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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Sun 20, 2021 12:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Crystal Bay, NV
The ceramic 500pf capacitors are rarely defective.
Incidentally, you can always replace with higher voltage caps (e.g. 5000v). It would also be OK to use higher capacitance values (e.g 1000pf).
The problem I see is in the sync circuitry.
Sometimes it best to give a project a rest, before you become too frustrated.
The TV=37 sets will work quite well and your problems are not caused by design.
Be sure to substitute any tube used in the sync circuits. Tubes can cause all kinds of problems.
====
Ron


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 22, 2021 6:44 pm 
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I think I solved the noise issue but I'm still left with the bending raster. One thing I noticed: the tube in the B- supply is a 35W4 and the original values of the filter electrolytics are 80 and 40 mfd. If I double these values, then the ripple and the bend decrease by about half. I can probably live with that. Can I safely leave the caps doubled?

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 22, 2021 8:42 pm 
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Location: Crystal Bay, NV
Yes.
Electrolytic capacitors have a very wide tolerance, typically -20% to +80%.
However, you may be covering up the underlying problem with excessive B- filtering.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Wed 23, 2021 3:12 pm 
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The 10 ohm resistor between the tube and the 80 uf cap should limit the peak currents to what the tube can handle. But it seems strange that you should need to increase the value of those caps. Check that the 470 ohm resistor between the 80 uf cap and the 40 uf cap is really 470 ohms. When the set was made the tolerance of electrolytic caps was +80/-20 %. New caps are now +20/-20 % with the new caps I have tested running low.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Wed 23, 2021 6:18 pm 
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This is a very classic symptom, well know by all old TV techs and the most common causes are either poor supply filtering/stage decoupling or a defective (H-K leaky) tube. The first cause can already be excluded since you replaced/bridged all the supply electrolytic filtering caps. Increasing the filter caps value will only (partially) hide the problem but doesn't cure it.
You also replaced the tubes "one by one"... but there is still a possibility that more than one tube is defective, or that the replacement tube(s) you used is also leaky. Some circuits are more sensitive to H-K leakage than others.
To avoid running into circles I would start checking the tubes again, using fresh (N.O.S) tubes and replacing them all at once if possible. Remember almost any leaky tube in the set (including in the tuner) can produce this symptom.
Parasitic/electrostatic/magnetic coupling of 60 Hz into a sensitive circuit is allways possible but only if you made changes in the original wiring layout, etc...
From your pictures the bending is too pronounced to be considered as "structural", there's no way the set left the factory with this problem.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Wed 23, 2021 7:06 pm 
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A heater-cathode leak in the video circuitry would probably result in hum bars. These have not been reported.

Interesting how old terminology dies out very slowly. E.g. Heater vs. filament. I still want to say condenser instead of capacitor.

I'm still betting on the syc or horizontal osc. circuitry for the source of the 60 Hz (not cycle) interference.

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Ron


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Thu 24, 2021 4:10 pm 
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Not specific to your Pilot TV but very interesting read nevertheless: In the March and April 1951 issues of Radio & TV News there's is a very detailed and informative 2-part article about horizontal pulling/bending troubles (starts at p.61). The author (John R. Meagher) goes deep into the topic and maybe you can find some useful informations and tricks related to your problem.

Both issues can be downloaded here:

March 1951:

https://worldradiohistory.com/Archive-R ... 1-03-R.pdf

April 1951:

https://worldradiohistory.com/Archive-R ... 1-04-R.pdf


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 06, 2021 2:02 pm 
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Thanks for all the suggestions and the links to the articles on horizontal pulling. I replaced all the tubes with a new NOS set and still get the same problem.

I have a question about the alignment of this set. I thought I'd check it out to see how it looked so I hooked up the sweep generator and the oscilloscope according to what's in Sams. Surprisingly, I find that the waveform at the output of the video can (right before the capacitor on the grid of the video amplifier) is upside down in comparison to what's shown in Sams. That is, as the generator sweeps, the signal goes through a negative peak in the middle. I double-checked this with my VTVM and the voltage is negative. Is this normal?

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 06, 2021 2:23 pm 
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Yes, the negative peak for a sweep at the output of the detector is normal. In sets with no AGC
like the Pilot, it could be either.

My set had hum problems too. These were cured by IF tubes with lower heater-cathode leakage as well as by better
HV filtering. I would replace the 40 uF cap with 100 or even 120 uF. I would not increase the 80 uF one above 120 uF.
Make sure that your RF modulator is not causing compression of the sync tips.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 06, 2021 3:16 pm 
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Thanks. And thanks for the tips. So, when Sams says to maximize the voltage during alignment, I should try to make the voltage as negative as possible?

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 06, 2021 5:51 pm 
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Dave Tilotta wrote:
Thanks. And thanks for the tips. So, when Sams says to maximize the voltage during alignment, I should try to make the voltage as negative as possible?

Dave


Yes, maximum is maximum, regardless of polarity.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 06, 2021 6:44 pm 
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Thanks, it's nice to get the confirmation. My only previous experience with aligning a set was on a Motorola VT-71. It seemed to go well, though.

I'm a bit hesitant to try realigning this Pilot but the response curve is poor. The baseline of the current curve runs from 22.25MHz to 24.75MHz. According to Sams, that's too narrow.

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Fri 09, 2021 12:26 am 
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Concerning the CRT have you done anything to prevent the initial turn on surge of the tube heaters from possibly putting too much voltage on the CRT heater and burning it out eventually?

I've seen where a transformer was used to supply the CRT heater voltage, but another maybe better way is two zeners back to back in series either the cathodes or anodes connected together and placed across the CRT heater pins. A zener rated at 6-7 volts will do. That way the heater voltage can only go as high as 6.6-7.6 Vrms.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Sat 10, 2021 7:09 pm 
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Tube Radio wrote:
Concerning the CRT have you done anything to prevent the initial turn on surge of the tube heaters from possibly putting too much voltage on the CRT heater and burning it out eventually?

I've seen where a transformer was used to supply the CRT heater voltage, but another maybe better way is two zeners back to back in series either the cathodes or anodes connected together and placed across the CRT heater pins. A zener rated at 6-7 volts will do. That way the heater voltage can only go as high as 6.6-7.6 Vrms.

They make a single part that does the job of those 2 Zeners it's called a TVS (Transient Voltage Suppressor) diode.

Some years ago the member that came up with it used a true RMS volt meter (most volt meter's measurement is only RMS for a wine wave and gives inaccurate readings for non-sine waveforms such as clipped wine waves). IIRC he came up with 8.2V as the TVS clipping voltage to limit power delivered to the heater to it's correct specification.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Sat 10, 2021 8:32 pm 
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Oh ok I didn't know those could be used for that.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Sat 10, 2021 9:01 pm 
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hello Electronic memory ,
I will have to remember that one (Transient Voltage Suppressor) diode. I don't have a Pilot but I have National 7inch .
Dave Tilotta you are on the right track and I am sure you can get fixed you may need to step away from the set.

Sincerely Richard


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Sun 18, 2021 12:34 pm 
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Location: British Columbia
TahoeTV wrote:
A heater-cathode leak in the video circuitry would probably result in hum bars. These have not been reported.

Interesting how old terminology dies out very slowly. E.g. Heater vs. filament. I still want to say condenser instead of capacitor.

I'm still betting on the syc or horizontal osc. circuitry for the source of the 60 Hz (not cycle) interference.

====
Ron


I always understood that in tubes with an indirectly heated cathode tubes it was a called a heater, in directly heated cathode tubes it was a filament. So in a type 80 or a 5U4 it has a filament, whereas in a 6V6 it's a heater, since it heats a cathode sleave.
Regards
Arran


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Tue 20, 2021 5:42 pm 
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TahoeTV wrote:
I'm still betting on the syc or horizontal osc. circuitry for the source of the 60 Hz (not cycle) interference.
Ron

Well, one more post on this. The cathodes of the sync tube and the horizontal oscillator (multivibrator) are connected to the negative power supply. So any ripple there will affect the sync. On one hand it seems that the set would not have had this problem as designed. That is why I wondered if the resistor between the two filter caps might somehow be shorted out. That would make the filtering single stage instead of a two stage filter. But when the set was made electrolytic caps had a +80%/-20% tolerance. Perhaps when the set was built all the caps they got were +80%. And the small amount of bending in that very small picture was not considered important. So now that caps are +/-20% and often run low, doubling the size of those caps is probably the right thing to do.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Wed 21, 2021 6:06 pm 
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Thanks, Tom, I'm inclined to agree with you. Here's what I've done, and found, about the low voltage supply:

1. I rebuilt the ballast tube with resistors so I can guarantee that the resistances are correct.
2. I measured the current draw from the 35W4 (the B- tube) and it's about 85mA after warm-up.
3. When I bridge a 100mfd cap across each of the four caps in the power supplies, the only one that makes a substantial difference (i.e., reduces the ripple by at least 2X) is the 80mdf cap (C2) on the 35W4 B- line.

As an aside, I tried an experiment of disconnecting the input cap to the grid of the horizontal multivibrator tube and found that the ripple was still there.

So, I think I'm concluding pretty much what Tom and dtvmcdonald arrived at and, unless anyone else has any other ideas, I'm going to double the B- caps (at least one of them) and call the TV restored.

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jul Fri 23, 2021 6:36 pm 
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I have a question about the B- supply in this set. Of the two filter capacitors on the 35W4, which of them is the input capacitor? Is it the 80mfd (C2) or the 40mfd (C3)? I've been assuming it's the 80mfd, but I'm not used to looking at a half wave rectifier wired like this so I may be wrong.

Here's the schematic again: http://www.earlytelevision.org/pdf/pilot_tv37_sams_62-16.pdf

BTW, the reason I'm asking is that I thought I'd try to make only a minimal increase in the size of the input cap.

Thanks in advance.

Dave


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