Predicta: My first official Restore

[Phil Nelson]

Originally Posted by 6GH8cowboy

The sync buzz is very prominent on graphics. Is there a known mod for this

My 17" Siesta doesn't buzz noticeably when viewing normal content. Based on that unscientific sample of one, I'd say you should be able to get reasonable audio without circuit modifications.

The 9L37 chassis used in 21" Predictas has a buzz control -- a potentiometer on the cathode of the audio detector -- where your set has a fixed resistor. If you feel like playing around, perhaps you could substitute a little pot there and see if it helps.

Regarding the couplates (hybrids, networks), I guess it's a judgment call how many you want to replace, if any. I replaced one in my first Predicta and three in my second Predicta. These were replaced to cure problems that I observed after finishing a basic recap. The couplates that I didn't replace are still holding up years later; your mileage may vary, of course. One downside of shotgunning all of the couplates is that you'll make a crowded board even more crowded.

You can read more about the second project (a Siesta) here:

http://antiqueradio.org/PhilcoH3412L...Television.htm

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

[josephdaniel]

I still need to get around to recapping mine
Every time I see a Predicta thread I look at how easy others make it look and then I look at that box of parts from just radios but then I come to my senses and realize that I will start and realize that I should wait until I have more experience! I don't even have a tube tester :p

[6GH8cowboy]

This is a 11L43 chassis and Im working from a 10L42 schematic.

The screen grid resistor on the sound IF tube V5- 6EA8 is a 10k 1 watt but the diagram says 22k. The SG voltage is correct.

Does someone know for sure what value this should be? I could just sub it but dislike soldering the pcb twice if it's for nothing.
Still fighting for good sound.

[Phil Nelson]

For what it's worth, both the Sams schematic and the Philco factory schematic give 22K for that resistor. One says 80V for that pin, the other says 50V.

Phil Nelson

[6GH8cowboy]

I went ahead and changed the bobo 10k resistor with a 22k and it helped a little. I then added the "buzz pot ( a 500 ohm trimmer) to the detector cathode and lifted one end of the existing resistor, that helped a little also. Between both, the sound is usable and fairly forgiving with strong video content. My gut feeling is the converter box is providing a very strong video signal and slightly low audio. I have noticed the low audio on the last two roundies I restored but they tuned up better than this predicta for the sound.
I'm going to let it cook for a couple of days.

I'm going to take a screen shot of the closing credits of Rock Center with their operating predicta on the shelf on this predicta.