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.

I did go ahead and put in mica caps where mica once were at. If I understand Wa2ise's explanation seems like another run at calibration is in order. I can do that!

One of Muntz's selling points was the fact, that once the set was set up in your home, it didn't need any adjustment, either by technician or user.
We all know how that worked out. They even had a set screw on the channel selector knob. :scratch2:

That's probably because the set's video IF had a passband as wide as a barn door. So the local oscillator in the tuner drifting wouldn't matter. Taking full advantage of the FCC not placing TV stations in adjacent channels. It should look awful on analog cable.

with a dvd player and not giving me problems this set has as good a picture as I have seen on a black and white set and good (very loud if pushed) audio.
Just wish it was not problem after problem. This has been the biggest lemon as far as problems.....

I had a cbs- Colombia set once that took 8 hours on one weekend to get running and that was it. no problems after that except a tube here and there..and it was not calibrated ....just horizontal set up with a scope to minimize fly temp.

I realized that I had the secondary of the ratio detector coil wired wrong. I redid it and for about 30 minutes the sound was clean ......and then the buzz slowly came back! that was after I had ran the set for about one hour.

well, I guess I should state that when the sound was clean there was a slight buzz that I felt was tolerable, barely there.....
just got curious and......there is resistance on my ohmmeter from either "leg" of the primary to the secondary of the audio output transformer.
that means that is where the buzz is coming from....a shorted transformer!

Not necessarily. Did you completely isolate (ie., disconnect both leads of either the primary or secondary) before taking the measurement?

since the output hooks to nothing but a speaker and that was not connected I did not unsolder the primary side

shoot!...... I just checked the diagram and one side of the secondary does go to ground........Ok, I will unsolder wires then.
right after shutting the set off it read earlier about 6400 ohms from primary to secondary.
after sitting it still shows about 8 meg so I had guessed that the tranny was shorted and heating making it worse.

Did you disconnect both primary leads?
If not, and one side of the primary remains connected to the B+ line, and one side of the secondary remains grounded, your ohmeter will be showing charge-up current of the filter capacitors (the meter's internal battery supplying the current).

Well, the audio transformer is shorted......and thanks to Bill from Moyer's The new part is on its way.
I hope I will be done with this set soon!

Just wanted to mention that I get a distinct buzz in the audio with my two '46 RCA TV's when I input the DVD player. Oddly the cheap, lesser quality DVD's like those of early kinescope's shows, have a much less pronounced buzzing. In fact those are the DVD's I mostly use when I want to play those RCA sets.

well, new transformer in.........and the sound needed adjustment.
but now I will need to do a total recalibrate.,,,,,still has buzz but it is in the local osc circuit and the adjustment causes jitter when I try and adjust it.
It acts dirty......touch it with an alignment tool and the picture freaks out. have to keep barely turning the control and then pull the tool away to see the results.

The funny part is after removing the old audio transformer it showed about 6400 ohms primary to secondary short.
How many parts in this set are junk??????? So far Most!

Is your alignment tool metallic?:saywhat:

Well, I now have plastic alignment tools...
Oh and I stumbled by accident onto the last problem with the buzz....
When I move the volume wires away from the other wires which are the holds and controls on the front of the set the sound cleans up. Heck the sound kinda buzzes if I touch the sound wires. So I bought shielded cable and will hook that up soon.