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Old 06-13-2014, 09:18 PM
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Tubejunke Tubejunke is offline
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Zenith Video Issues

My 64 Zenith from the get-go of bringing it home had intermittent color, but produced a beautiful black and white picture. That's in another thread, so I'm leaving that alone.

Anyway, my sets video display had deteriorated to the point of not being really view-able. The picture is not sharp at all and sometimes looks almost negative with what I call worms running through the lighter shades. Just before the problem began, oddly enough I had a VCR/DVD combo connected to it and the picture was great AND the color was back. Then next time I tried it I had basically what I have described only worse. Sometimes the picture would start to fade to black as if there were high voltage issues, but I could turn to an unused channel and have a perfect snowy raster. I started thinking perhaps something with AGC, but the tube tested well and the control pot works. For the record the sound is perfect.

I know that like most all old electronics it boils down to caps, drifted resistors and bad tubes, Check voltages, resistances, waveforms; the whole 9. I guess the beauty of VK is that sometimes we luck out and there is someone familiar with a given problem or a given set who can point us in the right direction. I have been helped and helped out others; that's why I love this site. Lord knows we don't all have a bench with all the nice lab equipment. Some of us don't even have a tube tester, but they keep trying and that's what counts,

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Old 06-13-2014, 09:31 PM
DaveWM DaveWM is offline
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a picture (or video better still) is worth a thousand words. The hardest part of diagnosing sets with out being there is trying to visualize the problem. Next biggest problem is not being able to tap around and adj controls.

All that being said if you think you have an agc issue you should look on the schematic for a small value eletro, like 5uf or so. they are often singles small round with red painted on on end. This generally manifest as overly critical agc (from blanked out signal to loss of sync with very little adj in between. I don't know if I agree with the prob as described as AGC.

likely issues are tuner (dirty) tube sockets (dirty) baluns (open or not connected right). I look to this for problems with poor video signals.
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Old 06-15-2014, 03:29 PM
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Thanks Dave! I agree whole hardheartedly on the tips that you gave me. But I have had more sound than video issues with all of my sets when it comes to baluns connections and the use of stand alone signal sources like digital converter boxes (free TV) and VCRs ect. I had two sets that I thought had sound circuit problems (low volume) that turned out to not be happy with the signal device configuration.

Specifically with this Zenith set, I may still have something like that going on, but I have swapped TV out cables. I haven't tried a different baluns. The only reason I mentioned AGC was the darkening picture. I forget if that would effect the sound, but the sound again is perfect.

Hopefully soon I can get a picture of the picture. That will be a big help.
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Old 06-15-2014, 07:08 PM
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zeno zeno is offline
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A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
AGC usually messes up the sound if overloading or washes out
(no contrast) the pix.
Anyhows always start with cleaning & adjusting then tubes next.
Its the cheap & easy way to go. Two of the IF tubes & several of the
other low level tubes are the same. Try swapping them & see if
you get different symptoms.
Dont forget the lost art of tapping. Light tapping of tubes often
show a bad tube or dirty socket. Give light taps one tube at a time
while watching the pix in a mirror.

nuf fer now
73 Zeno
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Old 06-15-2014, 08:50 PM
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Quote:
Originally Posted by zeno View Post
A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
AGC usually messes up the sound if overloading or washes out the pix.

Dont forget the lost art of tapping. Light tapping of tubes often
show a bad tube or dirty socket. Give light taps one tube at a time
while watching the pix in a mirror.
73 Zeno
Very true on all statements and methods. These resurrections or restoration projects are unique as we are often working with something with years of use in a smokey room and then dormancy by the decade. With that being said, we don't really need the laboratory equipment that only some of us may own. You need these rudimentary, common sense skills like tapping tubes. cleaning their pins and sockets, and feeling capacitors to see if they are getting hot.

I just talked to the guy who gave me this Zenith yesterday and he told me that he got it in 1997 and that it had been in a basement before that since the late 70s. I feel that I am lucky to have gotten the set to where it is right now. I must look at all that is functioning which is most everything. Just the few issues that I am mentioning isn't that bad in the grand scheme of things. Also, I have seen the set play almost perfectly a time or two, but it's momentary. I am crippled by lack of space to move the thing around a lot or pull the chassis. At least it has that nice access panel on the bottom that gets me to much but not all of the components.

You mentioned IF tubes. I have tested and swapped all the video IFs. No change. Like I have mentioned, I need a screen shot......
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Old 11-08-2014, 01:12 PM
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Quote:
Originally Posted by zeno View Post
A screen shot would be nice but... just because a tube checks good
it dont mean its good. The worms make me think an IF tube.
:
I have found at least one problem that I sort of suspected, but failed to mention. The 6GJ7 tube in the tuner has a dead short. I remember when I first applied power to the set, now a long time ago, this tube lit up blue like a Christmas tree light for a few minutes, not seconds, minutes; then it went to a normal operating appearance. I figured that it was just really gassy. I hoped that no permanent damage was done, but such is not the case.

I also figured that I would have another tube since I am up to my butt in vacuum tubes, but nooooooooo. Anyway, if anyone here has one or two of these (6GJ7) to spare I would gladly pay them the value of a good tube and of course shipping OR if anyone has any tube needs I would gladly give them back more than they would be giving. Especially if you need Compactron types. If you need those I will give you two (maybe more) to one for any tube needed for a 1964 Zenith model 5111 with a 25LC30 chassis. I know that there are a few in my set. The only one that I don't want more of is a 6BA11. They seem to be everywhere; in my hoard at least. Other than this set I have NO use for the later style tubes or "Compactron" types as I refer to them.
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Last edited by Tubejunke; 11-08-2014 at 01:15 PM.
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Old 11-08-2014, 02:46 PM
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Does it display the same symptoms with a different signal source? Just wanta be 100% certain the problem is in the set and not the signal source.
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Old 11-08-2014, 05:41 PM
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I have definitely gone down that road as that or similar issues have been an issue with several of my old sets, but none (or few) of my newer, everyday sets. Basically, each set has it's own properties as to how it will accept say a digital TV converter, or a VCR as a signal source. The Zenith we speak of doesn't seem to be as persnickety as some of the older black and white sets I have and on the Zenith I have swapped baluns, cabling, and tried different sources and they all bear the same result.

My beloved Philco had a tough time when analog signal left our world for the most part. There was never a problem with a set of rabbit ears, but when I got a digital converter or other things that had to be cabled in it simply didn't like to perform. Oddly I get good performance with one lead of the balun connected to the tuner and the other connected to one side of the built in antenna. It would seem that it wouldn't work, but that's how it does work. I have a 1950 RCA that had very low sound until I tweaked the signal process somehow. The picture was perfect with almost inaudible sound.

So, I feel or dare I say I know that I must replace any shorted tube. It may not make the set play perfectly, but I strongly feel that it will help a lot.
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Old 11-10-2014, 09:50 AM
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My RCA CTC9C and my Zenith 25MC33 both have problems with the converter boxes. The RCA had terrible herringbone and the peoples faces would turn black. The Zenith had bad times with any graphics on the screen and loud audio buzz. I mentioned in a earlier thread about using a agile modulator. It cured both tv sets. The Channel Master box was the best one, the Digital Stream the worst (except for DXing, where it is the best).
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Old 11-10-2014, 08:39 PM
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My sets had issues with the converters as well as other signal sources with varying symptoms. I remember you mentioning the agile modulator. What is that exactly?
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Old 11-10-2014, 08:51 PM
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Quote:
Originally Posted by Tubejunke View Post
My sets had issues with the converters as well as other signal sources with varying symptoms. I remember you mentioning the agile modulator. What is that exactly?
Blonder-Tongue agile modulators allow you to become your own tv station, placing video on a selected channel. Pretty professional stuff, used in many applications like upstream video feed to a cable system, MATV systems, and out-of-the-box tv stations. We used them aboard ship to feed baseband video to about 270 TVs in various spaces. We had 7 interconnected, broadcasting movies, a scrolling information screen, and live tv from our satellite AFRTS feed.

You can find them on ebay.
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Old 11-11-2014, 12:27 PM
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Electronic M Electronic M is offline
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The Blonder Tongue aglie modulators (as well as their non-agile/fixed channel BAVM models), and other broadcast grade equipment should produce a comparable quality signal to the old OTA stations. That gear was designed to meet the transmission NTSC specs fairly precisely...Compare that to the cheap modulators in consumer gear (like DTV and cable boxes, VCRs, and even cheap standalone RF modulators marketed to those with older TVs that wanted to use DVD players or newer game consoles) which transmit the video carrier in AM double sideband mode (one of the sidebands is supposed to be suppressed) which results in a signal more than 6 MHz wide that interferes with adjacent channels and causes horizontal video ringing (faux vertically oriented horiz drive bars) in Predictas, and other sets...Or over modulation of white that temporarily cuts off the carrier, thus freaking out the FM audio demod causing buzzing.

The best part about a powerful agile mod (like the blonder tonugue models) is that with a simple lamp cord antenna (cut to be a half wave dipole at the center frequency of the channel) one can send the RF wirelesly to all the sets in ones house, or perhaps ones block if the antenna is sufficiently optimized and the output is turned up to max...No more dragging the set to the signal source or vice versa.
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Old 11-11-2014, 01:53 PM
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The Blonder Tongue can be picked up cheap on our favorite auction site, Be careful of the options it has. Some will use balanced audio in and will have to be converted to single ended. I have the AM60-550A with no options. I use it on channel 10 VHF as that is the only channel out cable has with nothing on it.
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Old 11-12-2014, 02:14 PM
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Pardon me for being late to the party here...

I often have this dream of a closet full of digital convertors on the few useful channels (This!, Movies!, Retro, Antenna TV, etc.) pumping into Blonder Tongue aglie modulators. Then I spin through the dial with my SC 600 and the world is good again.

But I probably won't get to that project until the rest of the broadcast spectrum is sold off and the whole damn thing is a moot point. Perhaps I'm asking a technical question that is over my head, but it seems that the issue of digital converters "overloading" the signal or introducing too much noise is identified.

Could anything be hung off the back of a convertor to clean up the signal and/or reduce the strength? Even if that requires and actual circuit and power supply... Can anything be "tweaked" amongst the IF/Video/sound coils of our sets that would improve the situation?

I'm trying to impress my friends with an episode of original Star Trek when the commercials for trans-vaginal mesh lawsuits come on and the buzzing around the 1-800-VAGI-SUIT lettering blasts our ears. Or Tom Kruise tries to sell me a Hover-round™ ScooterChair, and the video overloads and gets dark. Bad Times Notice how the same menu screen goes from gray/black to all black, and the background content doesn't appear much different? At least to me.
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Last edited by Carmine; 11-12-2014 at 02:18 PM.
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Old 11-12-2014, 04:51 PM
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The text buzz issue comes from an internal design flaw in the RF modulator it's self, which likely*# can't be corrected by an outboard device. In am AM modulator traditionally modulation is to be kept less than 100% so the carrier is always on regardless of modulation, but if allowed to equal 100% the carrier is extinguished, and if it goes over 100% mod the carrier polarity reverses (creating a 180 degree phase shift).
Consumer modulators have a tendency to use the same carrier for AM and FM components, so if the AM video mod kills the carrier or shifts it 180 degrees in phase, the FM mod will still try to modulate what it is given resulting in noise added to the audio (drop out noise from 100% mod, and likely more critical PHASE distortion of the FM signal from >100% AM mod). Now a key here is that over modulated AM (from the VIDEO signal) creates phase modulation...Phase modulation and frequency modulation are nearly the same thing* and can both be received or transmitted with equipment designed for the other one with a simple signal processor inserted outside the RF system, so the AM modulator is interfering with the FM sound (via PM noise) every time the whiteness of the video gets strong enough to achieve 100%+ modulation (the white level in video corresponds the lower troughs of the signal so when those parts of the modulator envelope dip below 0V carrier on bright text or other bright objects the sud hits the fan). Outboard RF attenuation will do nothing to stop this since it has nothing to do with the level of the outputted RF, but instead the internal modulation signal level relative to the RF carrier before entering the modulator...You could attenuate the RF output to a hairs breath away from snowy reception and the buzz would still remain from the crap happening in the modulator.

A way to verify this (which I've done) is to take an old outboard Closed caption demodulator which has caption brightness and contrast user controls and feed the video output to a modulator you want to test. Start with the brightness/contrast at max and turn it down until the audio buzz, with a normal/dark program on, goes away.

*# It may be possible on some consumer RF mods to attenuate the video line in to them and or add a small DC offset to kill the buzz, but if there is any internal auto gain level circuits and or DC blocking circuits this will be useless.
The best solution is to get a cheap older pro-grade RF modulator with audio and video level controls to be able to properly set them your selves, and not have to worry about all the other (inexcusably)crappy approximations of the NTSC specs that the consumer modulators make.

*Phase mod and frequency mod circuits can produce the opposite type of output simply by having the input signal pass through an integrator or diferentiator before the modulator or after the detector (the specifics depend on what type the hardware is VS. desired mode of operation and whether you are on the receiver to transmitter end).
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Last edited by Electronic M; 11-12-2014 at 05:03 PM.
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