I have a Kenwood TS930S, its a fantastic old girl and has given me hours of enjoyment.

The only thing it hasnt got is FM which i want to add to drive a transvertor. I contacted a company who make FM boards for many Ham and CB rigs, and they say its not easily done after looking at the PLL circuit.

I dont give up that easy, can it be done?

Any idea's and suggestions welcome.

John G8TDU

 

I don't think so. Given the usual loop filters in a PLL, pulling one to a different frequency is not likely to achieve the performance required of FM transmission. I thought that an actual FM circuit might use varactor diodes to get the required deviation for NFM modulation. Even that technique may be out of date by now.

 

I came from the 'generation' where we adapted old AM rigs for FM, this usually involved just adding a varactor or diode across the osc and feed a little of the original AM audio into it.

This kinda approach tends to die a little hard when you want to FM something, and somewhere there will be an oscillator lurking, just waiting to get a varicap slapped across it. Well thats the theory anyway

Murphy's law is bound to take a grip somewhere along the line, and after a chat with the guy who makes FM conversion boards (amongst other things) he is a renowned and clever engineer, and after looking at the block diagram of the 930, he said it couldnt be done in a simple way.

I have posted another question about changing a CW signal into FM without going inside the original equipment. One solution was to 'Mix' the original signal up or down, and then 'Mix' it back to the original. During this up/down conversion, one of the oscillators can be FM modulated, thereby modulating the original signal.

As the frequency involved is 28 - 30 Mhz to drive a transverter i think this is achievable, but surely there must be a simpler way to get FM on this old set?

 

Have you got a circuit of the radio?

 

Hello Dave,
Yes i have, but the quality is probably fair. There are 'one or two' places which look possible to add FM, but im not 100% sure.

It would be ideal if i could FM all bands, that would then give me the freedom to use FM with any IF frequency with any transvertor.

John

 

To convert the receiver section from AM to FM, you need to add a ratio detector or Foster Seeley discriminator, i would use a pll chip like the ne567 or cd4060

usually the IF in an FM set is at 10.7Mhz, so in your set the if will be at 455-470Khz usually...

any ideas here??

 

Thanks for that Dave, its not the receiver which really is the Issue, although my choice would be a 3089 IF chip which would allow me to fit a crystal filter. Its the TX side which is proving a little harder to solve.

John

 

Whats the circuit of the modulator in your set to convert AM?