For you fellows that are into RF;

I have an RF heating tunnel that is set up to auto tune to 13.5Mhz. The controller (counter) obtains it's signal from an approx 4" dia, single coil antenna located about a foot from one of the adjusting tank plates. The controller's reciever LC ciruit is fed to an LS20, then off to the counters.

My problem is that one of four units, works fine when a TTL level signal is injected, but fails to pick up on the antenna. I suspect my recieving circuit isn't tuned, but not knowing is why I ask. Anyone point me to some basic calculations that could help.

 

Do you mean the tuned circuit connects directly to the logic gate without any buffering or front end detection?

 

For you fellows that are into RF;

I have an RF heating tunnel that is set up to auto tune to 13.5Mhz. The controller (counter) obtains it's signal from an approx 4" dia, single coil antenna located about a foot from one of the adjusting tank plates. The controller's reciever LC ciruit is fed to an LS20, then off to the counters.

My problem is that one of four units, works fine when a TTL level signal is injected, but fails to pick up on the antenna. I suspect my recieving circuit isn't tuned, but not knowing is why I ask. Anyone point me to some basic calculations that could help.

The first plus you've got going for you is that you indicate you've got four of these and three of them work. I'd start by comparing some voltages and, if you've got a scope, some waveforms.

There are many equations related to determining the resonance of a circuit and in your case it sounds like air core inductors and some sort of capacitors.

One thing gets me though, I haven't a clue what a "heating tunnel" is? I would assume, at least in my wording, it's some sort of induction heater?

 

Do you mean the tuned circuit connects directly to the logic gate without any buffering or front end detection?

That is correct.

One of the newer units has a 3 stage transistor input section. That unit failed some time ago, and by jumping those stages onto the nand gate, as in the other units, we got it going. My thoughts are that we may be looking at reconstructing that transistor section however, for all the units. Any circuits one can recommend?

 

Without seeing some sort of schematics of what's working and what isn't I don't see how to, after all I'm still in the dark as to what the heck these things do to begin with.

 

I suspect the antenna itself it tuned. With such close proximety to the transmitter more is not needed. It may be no amplification is needed either. What does the antenna feed into? Is there a variable capacitor (or fixed value) across where the loop leaves the tank. The loop itself is an inductor as well as an antenna.

You're unit sounds somewhat similar to a plasma asher, which I have had some experience with.

 

I suspect the antenna itself it tuned. With such close proximety to the transmitter more is not needed. It may be no amplification is needed either. What does the antenna feed into? Is there a variable capacitor (or fixed value) across where the loop leaves the tank. The loop itself is an inductor as well as an antenna.

Fixed cap with varistor. The antenna / cap is configured as tank or resonator. Last hour yesterday I reconfigured the antenna, and this morning on start up everything is working.

I noticed on the older units that the 74LS20 is mounted in a socket as opposed to soldered to PCB. Must be a sign. I have an extra 1/2 doz LS20's in stock this morning.

Thanks to all.