Distinctive audio buzzing high and low problem

[ketron281989]

Hi guys, I bought a Philco 50T-1402 a while back, pictures are in my gallary, this set was fully restored from the tv collector I bought it from. There is a real annoying buzzing that changes tones when a commercial comes on, or sometimes they just change for no reason, I think something is wrong, this weekend or next I am going to take out the chassis for the first time since purchase last month to examine. Could any of you help Identify what this noise could be, I don't think it is interference as my computer is fully shut off, I do run it off a variac, have a converter box and digital cable box hooked up, but they are about 3 or 4 feet away from the set, there is nothing else around that could provide electrical interference.

Do any of you know a simple fix or cause, or have any of you experienced this type of problem. I will work on getting pictures here soon, but for the time being any help would be appreciated.

Thanks guys
Jon

[wa2ise]

That buzz can happen in intercarrier sets. The sets that get the 4.5MHz sound IF off the video detector. If the video IF is not biased right, the whites in the video (which occur at minimum RF modulation) can result in no video RF carrier, and thus the sound carrier aso disapears momentarily. Which in turn results in a buzz at the vertical frequency, 59.94Hz (this because the pattern of white lettering repeats at this rate, the video field rate).

When the analog NTSC transmission system was designed, they decided to make the tip of the sync pulses be the max RF modulation, and peak white the minimum, but still 10% that of the sync tip. If the IF amps shave off that bottom 10 or more %, you will get buzz in intercarrier sets. Making the sync tip be the max allows the use of keyed AGC, as that part of the video signal is always a known reference, and has lower noise issues.

So I'd look at the IF strip for weak tubes or bad biasing, or bad B+. Badly aligned IFs could do it, but I would avoid messing with the alignment, as it's harder to get right than that of a radio.