FM-10A Frequency Stability

[Fletch]

Hi All... I just got my FM-10a today and built it. It seems to work reasonably well, and if left alone it is pretty stable. But I have noticed that moving the audio input cables, or switching to a different input source (i.e. from a cd player to a cassette deck) the frequency jumps all over the place and it needs to be re-tuned (I put a small hole in the top of the case for this purpose).

Any ideas why this is happening?

Thanks -Alan

[jay]

There is no rf decoupling between the audio, and power jacks on the FM10A. Essentially, all the cables connected to it are acting as part of the antenna system and moving them will change the reactances in the oscilator stage in turn changing the frequency.

[Fletch]

Yes, I had pretty much surmised that the cause was something like that. I guess the claims made by Ramsey on this old but venerable design, are a bit of a stretch.

But, on the cheap, I guess it's livable.

Time to save some pennies and get an upgrade to FM25B. :(

[mingkee]

in my opinion, stablity is worth to pay
FM10A is just a hobby kit, if you're a little bit more concern about stablity, FM25B is better choice, and comes with full kit (with housing and AC adaptor), and less hiss, and better audio, that FM25B is good for your next project
of course, FM100B is much better than FM25B, with audio mixer and level indicator, and metal housing provides way better grounding, and upgradeable to higher output with 1watt version, or with LPA1 or FMBA1 (recommended because it has better filtering), and it's autovolt, just use a plug adaptor, you can use anywhere in the world

[e92fm]

>FM10A is just a hobby kit, if you're a >little bit more concern about stablity, >FM25B is better choice, and comes with full >kit (with housing and AC adaptor), and less >hiss, and better audio, that FM25B is good >for your next project

do yourself a favor and buy a REGULATED power supply, AC adapters are nice but they fluxuate in voltages a lot verses power supplies, for ANY RF electronic kit I reccomend buying a regulated power supply

[mingkee]

regulated power supply not only supply stable voltage, it provides cleaner DC current for hum-free operation, moreover, the better regulator provides RF filtering as well
I like FM100B, it works well

[Fletch]

Hi All..

Hmmm... seems everyone thinks FM10 is not worth bothering with...

Anyway with some insightful care as to construction and audio processing, not to mention some slight modifications, although VCO controlled it can be made to perform admirably.

Not too sure where the power supply came into question... have run the unit first during testing on 12v gel-cell battery (pure DC) and now on a fully regulated computer grade switching supply at 13.8v.

Notes that instability can occur from antenna mismatch.. the unit doesn't like high SWR operation. The RF likes to find its way back through the VCO and audio chain and then when you move things around and sometimes even breathe the wrong way... LOL... you will find yourself 50-100KC off- frequency.

The best solution is to put the board in a metal case, remove the RCA jack antenna output and mount an SO-239 output jack on the metal case. Connect it from the board using a coaxial cable (try to find the small diameter flexible type coax used in test equip) to the SO-239 mounted on the metal case. Then use a well-matched antenna to prevent SWR (Standing Wave Ratio) problems where the antenna isn't resonating at the chosen frequency, causing RF (Your Signal) to be reflected back into the transmitter and wreaking havoc on the circuit.

These suggestions should improve even results from the highly touted FM25b.

DON'T EVEN THINK ABOUT USING A PLASTIC BOX --This includes the Ramsey supplied ones, unless you just want it to look cool and not work as well as it could.

-Alan