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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 16, 2022 2:07 am 
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Location: San Jose, CA USA
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Interesting that Channel 3 and Channel 4 are giving you a big difference in audio buzz. Must be some difference in how well the RF coils are tuned to the incoming signal in the tuner. I'd try touching up the tuner adjustments A25 - A28 to see if you can improve the buzz on Channel 3. If it won't improve, there's always Channel 4. :?

Predictas have a buzz control because they use a very cheap audio demodulator that is quite prone to buzz. This one really shouldn't be buzzing quite so much as what you have on Channel 3 now. If you turn down the brightness and contrast, does it reduce the buzz? If so, it may simply be that the tuning of the tuner coils on Channel 3 is giving you higher contrast than Channel 4 for the same settings. If the video detector saturates, it can cause buzz in the audio.

If you're curious to plot your IF response curve, it would be quite interesting to see where things ended up. Just step through a frequency range of about 21 - 29 MHz with the RF output level constant, stopping every 0.2 MHz or so to read the AGC voltage and record it. Then plot up a graph. See if it resembles Figure 4 in Sams at all. The AGC voltage isn't particularly linear in its response, so it shouldn't look exactly like Figure 4, but the basic trapezoidal shape of the ideal passband will still hold. You can try adjusting A7 and A8 if you find that you have a big hump or dip in the curve. Your learning process, however, will be mighty slow compared to touching things up while watching live on a scope with a sweep generator. Nonetheless, it's a great way to learn about the process.

Overall, you've really managed to improve things on this set!

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 16, 2022 2:14 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
Posts: 87
Location: Woodinville, Washington
Thank you for your help. I’ll try touching up A25 and A28 to see what happens. I didn’t explicitly note whether the buzz varies with the contrast setting, but I think it does. I’ll check carefully.

I’m not sure if I’ll have the patience to plot the IF response by hand but I will at least start the process and see if I can stand it. The hardest part is setting my analog signal generator using my frequency counter—a slow process compared to typing in a frequency in a digital keypad! Doing that every 200KHz will be a chore.

Back at you later or tomorrow. I’m really pleased with how far we’ve gotten!

Thanks again.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 16, 2022 3:21 pm 
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If you decide you want to try plotting the shape of the passband, you of course don't really have to make a measurement every 200 kHz. Something like every 500 kHz is probably sufficient for a first pass, and the measurement points don't have to be at nice round numbers. For example, if you want to measure about every 500 kHz, but end up with 21.15, 21.46, 21.92, 22.55, etc., that's just as good as hitting the exact 500 kHz intervals.

In any case, the whole project of manually measuring the passband response is simply an educational exercise if you're interested. OK to skip it entirely if things are working well enough.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 16, 2022 9:42 pm 
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It'd be interesting to use one of those NanoVNAs to plot the response in real time. Getting the levels right to avoid overloading the TV front end or the NanoVNA detector would be important, but that could be handled by some attenuators. Of course the NanoVNA is RF to RF, so the result would have to be taken before the video detector. Still, for about $100 it'd provide a nice little sweep system in the size of a paperback book.

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Mark Nelson
http://tv-boxes.com
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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Wed 17, 2022 9:24 pm 
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Location: Woodinville, Washington
Even without specialized equipment, such as a NanoVNA, I was able to create a plot of the IF response.

The attached graph shows the measured DC voltage VS. frequency (MHz). I measured the response in two places: at point A in Sams and across the AGC capacitor C34. Also on the plot is a stretched out version of the ideal response curve given in Sams—stretched horizontally so that the horizontal axes are both at the same scale.

I had no idea how this would turn out but I am frankly AMAZED at how well the measures curves compare to the ideal. I would have never have guessed the results would be so good!

Here’s the graph.
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A6E18F6F-C5E9-487A-8050-E02563DC2953.png [ 47.83 KiB | Viewed 371 times ]


Your thoughts welcome.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Wed 17, 2022 10:27 pm 
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Well, that's pretty outstanding! I don't think the procedure I recommended for you is really that good, so there is at least some luck involved, unless you spent some time tweeking some of the adjustments to get it this flat.

The only significant deviation from the recommended passband is that you've got your low end of the IF passband (which corresponds to high video frequencies) pretty flat right down to the 22.1 MHz sound trap. Some adjustment of A4 to bring that up a little probably would make it work well with the sound IF really centered on 22.1 MHz. Right now I have a hunch that the shortcut we used placed your sound IF a little lower (the second bump on the left end of your response at point A). On the other hand, all that nice flat bandwidth down to 22.1 MHz is a good way to get a very nice sharp pictures (with perhaps even the chroma cross-hatch visible if your CRT has good enough focus).

There is probably no good reason to touch anything at this point, except possibly the tuner adjustments to try to get Channel 3 behaving as well as Channel 4 as far as audio buzz goes. Or just use Channel 4!

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Tom K6VL


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Thu 18, 2022 12:07 am 
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Joined: Aug Mon 22, 2011 11:47 pm
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Location: Woodinville, Washington
You’re too modest. Your procedure was outstanding! I didn’t know that it was possible to get such a good waveform without a sweep generator. It took me two hours today to make and plot the measurements I included here. To do any more without a sweeper, I’d have to tweak a slug, go through the entire measurement exercise again to plot what happened, and then tweak again and measure, and then tweak, etc. By trial and error it would take forever. Your procedure saved the day!

I probably won’t touch anything more, except maybe tweak the tuner for buzz. My next step is to finally get the chassis back into its cabinet. I’ll also post a few before and after pictures to the forum.

For now, I’m going to live with the infamous HV fade that inevitably plagues these sets. I may add a small box fan to cool the HV cage, which does help, as you said earlier. Unfortunately, the cage only has holes in the top, so I won’t be able to blow air *through* the cage. Oh well. It’s still great fun to see this old TV working!

Thank you for all your help, Tom, and thanks to everyone who contributed to this thread.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Fri 19, 2022 3:00 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
Posts: 87
Location: Woodinville, Washington
As promised, here are a few pics:

Underside BEFORE restoration. Lots of bumblebee and paper caps! The HV bumblebees at the upper left were leaking oil, creating a gooey mess.
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Underside AFTER restoration. Bumblebees and paper caps gone! The black cardboard-covered electrolytic was restuffed. While there was very little done to the chassis before I got it, I noticed quite a number of places where wire insulation was burned by a soldering iron. Might this have been original? If so, the soldering was a bit sloppy.
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High-voltage section BEFORE restoration.
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High-voltage section AFTER restoration. Those electrolytics peaking out in the foreground have been restuffed.
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HV oscillator section with the notorious oscillator coil. Yes, it exhibits HV fade.
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Rectifier section of the B+ supply AFTER restoration. Selenium rectifiers have been replaced by silicon diodes. That purple jumper at the right was to be replaced by a dropping resistor. However, I decided not to add the dropper, as the B+ was about right even with the silicon diodes in place. I kept the selenium rectifiers and will put them in a bag to attach to the inside of the cabinet—just in case someone in the future wants to reinstall them for authenticity. But the silicon diodes work great and are invisible inside the rectifier cage.
Attachment:
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Completed chassis with CRT back in place.
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TV working! Photo taken in dim light. Those electrostatic CRTs were never as bright as modern picture tubes. Yep. That’s CNN. Hard to believe.
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Again, my sincere thanks for all the help I got with this restoration. I couldn’t have done it without you.

Finally, one more pic from Star Trek. A blast from the past.


Attachments:
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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sat 20, 2022 6:55 am 
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The 7” Philco I worked on a number of years ago for someone was the Bakelite version. Nice to see what the wood version looks like! Nice clean chassis, and nice neat restoration work. Should be quite satisfying to watch this set perform, at least until the HV fades. Picture looks good!

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Tom K6VL


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sat 20, 2022 3:06 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
Posts: 87
Location: Woodinville, Washington
Thank you. The wood cabinet is in beautiful shape, a couple of rubs in the finish, but that’s about it. The lacquer has a nice patina, appropriate for the age of the TV. I demonstrated the TV to a contractor working at our house yesterday. He thought it was the coolest thing. Of course I was careful to turn it off after a few minutes—before HV fade came along to mar the experience!


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sat 20, 2022 3:51 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
Posts: 87
Location: Woodinville, Washington
Oops. I forgot to add one more thing I had to deal with in this restoration. This is what the picture tube looked like when I opened the back of the TV.
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Just another day in the life of a restorer!


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