Philco Predicta predicament

[ShartZenith]

I have a Predicta that a friend helped me get working last Winter. The picture couldn’t be much better except it slants down to the right ever so slightly. The nagging problem is the audio.
Sometimes there’s a constant humming sound that is loud enough that it overshadows the TV audio. It’s not on any particular channel and very intermittent. I have noticed it tends to start doing the humming if the show goes to a commercial or changes to another feed of any kind. Even when I intentionally put up the signal strength screen from the converter box it starts to hum, then goes away when I take the signal strength screen off.
I can also tune out the humming with the knob behind the channel selector, but that doesn’t always work and sometimes when it does quiet down the humming the picture is no longer ideal. We’ve recapped everything and replaced numerous weak or bad tubes when we worked on it last winter. What am I up against here? I will share your answers with my friend who did the lion’s share of the work on the TV and is the “techie” type. I’m not. Thanks for any and all help.

[miniman82]

It's an obvious alignment issue, but unfortunately you can't fix it accurately without the alignment setup. That consists of an alignment generator, bias supplies, oscilloscope and hand tools. Where are you located? It's possible a member here can do it for you, for a small beer fee.

[ctc17]

Some sets buzz like crazy if I use the rf out on my directv box but if I run the composite to a quality modulator its goes away completely. I bet it buzzes when theres fine text or bright text? Try another rf source

[marty59]

Not sure on your "seventeener" but on the 9L37 21" chassis the AGC is rather crude being controlled by a local-distant switch and variable by the contrast pot as being in part of the feedback loop. Minimize these controls and see what effect you get.

[IsthmusTV]

Also, I believe that at least some versions of the Predicta chassis have a "buzz" control that can be adjusted to minimize inter carrier audio buzz.

[Don Lindsly]

Chassis or model number would be helpful.

[Phil Nelson]

Originally Posted by IsthmusTV

some versions of the Predicta chassis have a "buzz" control that can be adjusted to minimize inter carrier audio buzz.

My barberpole set (ch. 9L37) has a buzz control on the cathode of the audio detector. I recently got a 17" tabletop like this (ch. 10L4x) and it appears to have no buzz control.

Phil Nelson

[ShartZenith]

Yes, it does buzz or hum whenever some text comes on the screen when I prompt it from my converter remote, i.e., sound volume adjustment, signal strength. It always will if the program changes from the current view to a commercial or even between different commercials. When it goes back to the regular program feed is settles down almost immediately and the buzzing/humming ceases. It also tends to buzz right when I turn the set on until I adjust the tuning knob behind the channel selector back and forth slightly. It's a model H3410L. Not sure of the chassis number. It's behind the back cover I think and I'm running it at the moment and don't want to shut it off. Should be one of these three according to the schematic we used, 10L41, 10L42, 10L43.

[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

[ShartZenith]

Thanks everyone for their input on this. My radio/TV friend and I will do as much as this as we're technically capable hopefully in the near future. Meanwhile I'm starting a new post about one more problem on the Predicta I've noticed while running it nightly this week.