Buzz in sound from video - a porthole

[kramden66]

yes i get nice snow , the problem is not weak video it is a buzz caused by the video signal , it is not over modulated , tried a vcr , rf modulator and another rf modulator , all the same , what i said was if you turn the agc to wear it is weak and is so grey you are starting to lose sync the buzz is gone , increase the agc just a little and buzz is there but fain , increase to where it starts to be less gray and more black and white and buzz .
perhaps this means something , the buzz control when turned hits a point where the sound goes out and comes back , it acts more like a coil , perhaps the quad coil isn't right because it really has no effect on the sound yet it really should , or the little resistor looking cap on it is no good.

[old_coot88]

So does the quad coil have continuity as measured with an ohmmeter (on the RX1 scale) or not?

[wa2ise]

Originally Posted by old_coot88

So does the quad coil have continuity as measured with an ohmmeter (on the RX1 scale) or not?

It should measure around 10 to 30 ohms, I'd think. Definitely less than a few hundred ohms.

If it shows an open, check for corrosion of the coil leads at the terminals (inside the transformer can).

IIRC, any coil and cap that can be adjusted to resonate at 4.5MHz would work as a replacement. This kind of FM detector is very common in tube TV sets, so finding a replacement should not be hard to do.

[earlyfilm]

Originally Posted by kramden66

. . . . . . perhaps this means something , the buzz control when turned hits a point where the sound goes out and comes back . . . . . .

Yep!

It means that in all probability the slider in the buzz control is not making contact with the resistance element and when that happens, it opens the cathode on the audio detector and your sound goes out.

This either indicates a dirty control or a bad control.

It is also possible and quite probable that the sweet spot on the buzz control is in that open spot.

To test the control:
Put an ohm meter on both ends and it should read 600 ohms. Put an ohm meter on the cathode end and on the slider and when the control is rotated, it should vary between 100 and 600 ohms with no open spot.

James