Adjusting video level for IF

[TinCanAlley]

Does anyone know where the video level adjustment (assuming it's a coil on the IF module) is located? On bright scenes and white text I'm getting an overload of signal causing a lot of buzzing. I looked up the problem and it says I need to lower the video level (usually located near the IF section).

I found that IF module has a "video input IF" coil. Is that it? The instructions says to adjust it while viewing a bright scene until the noise is gone.

Thanks!

[zeno]

Do NOT mess with any coils at this time, you will probably
regret it. You can try the AGC delay. There was also a cap
that went bad in the AGC IIRC it caused snow. Also what are you
using for a scorce ??? You can also check the AGC at the
IF module, yellow wire & be sure its in range both with &
without signal.

73 Zeno

[TinCanAlley]

Originally Posted by zeno

Do NOT mess with any coils at this time, you will probably
regret it. You can try the AGC delay. There was also a cap
that went bad in the AGC IIRC it caused snow. Also what are you
using for a scorce ??? You can also check the AGC at the
IF module, yellow wire & be sure its in range both with &
without signal.

73 Zeno

I have a dvd hooked up to an RCA RF modulator. When the scene gets bright (credits, sunny day, etc..), the noise starts.

I'll check the AGC. The SM says to tune to a weak station without snow. Adjust until snow appears and back off 'til it's gone. That's gonna be a bit hard since there are no broadcast stations anymore. Still, I can adjust it when there is buzzing and see if it helps.

[wa2ise]

It might be the RF modulator doing it. You can lower the video signal level feeding the modulator by adding say 75 ohms to ground. Or lower resistances. The buzz happens when the picture carrier gets completely cutoff, white level is the minimum RF modulation level, sync tip is max RF carrier level. Get too white and the picture carrier disappears, and the intercarrier system no longer sees a 4.5MHz sound IF. "Holes" in the sound, yields the buzz.

[Username1]

When I was working for a tv shop back in the 80's and cable was growing, Panasonic had a few of their sets that would get an overloaded signal when fed by cable, but work fine with ota reception. Their fix was a replacement of a few if parts, and then an in field "turn of one coil" that would reduce or eliminate the problem. I also have a few tvs that are sensitive to excessive noise when fed from such things as dvd or vcr's.

If you have identified the coil you wish to turn then you should try it. But do it in a way that prevents you from ruining the alignment. Turn the coil one or two turns, keep track of that you did, If its the right coil, there should be a significant change. If not, then turn it right back to exactly where it was, and do the same thing with a different coil. Don't chose a random coil, and don't do more than one at a time.

If its an old tv, some had buzz controls, others had both the last video, and sound detector coils that effected buzz from sharp white writing on the screen. AGC is a good place to start, check that first, since you will be feeding old tv's now with non broadcast signal sources, you can de-tune the AGC with little detrimental effects.

Stay away from first, second, third if coils, you don't want to mess those up. You want the stuff after that, where the detector is just before the sound amp. Those coils decide where the circuit is choosing the frequency to pick the sound from, if its up or down from there it is, you will get poor sound, or the buzz.

wa3ise has a good idea though, you could use a resistor impedance matching network to drop part of the signal across a load before the tv sees it. This can cut the signal level down, and then you can make up the rest with the AGC control.

All are possible fixes, you should do the least troublesome first, like the resistor network, AGC, turn coils last, but do it in a smart way, where you can put it back the way it was without en entire re-alignment. You might even want to look into a signal attenuator you can get at solidsignal for a few bucks.