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 Post subject: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 3:03 pm 
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Joined: Feb Thu 18, 2021 3:09 pm
Posts: 114
I recently restored a Zenith B1411P. The picture tube tests great. Replaced caps, bad tubes, horiz afc diode, germanium diode and peaking coils. I left C1 electrolytic in there since it still tested okay. When I turn the set on, it takes about 30 seconds for the picture to finally come up on the screen so high voltage it taking a while to get there. After a few minutes, the picture starts to get dimmer. Not to an unwatchable point, but to a point where a can't really see the details in any of the darks. Should I go ahead and replace C1? Or try a new horizontal output tube?


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 3:43 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 3497
Location: Dallas, TX
If the HV goes low the picture will "bloom", the picture will expand. I bet the HV is OK.
Monitor the DC voltages on the cathode and grids of the CRT gun. If you see a voltage change when the picture fades that will give you a clue.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 4:11 pm 
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Joined: Feb Thu 18, 2021 3:09 pm
Posts: 114
I'll test the voltages when I get home. Here is a video of how long it takes for the high voltage to start and an image appear. It looks like the image is dim from the start now. Could C1 be reforming? Video is compressed down so it's a bit blurry but it took about a minute for high voltage to start.


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 5:24 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 3497
Location: Dallas, TX
The long power up and the dimming afterwards probably are two different problems.
I would replace C1. That may be the cause of the slow warm up. I think that TV is does not use a power transformer and uses selenium rectifiers. The rectifiers might be bad.
There also could be resistors that have drifted. Also sometime a resistor will change value as it warms up.
BTW, why did you replace the peaking coils? They are not a high failure rate component.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 5:45 pm 
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Joined: Feb Thu 18, 2021 3:09 pm
Posts: 114
I didn't mention in the first post, I replaced the selenium rectifiers with 1N4007 diodes. I'll check resistor values and replace C1. I replaced the peaking coils because in my 1954 RCA, the coils were bad. When I was having trouble with the horizontal and vertical during the restoration, I assumed it was the peaking coils again.


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 6:12 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 2456
Location: Lafayette, CO
With so many questions, l would want to measure B+ at several points. In particular the horizontal oscillator. Sound appears fine but l would include that. Measure the B+ for about 5 minutes at each point. My 2cents' worth......Craig


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 Post subject: Re: Zenith picture gets dim over time
PostPosted: Aug Thu 12, 2021 9:36 pm 
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Joined: Feb Thu 18, 2021 3:09 pm
Posts: 114
I replaced C1 and it seems to be much better. Image isn't getting dim like before. Tested C1 and it was around 248uf instead of 140uf. Thank you everybody!


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