sound buzz

[Bobby Dip]

Originally Posted by wa2ise

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.

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.

[Tom Albrecht]

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.

[radio nut]

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!

[radio nut]

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.

[radio nut]

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.

[radio nut]

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!

[old_coot88]

Originally Posted by radio nut

...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?

[radio nut]

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

[radio nut]

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.

[old_coot88]

Originally Posted by radio nut

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).

[radio nut]

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!

[decojoe67]

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.

[radio nut]

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.

[old_coot88]

Originally Posted by radio nut

,,,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.

Is your alignment tool metallic?

[radio nut]

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!