L7 is listed as the FM coil, is L7 located near the osc circuits V5?
Presume V3a is an audio oscillator.

[ms660]

FM is for ranges F and G (20-100MHz) according to the switchery in the schematic.

Lawrence.

[Radio Wrangler]

I suspect you'll find that L7 has polepieces with the cores of the inductors for ranges F and G between them, so that FMing is done by partially saturating the VHF cores.

FM was a devil to do in the valve era. Saturable reactors, motor-spun butterfly capacitors, you name it. The humble 'reactance valve' technique was non-linear and limited in range.

Most FM transmitters used "The ARMSTRONG Method" And yes, there really is one. Invented by Edwin Armstrong the inventor of FM himself (do narrow deviation mod on a low freq oscillator followed by a chain of multipliers. They multiply the centre frequency and the deviation as well)

David

[ms660]

Originally Posted by Studio263

its hard to work out what the switches do in each position.

For the AFM2, the range switch setting in the schematic is on the highest range (Range H) When the rear switch wipers are rotated clockwise on the schematic the front switch wipers will rotate counter clockwise by the same amount, ditto for the function switch wipers, the function switch is shown in the CW position, that tallies up with the fact in that position as shown, the output from the AF Osc amplifier is not connected, the AM modulators AF input is in effect grounded and also that three clicks clockwise of the front switches from the said position connects the AF In socket to the grid circuit of the AF amplifier as it should do (eg: Ext. Mod.) Once that bit's figured out the rest should tally up ok....

….so far as I can make out.

Lawrence.

[ms660]

Error alert.....I think clockwise not counter clockwise.

Lawrence.