[Penthode]

Intercarrier receivers like the Predicta must have a correct video carrier to sound carrier ratio at the video detector to ensure proper mixing to 4.5MHz sound IF and demodulation. Also, the 4.5MHz IF should have some form of limiter prior to or at the sound demodulator. I would expect the Philco to use a quadrature FM detector which inherently limits to a degree.

Later solid state sets generally use more sophisticated sound processing and demodulation and are less liable to experience the buzz. But with the cheap set top DTV boxes I see two problems:

1) Video over-modulation: In the early days of intercarrier receivers, the video amplitude modulation was regulated so that at minimum carrier (peak white), the residual carrier level would not be allowed to drop to less than 10% of the carrier amplitude at peak sync. If the peak white causes the carrier to drop below 10%, the 4.5MHz sound IF which requires the video carrier as the local oscillator will effectively drop out at picture horizontal and vertical scan rates. The vertical 60Hz drop out is most noticable which leads to the characteristic buzz.

If you take the output of the box and feed the audio and video to a better modulator where the video amplitude is better controlled, then the buzz can be minimized.

2) I pointed out earlier that I have found some set top boxes (Zenith for one) do not seem to modulate the audio FM carrier adequately, hence the audio level is low. This makes the buzz and other noises more noticable. Again a separate modulator where you have better control will give better results.

I have been frustrated by the bad performance on old sets of the internal RF modulators of these DTV boxes. In addition, the modulators (Philips and RCA) you buy at Walmart, Home Depot and other places are similarly bad. I ended up using older modulators from 80's scrapped VTRs which allow some adjustment. Also, I have bought on eBay old Blonder Tongue modulators. These have the advantage of allowing you to alter the video and audio modulation levels to minimize the problem of buzz.

However, with the Predicta and other old intercarrier sets, do not fail to do a proper Sound IF and Quadrature demodulator alignment. The manual says this is best done by ear for minimum noise and least buzz and distortion.

That is why I like my old split sound RCA sets.

My two cents....

Terry