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 Post subject: Offset raster?
PostPosted: Oct Wed 04, 2017 2:10 am 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 789
Location: Santa Rosa, CA
Continue to work through the problems in the Admiral 20A1 I mentioned
a while ago. This one has me stumped....the raster is offset by a full
inch on the right side of the 10BP4. Reading through service info and
all say that this is usually caused by the focus coil or yoke not being
positioned properly. Possibly the ion trap magnet. I've tried all of these
things with no improvement. The yoke and focus coil are centered
on the neck of the CRT and the chassis has been completely rebuilt
(caps, resistors, tubes).

The image is not distorted in any way so I doubt there are shorted
windings in the yoke or focus coil. I should mention while the offset
is primarily horizontal, there is also some offset vertically.

What am I missing?
Steve


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 6:38 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16747
Location: Albuquerque, NM 87123
I need clarification: You say that you tried positioning the focus coil and yoke and the ion trap and got no improvement. Did they do anything?

Your description tells me you have a capacitance problem around the screen grid of the horizontal output tube.

Have you tested the new capacitors you put in? Forget about your vertical until you solve the horizontal.


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 2:01 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 789
Location: Santa Rosa, CA
Johnnysan wrote:
I need clarification: You say that you tried positioning the focus coil and yoke and the ion trap and got no improvement. Did they do anything?

Your description tells me you have a capacitance problem around the screen grid of the horizontal output tube.

Have you tested the new capacitors you put in? Forget about your vertical until you solve the horizontal.


Wasn't clear about the problem. The raster is shifted to the left about 1".
Moving the focus coil did make a difference but not a lot and in the
wrong direction. The yoke is as far forward as possible and shifting it back
slightly made the whole raster smaller as expected.

Moving the ion trap around changes the brightness of the raster but does
not seem to shift the position of the raster. The best position was where
it was originally.

Maybe something has become magnitized? I will check the screen grid
cap, thanks for the tip! While I've restored quite a few 1940's TV's,
most have been electrostatic CRT types so not as familar with the
problems encountered in magnetic deflection sets. Always a learing
experience!
Steve


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 2:46 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 629
Location: Pewaukee, WI
The guns in these tubes do not always shoot straight. Here is an idea: if the CRT is round loosen the mount straps and rotate the entire CRT ~180 degrees (let the ion trap move in tandem with the CRT) then try the focus coil/yoke adjustments again.


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 4:48 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9626
Location: Redlands CA
The Ion trap should always be adjusted for maximum brightness regardless of its effect on centering, not doing so can damage the CRT.

I noticed you said the focus magnet was centered on the tube, it doesn''t have to be, sometimes it has to be angled or slid up and down to get the image centered, I don't remember offhand which adjustments the Admiral has but most sets of that vintage have slotted holes that allow a wide range of movement on the coil.

The Horizontal being misadjusted can also pull the raster off center quite a bit before it flops over.

One last thing I can think of, make sure the Focus coil is not flipped front to back.


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 4:52 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9626
Location: Redlands CA
I just looked at the schematic, this model has the spring loaded screws to adjust the coil right?

You should be able to shift the raster up, down and sideways by adjusting those screws in or out, the coil may not be centered and may actually need to be tilted quite a bit to get the raster centered.


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 Post subject: Re: Offset raster?
PostPosted: Oct Wed 04, 2017 7:24 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 9306
Location: Powell River BC Canada
Can you post a photo of the face of the CRT without the mask ?

Also, the 10BP4 uses a double ion trap. Photo ?

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Offset raster?
PostPosted: Oct Thu 05, 2017 12:29 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 16747
Location: Albuquerque, NM 87123
How did you change capacitors? Did you replace with the same values that were in the set originally, or did you follow what the Sams said?


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 Post subject: Re: Offset raster?
PostPosted: Oct Thu 05, 2017 1:52 am 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 789
Location: Santa Rosa, CA
Eric H had the right answer. Screwing around with the focus coil brought
the raster back on the screen. It is certainly NOT perpendicular to the
neck of the CRT now. I was worried about going too far and breaking
the neck. the funny thing is both of the period general service manuals
I have say the focus coil needs to be "centered on the neck". Wrong!
The wisdom of the guys on this forum prevails again!


And to answer another question, I always replace parts based on what
was there originally, not schematics which are all too often wrong.
The Sams package did a very good job of covering all of the many
production changes which helped me verify some parts.

Thanks guys....on to the next issue.
Steve


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