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 Post subject: Bending Raster in Pilot TV-37
PostPosted: Jun Sat 12, 2021 7:19 pm 
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Location: Raleigh, NC 27675
Thanks to BrendaAnnD, I got a 3KP4 for my Pilot TV-37 and that's allowed me to continue with the restoration of this set. All the caps have now been replaced as well as the out of tolerance resistors. I also needed to replace the 1N34 video detector (I used a 1N270 which I have a box of). Interestingly, all of it's original tubes were good. It works great expect for one remaining issue that's driving me buggy.

The raster has a vertical bend that slowly crawls up the sides. It's not huge but it's noticeable, at least at the optimum viewing distance of 2ft. To troubleshoot this, I've replaced the tubes, one by one, to rule out H-K leakage. I've also scoped the B+ and B- rails and they look normal, at least I get the same results as shown on the radiomuseum site: https://www.radiomuseum.org/r/pilot_candid_tv_tv37.html. I assume this is not normal. Is the problem in the horizontal section? How do I troubleshoot this?

Many thanks.

Dave

PS. Schematic is here: http://www.earlytelevision.org/pdf/pilot_tv37_sams_62-16.pdf


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Sat 12, 2021 10:44 pm 
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Location: Belrose, NSW, Australia
Your TV has a very narrow deflection angle and no magnetic screen on the CRT (at least, not in the SAMS pictures). So it is VERY vulnerable to external magnetic fields.

You might have a 60Hz magnetic field from a transformer or similar nearby. What happens if you move or rotate the TV? Does that make any difference?
It might be coming from the field coil on the speaker. If so, I would expect the picture bend to have somewhat of a triangular or sawtooth shape.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Sun 13, 2021 8:36 pm 
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Thanks, excellent suggestions. I temporarily moved the CRT out of the cabinet, away from the speaker and chassis, and it didn't make a difference - the ripple is still there. Btw, it's a sinusoidal modulation.

One thing I did just note, though, is that the raster bend goes away completely if I turn the contrast/gain control to minimum. Of course, at that point, I have no picture.

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Mon 14, 2021 1:09 am 
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A picture would be helpful, to see if the picture is shifting on the raster or the whole raster is bending. Very different faults!

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Mon 14, 2021 2:04 pm 
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Thanks for the help. See below for pictures of the wave.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Mon 14, 2021 6:19 pm 
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Yes, that is AC hum in the picture. I have built many electrostatic television sets, and from experience, it is from the LV power supply. Nice picture by the way!

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Mon 14, 2021 6:34 pm 
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OT, sorry :oops:

You guys keep this up, and I will be "forced" to get out the TV-37 parts that I have and see if I can get one going! Don't know if I have a good CRT...

Seriously, I appreciate all of the comments above.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 15, 2021 1:31 am 
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I just looked at the schematic. There are two power supplies. There is a positive power supply for the B+. And a negative power supply for the B-. Did you check the ripple on both supplies? Either one could cause the problem. The two supplies together perform the same function as a voltage doubler power supply.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 15, 2021 2:07 am 
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Greetings to Dave and the Forum:

Quote:
One thing I did just note, though, is that the raster bend goes away completely if I turn the contrast/gain control to minimum. Of course, at that point, I have no picture.


This is interesting. It suggests incidental coupling between the video circuitry and the horizontal oscillator circuitry. I would look for missing or mis-installed bypass capacitors in the video or horizontal oscillator circuitry. I would also look at lead dress and see if there is any stray capacitive coupling going on.

Regards,

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 15, 2021 3:15 am 
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Jthorusen wrote:
Greetings to Dave and the Forum:

Quote:
One thing I did just note, though, is that the raster bend goes away completely if I turn the contrast/gain control to minimum. Of course, at that point, I have no picture.


This is interesting. It suggests incidental coupling between the video circuitry and the horizontal oscillator circuitry. I would look for missing or mis-installed bypass capacitors in the video or horizontal oscillator circuitry. I would also look at lead dress and see if there is any stray capacitive coupling going on.

Regards,

This set does not have the automatic frequency control type of horizontal sync that newer sets have. Instead the sync pulses directly trigger the horizontal oscillator in a like manner to the way the vertical sync is done. If the height of the sync pulses are moved up or down the horizontal oscillator would tend to trigger earlier or later. That is one possible way this problem could occur.

As for lead dress. If a coupling cap or other component is laying close to a filament string wire, that could be one way for 60 HZ to get into things.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 15, 2021 11:19 am 
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Tom is on the right track.

Note that the whole raster is bending, not changing in width.
Given the way the hor sync works I'd be looking for hum in the sync.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Tue 15, 2021 5:25 pm 
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Thanks, everyone, for the suggestions.

I checked the low voltage supply and I'm not sure if what I'm seeing is good or bad. The B+ rail, which is supplied by the 25Z6 tube and filtered with a C-L-C network (where the L is the speaker field coil) is at 125V with a 0.7V (p-p) ripple. The B- rail, which is supplied by the 35W4 and filtered with a C-R-C network, is at 120V with a 1.2V (p-p) ripple.

I'm not sure how to interpret the ripple: too high? The B- seems okay based on the info here: https://www.radiomuseum.org/r/pilot_candid_tv_tv37.html

The B+ ripple, however, is too high if it's supposed to be 140mV (per the above site). Is a ripple in the range of 0.5% to 1% reasonable for these types of circuits?

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Wed 16, 2021 12:17 pm 
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That ripple is still reasonable. The 140mV might be an RMS reading, not P-P. In which case it's close.

I'd scope the video feed to the sync separator, then after it. I suspect you'll see it there.

I'm suspecting a heater-cathode leak in a tube. Any stage that has an un-bypassed cathode in the signal path would be a candidate.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Wed 16, 2021 5:13 pm 
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My set has a similar problem. I know what it is.

Its hum in the video that discombobulates the crude sync separator.

One cause was a minor heater-cathode short in an IF amplifier tube.

The rest has never been found, but I believe it simply is accumulated
pickup from capacitive hum pickup in all the IF amplifier stages.


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Thu 17, 2021 5:50 pm 
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Thanks again for the suggestions. I'm beginning to agree with dtvmcdonald in that it may just be a design problem.

I scoped the output of the video detector at point "A" in the Sams. This is at the capacitor before it gets coupled to the video amp. The trace is shown below. If I'm reading the trace correctly, the sharp spike which goes to the bottom of the sinusoid at the very bottom of the trace is the horizontal sync pulse, correct? Also, I assume that the sinusoidal modulation at the bottom (around 60Hz) shouldn't be there? The horizontal sync pulse "rides" that 60Hz signal and travels slowly from the top to the bottom of the wave... suspiciously like the bend in my raster.

I'm feeding the TV from a converter box and, interestingly, that 60Hz modulation goes completely away when I turn it off. And I've tried different boxes and I get the same results. Same, also with my crosshatch generator.

Does this mean that the 60Hz signal is originating in the IF strip? I've swapped out every tube so I'm puzzled.

Dave


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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Thu 17, 2021 6:30 pm 
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Your analysis is correct. The 60 Hz could originate in either the IF strip or tuner. It could come from either ripple on the B+ or coupling from the filament wiring.

Given how sinusoidal the 60 HZ looks, I think it is not from the B+. But to be sure, try tacking in some extra filter caps in parallel with the ones in your set to reduce the ripple. See if that makes any difference.

For coupling from the filament wiring, follow the wiring around in the chassis and make sure that everything is as far away from the filament wiring as you can make it.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Thu 17, 2021 11:35 pm 
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Greetings to Dave and the Forum:
Quote:
I'm feeding the TV from a converter box and, interestingly, that 60Hz modulation goes completely away when I turn it off. And I've tried different boxes and I get the same results. Same, also with my crosshatch generator.


Sounds like a ground loop to me. I would try this: Power the TV set with an isolation transformer. Connect the chassis of your converter box or your cross-hatch generator directly to the TV set's chassis. See if the hum persists.

Regards,

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Fri 18, 2021 11:51 am 
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Location: Belrose, NSW, Australia
I concur, Jim.

You could also use a 1:1 isolation transformer on the RF feed. About 10 turns bifilar wound (two wires side by side) through a ferrite bead or a ferrite core removed from a balun. Baluns are usually not isolators, so you'll need to make this yourself.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Sat 19, 2021 12:56 am 
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Jthorusen wrote:
Greetings to Dave and the Forum:
Sounds like a ground loop to me. I would try this: Power the TV set with an isolation transformer. Connect the chassis of your converter box or your cross-hatch generator directly to the TV set's chassis. See if the hum persists.

Regards,

I just checked the schematic. The antenna is transformer coupled to the RF amp. Unless there is some funny failure of the antenna coil there should be no way to have a ground loop.
This strange set has a RF amp, mixer and oscillator for the high band and another RF amp, mixer and oscillator for the low band.

Edit: This set has one side of the power line connected to the chassis. If there should be AC leakage through the antenna transformer, the amount of leakage would vary greatly depending on the orientation of the power plug. Try plugging it in both ways and see if there is any difference in the bending.

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 Post subject: Re: Bending Raster in Pilot TV-37
PostPosted: Jun Sat 19, 2021 7:40 pm 
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Thank you for the continued help -- it's very much appreciated. I think this problem has now gotten worse and I'm ready to turn this TV into an aquarium housing. :x :wink:

But first...

I checked the filament wiring against a photo of the set before I started working on it and everything is in exactly the same place, at least where possible. There's actually not a lot of filament wiring in the set, especially from the tuner to the sync separator. The filament of one tube is connected to the filament of the next tube by either 3 ohm resistors or 1.5 in lengths of wire.

I'm using an isolation transformer to power the set but I did try connecting it directly to the house AC. Reversing the plug made no difference.

OK, suddenly I started getting random noise "lines' across the picture. This happened while watching it so it wasn't anything I did. See the picture below.

FYI, I did not change the 300pf, 3kV caps in the horizontal output section because it's my understanding that they're typically okay. I also couldn't find any. Was this an error?

Dave


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