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 Post subject: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sat 06, 2022 11:20 pm 
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Location: Woodinville, Washington
Hi guys. I have saved up some questions about the infamous Philco 50-T701 electrostatic deflection TV that I’m currently restoring and I’m hoping that you can help. I’ve read all of the posts on the HV fade issue—more than once—so I’m not seeking miracles, but I’d like to get it working as best I can. Sorry for the long post. I’ve collected these questions over the past months when working on the restoration.

First some background. Most of the questions relate to the HV section, so I’m including that part of the Sams schematic in this post. I have a question later about sound reception as well.

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I’ve replaced all of the paper and electrolytic caps in the TV and all of the significantly out of tolerance resistors. I also replaced the selenium rectifiers with silicon diodes (have not yet fitted a dropping resistor). When I got the set, all of the 6AG5’s had been replaced by 6CB6A’s. So I got some NOS 6AG5’s and put the tube compliment back to the way it was originally. (More on this later.) I also gave the set a good cleaning. All of the coils have continuity and the correct DC resistance.

I did all of the voltage measurements shown in the voltages table in Sams. These were all good with a few exceptions. First, the plate voltages on V13, the vertical amplifier, are too low. Pin 3 reads about 160V when it should read 235V. And pin 6 reads 220V when it should read 470V. Since these plate voltages are derived from the HV power supply, my working hypothesis is that the HV supply is not producing enough voltage. Please correct me if I’m wrong. Second, the grid voltage on the 6V6GT HV oscillator (V16) measures a bit off: -36V when it should be -50V.

With the CRT removed, I am getting 3,600V on pin 9 of the CRT tube socket. (I have not looked at the problem of HV sag yet, which I assume will be there when I’m ready to look.) I don’t know if the CRT has good emission, but it does work and I get a dim and washed-out picture.

Question #1: Is 3,600V too low and is this the reason for the low plate voltages on V13?

Question #2: Is -36V on the grid of the 6V6 a problem? Or is it close enough? Could it, too, be related to low HV?

According to Sams, there should be a 680 ohm resistor (R117) between the screen grid of the 6V6 HV oscillator and ground. My set had two of these in series, rather than one resistor. I’ve read other posts that speak of a switch to remove one of these resistors from the circuit. My set has no switch, just two resistors in series. I replaced both and kept them both in circuit.

Question #3: Should I leave both resistors in series or remove one as per Sams?

More puzzling to me is where the lower end of the HV transformer secondary seems to connect. According to Sams, this lead (labeled “white” on Sams) should be connected to the screen grid of the 6V6. But in my set, that lead (if I’ve identified it correctly) connects to the 6V6 cathode (and the top of the small inductor L36).

Question #4: Should I move the white lead from the cathode and connect it to the screen grid as per the schematic? I’m not entirely clear about the theory of operation of the HV oscillator, so I’m reluctant to move things around without a reason. Other posts recommend connecting the lead to B+, if I’m reading the recommendation correctly.

Question #5: If yes, should I connect the white lead to the 6V6 screen grid or should I connect it to the higher voltage end of R119? Will moving the lead give me some more HV?

Finally, a different part of the circuit entirely. The sound. When my RF modulator and the TV are both tuned to channel 3, I get a picture but no sound. But I can find the sound by tuning the RF modulator to channel 4. (The modulator works fine on my Predicta, so I’m assuming problem is with the TV, but I’m not certain.)

Question #6: Is there a way to bring the audio and video tuning into synch w/o a sweep signal generator (which I don’t have)? I wonder if the person who replaced the 6AG5’s with 6CB6A’s messed with the IF alignment to be able to work with the 6CB6A’s. It doesn’t look that way but I’m not sure. Running the TV with the 6CB6A’s doesn’t bring the sound and video tuning back together either. In fact, I can’t get a picture at all with the 6CB6A tubes in the RF and IF sections, which is where they were when I got the TV.

Thank you in advance for any insights you can provide.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sat 06, 2022 11:40 pm 
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I’ll take a stab at your questions:

1. 3600 V is a bit low. More like 5000 V would be good.

2. Low bias on the HV oscillator could be a sign of loss in the HV coil and a general symptom of your low HV problem.

3. I would try removing one of the screen resistors. This should increase your HV.

4. I’d connect the white wire to the screen grid. This gives an extra 100+ volts of HV.

5. Connect it to the B+ end of the resistor rather than the screen grid end.

6. A sweep generator sure would be helpful, but if you don’t have one, do you have a normal RF generator that will cover the IF frequencies? If so, go through the non-sweep alignment instructions in Sams for both audio and video IF and the two should then converge properly.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sun 07, 2022 1:06 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
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Location: Woodinville, Washington
Thank you so much. I will make the changes suggested and get back to the forum. Regarding RF generators, all I have is an AM signal generator—the type used for restoring AM radio receivers.

Back with some results later today…


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sun 07, 2022 1:44 pm 
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Location: Lafayette, CO
I would replace the original that high voltage coil with one from a Motorola. There is a gentleman in your area who works on old televisions...worth a visit. Craig


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sun 07, 2022 3:12 pm 
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Joined: Aug Mon 22, 2011 11:47 pm
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Location: Woodinville, Washington
Yes. I’ve read about using a VT71 replacement. Definitely a possibility.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sun 07, 2022 4:35 pm 
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An AM generator can work. You don’t need any modulation to do the non-sweep alignment. It would be good if the frequency of the RF generator is reasonably well calibrated. If you happen to have a shortwave receiver with good calibration, that can be used to determine the RF generator frequency. But you may not have anything like that available. If both the audio and video IF frequencies are covered in a single band of your RF generator, you might still be OK even if RF generator calibration isn’t very good. All frequencies may be shifted a bit in the same direction, which may be OK.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Sun 07, 2022 7:24 pm 
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Location: Woodinville, Washington
Soon after posting my reply I realized that my AM signal generator, used with my frequency counter, just might be able to work. Thank you for your helpful comment.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 12:43 am 
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With a frequency counter, your basic RF generator will do just fine. Alignment can be a little tricky, so questions will of course be welcome if you run into any problems.

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Tom K6VL


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 3:44 am 
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I would not touch the video IF alignment, but it's worth checking to see that the sound IF is aligned properly.

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Dennis

Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 5:48 pm 
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Thank you for the further comments. Regarding the alignment, I won’t touch anything upstream of where the audio signal splits off. Judging from the red wax on the adjustment slugs, almost nothing has been messed with. (But it’s hard to be certain in a few cases.) Once I start working on the audio, I’ll get back to the group.

After making the changes Tom suggested, I waited until morning to make sure that the coil had cooled down from my various experiments and here’s what I got:

—HV maxed out at 4.68KV on pin 9 of the CRT (CRT out of circuit) about 45 seconds after power-up to 117VAC.
—Grid bias on the 6V6GT is -47V (-50 is nominal)
—Plate voltages in the 7F7 vertical amplifier (which gets their voltages from the HV circuit) are 170V on pin 3 (235V is nominal) and 280V on pin 6 (470V is nominal). In both cases the voltages jump around about 20-40V each direction and never settle. I don’t know why that’s the case. HV caps C98 and C99 are new and resistor R118 between them measures almost exactly 100K.

Are these numbers better than before making Tom’s recommended changes? I think so, but I’m not certain as I didn’t always measure the HV when everything was cold. Unfortunately the HV fades almost in front of my eyes. Yesterday, it dropped from 3.4KV to 2.8KV within 15 minutes. In any case, the above numbers provide a baseline for where things are at.

I’ll work on the sound issue and get back to the group…


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 6:33 pm 
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OK. I just read the SAMS instructions for the sound IF and I already have some questions.

Question #1. The first question regards the theory of operation. Why is it that I’m adjusting the sound IF? Are the sound and video not in synch because the two are not exactly 4.5 MHz apart? Assuming that the video IF alignment is OK, will getting the audio IF aligned bring the two signals back into line? I don’t fully understand the rationale here.

Question #2. The SAMS instructions call for an FM modulated signal with a 450 KHz sweep. Given that I have only an AM signal generator, do I simply inject a 22.1MHz AM signal in its place and peak the A9-A12 using the AM signal? That would make sense to me since I’m measuring the output at at pin 6 of the 7X7 ratio detector. Or, perhaps I should measure the output at pin 7, before the ratio detector? (Or would measuring at pin 7 load the stage and mess things up?)

I’d love to get a bit more clarity before setting things up and turning slugs.

Thank you!


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 7:48 pm 
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Most of the earliest TVs from the 1940s have what is known as a “split IF” as opposed to the far superior “intercarrier” approach that took over shortly thereafter. With split IF, the sound IF is tuned to the sound carrier frequency coming off the mixer which is 22.1 MHz for this set. With the intercarrier approach, the sound IF signal is picked off right after the video detector and always has a frequency of 4.5 MHz.

This set is a bit unusual in the the sound pick off is after several video IF stages, so those stages need to pass both video and audio frequencies. Many other sets did the pick off right after the mixer, so the video IF is just for video and the sound just for sound. Of course for the intercarrier method, the video IF stages also need to pass the sound frequencies, so it’s not so different.

In split IF sets like this one, the sound and video tuning have almost always drifted quite far apart and won’t tune together without an alignment.

I see that the Sams for this set does not include a non-sweep set of alignment instructions, which is unusual for early sets. Nonetheless, from looking at the instructions it would appear that the following can work.

First, as Dennis suggests, you can try leaving the video IF alone and see if sound IF alignment alone will solve your problem. Without a sweep generator, do it this way.

1. Set RF generator to exactly 22.10 MHz.
2. Partially remove the shield from the mixer tube 7F8 and connect hot side of RF output to the ungrounded tube shield, and connect the shield side of RF output to chassis right nearby.
3. Measure voltage across detector stabilizing cap C9, and adjust A9, A10, and A11 for maximum voltage. Adjust RF output level so that you are getting a few volts across C9, and readjust downward as you align and signal level comes up.
4. Now measure voltage at point C on the Sams diagram, and adjust A12 so that the voltage at point C is 1/2 the voltage across C9. This should be the middle of the range of voltage you see point C sweep through as you adjust A12.

This should improve your sound, but if it does not coincide with picture, then you have to do a video IF alignment, followed by the audio alignment again.

For video IF alignment, this will probably get you close:

1. Feed RF generator to ungrounded shield over 7F8 mixer as above.
2. Measure DC voltage at point A on schematic.
3. Set RF to 22.10 MHz and level high enough to see some voltage. Adjust A1and A2 for MINIMUM.
4. Set RF to 26.5 MHz and adjust A3 for maximum voltage. Turn down RF level if you get more than a few volts.
5. Set RF to 24.0 MHz and adjust A4 for max.
6. Set RF to 25.3 MHz and adjust A5 for max.
7. Leave A6 alone for now.

Now and try it and see what you have. We may be able to help further depending what you see.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 9:05 pm 
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Thank you, Tom, for the detailed instructions and for the primer on TV theory. Much appreciated.

Let me add a couple more things:

The fidelity of the sound I’m getting through the TV is excellent—really clear and natural. The problem is that it seems to be a full channel apart from the video. When my RF modulator and the TV are both set to channel 3, I get a picture but no sound. To get sound, I have to switch the modulator to channel 4.

Before your latest post came in I did an experiment and injected a 22.1 MHz Modulated AM tone into the grid of V5 (through a dummy antenna). When I did that I was surprised to hear the modulation tone through the loudspeaker. (I can see how the diode section of the 7X7 tube would detect the AM signal but I didn’t expect the tone to appear at the loudspeaker after going through the ratio detector, but it did). In any case the fact that I heard the tone tells me (I think) that the sound IF is reasonably close to the specified 22.1 MHz. Otherwise I wouldn’t have heard the tone. Is that right? Does that imply that the problem is more likely to be the video IF alignment? Just a thought.

More to come after I work through the procedure.

Thank you again.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 9:20 pm 
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A ratio detector will give some output to an AM modulated signal, although distortion may be high when properly centered. It’s interesting that you get the sound one full channel off, which is a 6 MHz error between Channels 3 and 4. Quite a big error. Given that you get sound close to 22 MHz, it does seem the video IF is off by a mile. I sometimes see that on these early sets.

Give the alignment procedure a shot, as long as you are open to the possibility that it won’t work well enough and you might end up either needing to get your own sweep generator or take it to someone who has one. I’d still do the audio IF alignment first, even though you are currently getting an AM tone through at 22 MHz.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 11:22 pm 
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I performed the audio IF alignment as per your instructions, peaking the DC voltage across C9 by adjusting A9, A10, and A11 and then setting that same DC voltage to half of the earlier peak by adjusting A12. The good news is that the audio sounds great. The bad news is that it still *seems to* tune a full channel away from the video. I say *seems to* because by the time I was done with the audio alignment the video was so bad (HV sag, of course) that I couldn’t be sure. So, I decided to take a break, let the TV cool off overnight, and look again at the video/audio synch tomorrow.

One more observation is that the two voltages I measured across C9 are about 20V and 10V. Even loose coupling the hot lead of my signal generator to the oscillator as you describe and turning the signal level of the generator all the way down, I couldn’t get a DC voltage much lower than I just described. The transformer slugs all peaked, which I assume is good, but I wonder if the input from the signal generator triggered AGC action. Anyway, the bottom line is that I did align the audio IF as best I could.

Thank you again for your help.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Mon 08, 2022 11:52 pm 
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If your generator is an old tube unit or other potentially dirfted non-digital-synthesized generator it's wise to verify the generator frequency on a digital frequency counter or a digital radio reciever. I have both but usually prefer my Sony ICF-7600GR shortwave radio. It can tune anything between 150KHz and 30MHz AM and the Japanese and American FM bands. It's good for calibrating the output of my tube Heathkit generator.


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 09, 2022 12:21 am 
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It’s a solid-state but not very sophisticated Lodestar model SG-4160B AM RF generator. I’m using a Velleman DVM13MFC2 frequency counter, also not very sophisticated, to verify the calibration. I do have a Lowe HF-150 SW receiver as well, but haven’t used it for purposes of calibration for this application. Commercial SW carriers would be good for calibration as they are (most likely) right on frequency.

I find it ironic that I have to doubt the accuracy of my test equipment, but even test equipment needs testing. And so on. :-)


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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 09, 2022 12:29 am 
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Regarding the HV fade, I have had good success on some sets by installing a small muffin fan to cool the HV coil. You can test whether that might be fruitful here by removing the cover and blowing some air on the coil with pretty much any old fan unit you may have available. It’s not a great fix since an installed fan makes some noise, but might be a useful interim fix.

I’ve only restored one of these particular 7” Philco sets, and I happened to have a NOS Merit replacement HV coil for it which worked well, if I recall correctly. Either that or I ended up subbing in a different type of coil altogether.

Regarding your RF generator and frequency counter: most likely this combination is trustworthy for frequency. Even cheap frequency counters tend to be quite accurate if they are able to lock onto the signal and produce a stable reading. The nature of the digital design with quartz crystal clock means it never drifts in terms of frequency calibration.

Regarding the high signal level during alignment, try reducing the capacitive coupling from the signal generator to the mixer tube. This can be as simple as lifting the tube shield higher on the tube, or putting the tube shield fully back on and grounded, with just a short insulated piece of wire inserted between shield and tube to connect to the RF generator cable center.

That being said, your alignment with 20 V of signal level as measured across C9 might be OK.

Will wait to hear your diagnosis of video/audio line-up after your HV recovers, but it looks like you may indeed need to align the video IF.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 09, 2022 2:46 am 
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NormanG wrote:
Finally, a different part of the circuit entirely. The sound. When my RF modulator and the TV are both tuned to channel 3, I get a picture but no sound. But I can find the sound by tuning the RF modulator to channel 4. (The modulator works fine on my Predicta, so I’m assuming problem is with the TV, but I’m not certain.)

When you change the modulator to channel 4 to get the sound, how does the picture look compared to how it looks with the modulator on channel 3?

I am not sure doing an alignment of the video would get you anywhere. If you get sound with the modulator on 4 then the tuner is placing the sound carrier where the sound IF wants it with the sound IF's current tuning. I don't see how adjusting the video IF could affect that. What adjusting the video IF would do is make the picture better. But you would still have to have the modulator on the wrong channel.

I suspect that the oscillator adjustment in the tuner is shifted by one channel. Perhaps running the modulator on channel 4 is an adequate work around for that. Do a video IF alignment to get the picture to be good.

If I remember correctly, I had to do a video alignment on my Philco. There were some physically large capacitors very close to some of the coils in the IF strip. When I replaced those caps with modern physically small caps, the tuning of the IF strip changed.

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 Post subject: Re: Philco 50-T701 restoration questions I’ve saved up…
PostPosted: Aug Tue 09, 2022 3:22 am 
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Agree that the oscillator is also off. Once you get both video and audio IF alignment correct, then adjust the channel 3 oscillator coil to get everything coming in properly on channel 3. Very common to have the oscillator off by a mile on these very old sets.

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