sound buzz
 

Once again my Muntz is giving me fits. I thought I was totally done with this set but it has other ideas.......
Turn it on and the sound is great......give it about an hour and it slowly starts buzzing through the speaker.......
text on the screen is worse.
The discriminator coil is a replacement from moyer's. All out of tolerance resistors have been replaced, and ALL of the caps in the sound circuit are new.
Although ceramic have been put in for bad micas.
Once warmed up well the buzz starts as soon as the dvd player I use gets turned on..
Any ideas?
Also This set does not have any power cord to ground interference caps.

Also can a tube cause this kind of problem?

One other thought.....The ratio detector coil I put in from moyers is of course new old stock.
It has 2 -5 picofarad caps in it. could those be the cause?
When I first installed it the sound was clean. And at that time was fighting other issues and have started putting many hours on this set.

You say you replaced mica with ceramic? Well ceramic caps can drift with temp, and discriminator circuits are pretty sensitive to drift. Hard to say without analyzing the circuit, but it sounds like a bad combination.

Could be the DVD player is producing RF noise at your sound IF frequency and messing with the audio.

Is there any change in the picture over this long period?
Does adjusting the fine tuning fix the buzz or at least change it?

this muntz does not have fine tuning.....high end huh?

Well the sound IF coil was junk......tested dead on the primary. The sound still has a slight buzz but even a recalibration does not get it all out. BUT what buzz is present is tolerable and comes on as soon as the cheap dvd player is turned on. one of these days maybe I will get cable and see if its cleaner.

Since your Muntz obviously is too old to have a video input, I assume that you are using some sort of a modulator.

Instead of feeding the TV directly, connect a rabbit ears to the TV and just let a 3 foot wire hang from one terminal of your modulator, with no direct connection of the set. This should lower the signal level enough to eliminate buzz from IF overload. It also will isolate the TV from your modulator in case the grounds are different.

And about cable, my newer box has no RF output !!!

James

tried your tip and no go.....but I found that even with the dvd off and nothing hooked to the tv it has a buzz all the time. I wonder if I need a line interference filter for the power cord....?

In these parts there is still analog cable so I can just connect my sets straight to the wall(though I normally use a VCR as a tuner as the channels I most watch are not on VHF or UHF frequencies).

I assume that the set is an intercarrier design. What usually happens to create such buzz is the video carrier momentarily disappearing when there is peak white in the video. Peak white corresponds to the lowest RF level in the modulated TV channel, and a misbiased or misaligned video IF stage going into cutoff killing the video carrier when the peak white happens. The video detector creates the 4.5MHz sound carrier by beating the sound carrier against the video carrier, and if that video carrier isn't there, you get no sound carrier into the 4.5MHz sound IF. These sound "holes" end up sounding like the buzz you're hearing.

I have a TV that suffers from this, but I cheated by reducing the amplitude of the video feeding the RF modulator. Leaving more RF amplitude when peak white happens.

The contrast control can emphasize this buzz too, if it's located before the sound pick off point.

Thanks for the explanation, WA2ISE. I used to have a B&W set back in the 70's that buzzed whenever the station put up digital lettering. Sounds like it's not the set's fault, if the station is over modulating!

Bobby D.

Getting the discriminator proplerly centered on the 4.5 MHz carrier helps reduce its sensitivity to amplitude variation. As described above, the dropout of the video carrier is lowering the amplitude of the sound carrier. Getting the discriminator exactly centered gives you the best chance of minimum sensitivity to the buzz.

I imagine the reason it deteriorates with time as the set warms up is that the tuning is drifting. You may find that simply retouching the discriminator secondary is enough to reduce the buzz after the set warms up. If that turns out to be the case, changing the type of capacitors used in the discriminator to a zero temp coefficient or negative temp coefficient capacitor may help keep it stable. It's OK to use ceramic caps in these tuned circuits if you find the zero or negative temp coefficient. Otherwise mica is probably better.

Make sure all stages of the sound IF are very nicely aligned. If you have a sweep generator, go for the most beautiful symmetric discriminator transfer function you can get, with the zero crossing right on 4.5 MHz.

You may even have to tweek the video IF as well, since too much rolloff in the video IF can alter the symmetry of the audio demodulation response. (Even if the 4.5 MHz IF is beautifully aligned, asymmetry can be introduced by the stages prior to the 4.5 MHz IF.)

All that being said, there are some sets (for example, Philco Predictas) where I also cannot completely get rid of the buzz when there is white lettering on the screen from a DVD player or computer generated graphics in a broadcast signal.