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 Post subject: 1954 Philco A-T1888
PostPosted: Sep Wed 29, 2021 6:00 pm 
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Joined: Apr Thu 09, 2020 7:33 am
Posts: 189
Hello everyone,
What causes darkness on both sides of video image?


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File comment: Hello everyone. I recapped a Philco dual chassis model A-T1888 and it works but it has a darkness on the left and right side of video image. It only does this going through channel 4 or 3 input but straight video into the grid of video output tube 12BY7 it looks great and no darkness. The peaking coils have continuity but that just means they're not open
IMG_20210929_114113108.jpg
IMG_20210929_114113108.jpg [ 457.04 KiB | Viewed 693 times ]
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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Sep Wed 29, 2021 6:22 pm 
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Location: Albion, CA, USA, 95410
I see some contrast banding. Usual causes are contrast and brightness not set correctly. Can be issues within the set. Can also be from overdriving the input with your RF modulator. Older sets have some contrast banding because of the early circuit design.


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Sep Wed 29, 2021 11:01 pm 
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Location: Redlands CA
What setup are you using to feed it a signal?

A lot of converter boxes cause a squished image with black bars on the side on 4:3 sets, which might explain why it's normal when you bypass it and go direct?


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Sep Thu 30, 2021 1:34 am 
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Thank you for the help. I'm using a VCR to convert video from a DVD player to channel 4. I think you guys are right it's probably supposed to do that with modulators. I wonder if IF alignment is off and what would out of alignment IF would look like thanks. I did a lot of TV repair but I never had the pleasure of performing a complete alignment. The shop I work at had to move to a new location and it was a lot smaller building and the boss threw away all the old tube equipment used in alignment. I didn't mention that the picture tube had almost no emissions. I had to rejuvenate it. Its a 17YP4 its too bad the rebuild places have all closed


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Sep Thu 30, 2021 3:46 am 
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Location: Pewaukee, WI
What I see in the picture looks like lack of DC restoration. It may actually be a normal opperating characteristic of the design of the TV.

If your TV has a DC restorer circuit in the video output circuit check all components associated with it. If there is no DC restorer circuit (common in cheaper sets) you can search this forum for adding DC restoration if you would like to correct that performance issue that came baked in the original design.


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Sep Thu 30, 2021 5:07 am 
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Location: Ann Arbor, MI
in that posted picture, I don't darkness on both sides of the picture. I do see a darker horizontal band the full width of the screen with a bright object in the middle. Is that what you are talking about? Some weaker CRTs can have a momentary loss of emission when called upon to produce high brightness in part of the picture. Try cutting back the contrast and brightness a little and see if that goes away.

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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Fri 01, 2021 3:10 am 
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Thanks for your help. When I first got this TV to power up the picture tube didn't have any light at all practically 0 emissions. I have a tube tester but won't fit that old tube so I put my head in gear and devised a way to sort of wake it up. I have a small hand held ccfl tester that puts out an arc about 1/4 inch in length but very little current and is RF ac. I disconnected the green wire which goes to the g1 on the tube and while the TV was on with high anode voltage and about 520 volts on the accelerator grid or second grid I touched the control grid with my tester and snapped it back to life but I have no way of testing to see if it has proper cutoff...etc. but it looks good when tuner and IF is bypassed and injected with video at the grid of the video output tube. I think you're right about DC restoration. This is the first time I encountered replacing bumblebee caps and I might have mis read some maybe. There are some that were yellow purple yellow but the other side would be brown and I think purple then yellow. Not sure about the second purple band it's old and hard to see. Its either .47uf or .17uf but there is no such thing as .17uf. There is a capacitor in series with a resistor that is in the plate circuit of the video output tube and the other end of the series network goes to the 245volts. I wonder if that's the DC restoration filter cap. This was a really tough set. Everything was wrong with it. It even had an open vertical winding in the yoke due to sitting in a storeroom from corrosion. I had to do surgery on the yoke. The guy who owns it really wanted it to work again because it was his aunt's TV. Plus I like a hard challenge but I think I need to give up now that I've got it working this much. My boss is looking at me like when am I going to get back on LED sets hahahaha


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Fri 01, 2021 3:42 am 
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Adjusting the contrast will help but it's still there. This TV has a very strong contrast when turned all the way up it even makes the sound go up a little but it would look terrible because the tube is weak. I have to turn the brightness all the way up and the contrast down to about middle to get a fair picture. I would zap it again but that might ruin whatever is left. I told the customer to watch it like that until he might find a good tube.


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Fri 01, 2021 3:55 pm 
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Location: Ann Arbor, MI
Perhaps when you directly injected the video you had lower contrast and didn't notice.
But another thought. Does this set have an adjustable ACG (Automatic Gain) control. If so, try adjusting it for less contrast and then advance the contrast control to bring the contrast back up. See if that makes any difference.

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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Sat 02, 2021 3:24 am 
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Most B&W tube sets that had DC restoration had a diode tube doing the job. By the 60s color sets found complex ways of baking DC restoration into the amps.


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Sat 09, 2021 4:32 am 
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Joined: Apr Thu 09, 2020 7:33 am
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Thank you all for the good advice. The customer was happy and decided to take it like it is. When it needs service again I will try again to tackle the banding problem. The shop I work at is Campbell TV service in Shreveport and I repair present day TVs most of the time yuk!!. We stopped taking plasmas, dlp and projection TVs and now only work on LEDs and some older LCD with ccfl tubes and about to phase them out due to hard to find parts for old TVs. Those will never make it to the antique hall of Fame those just get used up like a plastic pop bottle. Sometimes something antique and fun comes in


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 Post subject: Re: 1954 Philco A-T1888
PostPosted: Oct Sat 09, 2021 6:59 am 
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What the banding represents is poor video amplifier low frequency response. Poor DC restoration if the set has the DC clamp circuit. Otherwise the video coupling capacitors have too low a value or an open cathode bypass capacitor. Even a bad filter capacitor could cause this.

Need to see the schematic to itemize the faults items.


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