Zenith 8S563 acting strange

[TUD1]

My Zenith 8S563 has been working awesome ever since I fixed it last spring. But now, it's throwing me a curveball. I turned it on tonight, and I was getting really weird crosstalk from my AMT3000 which is set at 1410 KC, and another stong station downtown - 960 KC! How would that be happening? I spun the dial all the way around (from the inside since I still need a tuner belt), and that seemed to fix it. My transmitter now comes in strong with no crosstalk.

[Olorin67]

1410, minus 960, is very close to the IF frequency (usually around 455khz) i think you were picking that up. 2 signals can mix, and you get sum and difference frequencies, that's how superheterodyne radios work, the oscillator is 455 khz above the frequency you are trying to receive, then the IF amplifier amplifies the resultant 455 khz component.

[TUD1]

Ah. I thought it would be something like that. But what causes it to behave that way? Weak tube in the IF department? I was thinking that it could be a dirty contact somewhere, or possibly the tuning condensor coils touching.

[Electronic M]

Originally Posted by TUD1

Ah. I thought it would be something like that. But what causes it to behave that way? Weak tube in the IF department? I was thinking that it could be a dirty contact somewhere, or possibly the tuning condensor coils touching.

'There is nothing wrong, do not adjust your set.'

ANY IF frequency signal WILL get into the IF of the average consumer set (I have some interesting examples if you doubt me).

As has been said your transmitter is mixing with that local station in the air and generating (in the space of your transmitter's range) a signal that matches most AM sets' IF frequency.... The best way to fix it would be to re-tune your transmitter to shift the difference signal away from the IF.

It would be an EPIC feat of redesign to make the set immune to external IF images....You would have to shield the living crap out of it and add filters in the antenna and RF pre-amp to prevent frequencies below AM from getting to the mixer/converter stage.

As someone who has dicked around with AM and TV band transmitters, and a variety of signal generators I've seen this and a variety of other interesting effects.

[TUD1]

But it's never done it until now, and none of my other radios have ever done it.

[Olorin67]

Shielding might help, make sure the sets tube shields are all in place and making good ground contact. Not much you can do other than that. and pick another frequency. if your set has tubes with top caps, that might make it more susceptible, since they can act as an antenna.