Advance FM/AM signal generator type 63a

[Winterpip57]

Hi all I'm new here, does anyone know if I can reduce the deviation from 22ks down to 2.5 ks by moding it please, note note I'm ok with transistors etc ,but tubes I'm a beginner .and cautious because of the high volts in tube items..
Many thanks....Paul

[Cruisin Marine]

Do you have a deviation/modulation meter?

[Winterpip57]

Hi ,no i havent...paul

[Winterpip57]

If i know how to mod, then i can monitor on a rx and get it aprox right...paul

[Radio Wrangler]

There is a potentiometer, R6 which will affect the amount of deviation, but this generator dates back to a period when all FM radios used much wider deviation than 2.5KHz. Broadcast radios using 75kHz deviation, PMR sets in taxis, police etc had 50lHz channel spacing and deviations of maybe 20kHz.

I can't say for certain, but it is unlikely that it can be easily reduced so far, also the amount of deviation will change from band to band and as you tune around each band, and you would need something to measure the deviation with to know what you've got.

Such narrow deviation was introduced chiefly on the amateur 2 metre band when growing congestion caused the channel spacing to be cut from 25kHz to 12.5 kHz. This is now so narrow that very little of the signal/noise ratio advantage that FM has over AM remains. Amateur repeaters now automatically check the deviation of incoming signals and if it is too large, then the repeater closes and ignores the signal.

Signal generators for narrowband FM are usually more complex than this and employ a fixed frequency oscillator mixed with a variable one. The variable oscillator gives the generator a wide tuning range, while FM is applied to the fixed oscillator. This gives a deviation which remains constant across the band.

The advance sig gen is adequate for repairing FM broadcast radios, but the narrowness of modern communications FM systems makes them need more sophisticated, laboratory grade instruments.

David

[Cruisin Marine]

I don't know the genny, but a quick search on the forum showed this thread https://www.vintage-radio.net/forum/...ad.php?t=97710
It seems you can achieve this by using the FM2 input, refer to post number 10. You will need an external audio source/generator to do this, but it is the best solution as it doesn't involve any modifications, which would be a bit hit and miss/a bit touch and go without a modulation meter.
Of course an audio generator is a must to have, so not a waste of money, an asset indeed.

[Winterpip57]

Hi thanks for that, i will look at that thread, must addmit thatdos look like a better idear,..many thanks...paul

[Winterpip57]

Also i would like to thank david for all the info, just need to find that pot now...twidling time😁..many thanks...paul

[Radio Wrangler]

When you're working with VHF receivers so narrow, you will run into difficulties with a free-running oscillator as a sig gen simply drifting with slight temperature changes. and the tuning knob won't be low enough geared.

These are things you have to experience to really grasp the magnitudes involved.

2.5 kHz in 145MHz is +- 0.00172% which is 17.2 parts per million which is the realm of decent crystal oscillators. Cheap crystals are more like 100ppm down to 10ppm for good ones and below 1ppm for temperature-compensated types.

David