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 Post subject: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Sun 27, 2022 5:26 pm 
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Joined: Nov Sun 27, 2022 5:19 pm
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Location: Madison,Wi.
I Have a beautiful RCA 19" remote controlled portable black and white tv with the KCS 149AA chassis. It has a like new crt. It has one problem I can't locate. The background has has dark vertical striping (Please see the attached photo). At first I thought it was yoke ringing. I substituted the yoke with no new results.
I replaced the Horizontal output, damper and video out recapped the electrolytics and film capacitors. That took care of other minor issues. It originally had very bad horizontal linearity. The middle was squished and the left was 150% of the right side and the linearity coil did nothing. A bad cap in that circuit took care of that problem. The cage around the flyback is all there. I was thinking there was radiation into the video section. Could the flyback be of fault?
It runs cool and current draw is normal. It's a very uneven pattern. The ringing of yokes is even usually on the left only with notable jagged scan lines. Has anyone run into this before? I have earlier set with the KCS 143 chassis which is basically the same but is series strung. It doe not have this issue.The set produces a great picture otherwise. Any help would be greatly appreciated. Thanks...Jeff[img][img][/img][/img]


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KCS 149AA VIDEO RINGING.JPG
KCS 149AA VIDEO RINGING.JPG [ 746.4 KiB | Viewed 1439 times ]
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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Sun 27, 2022 10:34 pm 
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Joined: Apr Sat 06, 2013 1:18 pm
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Location: Mason, New Hampshire
Hope you have a scope. Best way to trace this is with NO signal
by unplugging the tuner IF. On a scope at some point it will show up.
Watch out for the boost filter cap. It often feeds the CRT, vert & audio.
And watch out for cold joints on the PCB mounting tabs. Was common
on many brands.
Let us know !

73 Zeno 8)
LFOD !


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Sun 27, 2022 11:34 pm 
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Location: Chapel Hill, NC
"Jail bars" that look something like that can be caused by ground problems. The high current deflection section can modulate the video (or other) grounds. Sometimes these can be detected by moving a short heavy ground lead around to various Horizontal flyback circuit ground points. If the pattern changes when a suspect ground is bridged, then clean and redo the soldering there.

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http://tv-boxes.com
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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Mon 28, 2022 12:39 pm 
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I've done some checking by pulling tubes and watching the screen. All the way to video out the lines are there and don't shift when rocking the horiz hold. When the video out is pulled the lines are still there but will shift with the horiz hold. What does this suggest?....Jeff


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Mon 28, 2022 2:56 pm 
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Location: Great Bend, Kansas 67530
I would check the filter caps in power supply first. And electrolytic caps in video and boost circuits if there are any.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Mon 28, 2022 6:28 pm 
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The set has been totally recapped. The electrolytics, film caps, and a couple paper caps. Every thing but the discs. This issue was there before the set was recapped. You can't trust electrolytics this old even though they were working. They can short anytime. Thanks...Jeff


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Mon 28, 2022 6:49 pm 
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Location: Great Bend, Kansas 67530
I didn't notice in your original posting that you recapped it! Sorry.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 6:20 am 
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jeffilm80 wrote:
The set has been totally recapped. The electrolytics, film caps, and a couple paper caps. Every thing but the discs. This issue was there before the set was recapped. You can't trust electrolytics this old even though they were working. They can short anytime. Thanks...Jeff


I beg to differ. I have RCA sets I have restored from the late 40's thru the 50's and I have seldom have had to replace an electrolytic can. I restored an RCA 721TS from 1947 over two years ago maintaining all original electrolytics apart from a 40uF radial from under the chassis and it has now over 1000 hours.

If the bars move in the opposite direction when you adjust the horizontal hold, the bars are being displayed in the retrace interval. In other words, the tube is not cut off during horizontal retrace. If the video output is pulled and it is still there, then somehow it is being coupled.

It could be an electrolytic. A schematic diagram would certainly be useful to help diagnose further.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 7:51 pm 
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Here's the schematic. I Hope it helps. The Forum reduces it's resolution when uploaded. Ive just uploaded the Sams sm....Jeff


Attachments:
RCA KCS 149 AA-AB 19 INCH BW 1966 SM2.jpeg
RCA KCS 149 AA-AB 19 INCH BW 1966 SM2.jpeg [ 606.98 KiB | Viewed 1250 times ]
RCA KCS 149 AA-AB 19 INCH BW 1966 SM.pdf [1.39 MiB]
Downloaded 17 times


Last edited by jeffilm80 on Nov Tue 29, 2022 9:08 pm, edited 1 time in total.
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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 8:42 pm 
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Location: Great Bend, Kansas 67530
How about that rectifier in the grid circuit of the crt?


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 8:56 pm 
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Location: Mason, New Hampshire
Boost goes right to CRT G2
Also the blanking goes to CRT G1. Both worth looking into.
If you use a scope be sure the probe is rated at 1 KV for the boost.
& be sure the CRT cathode wire is dressed right.

Reason boost goes to audio is if the HV fails so will the audio. This
kept many sets from self distructing when the customer kept listening
to it.

Zeno


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 9:14 pm 
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I was thinking of looking at the blanking circuit today but time ran out. I did reflow all the ground connections. I also uploaded the sam's...Jeff


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Nov Tue 29, 2022 10:38 pm 
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Easiest check would be to simply ground the CRT control grid and look.

The vertical blanking is diode or'ed with the horizontal blanking. From your photo, it looks like the vertical retrace blanking is working but not the horizontal. I would check to ensure the winding on the width coil is good and that you see a horizontal pulse where indicated on the drawing.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Thu 01, 2022 3:53 am 
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Location: Belrose, NSW, Australia
The way I'd track this down would be to take a cap in the region of 0.47uF on clip leads and use it to selectively ground the signal path to AC all the way back from the CRT. The cap will charge to the voltage at each point.

This way you can find where it's getting in.

If it seems to be everywhere, it's probably superimposed on a B+ rail or a common ground return.

When you recapped it, did you connect the new caps to EXACTLY the same points as the originals? Putting a cap on the "wrong" ground point is a great way to inject unwanted noise.

Yet another reason NOT to shotgun re-cap. Replace the critical parts first and re-check the performance after each few parts.

EDIT - just had a look at the circuit.
This design is unusual for the time in that it has horizontal blanking. There is a diode off G1 of the CRT. If that was open circuit, the little transformer would inject ringing during the forward scan. I bet that's it! The diode is also vulnerable to CRT flashovers so all in all not surprising.

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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Thu 01, 2022 4:52 am 
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I did some testing today I found that R82 3.9 megohm across C78 .01 was open. This is in the G2 circuit. As far as the diode goes it seems to check ok. The part number listing for it is left blank in the Sams, Its a small glass type. I had to order more resistors today as I found a few others that were out of tolerance...Jeff


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Thu 01, 2022 7:06 am 
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Location: Belrose, NSW, Australia
Replace that diode anyway. Use a 1N4936 or 1N4937. These two are easy to get.
It must be a fast diode, a 1N4006 for example won't do.
Your problem will NOT be out-of-tolerance resistors, unless perhaps 50% or so. Still, 20% or more out can mean the part is reaching end of life.

_________________
Wax, paper, bitumen, cotton, high voltages - what could possibly go wrong?


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Thu 01, 2022 8:52 pm 
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Joined: Apr Sat 06, 2013 1:18 pm
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Location: Mason, New Hampshire
You can run the 6 digit RCA part ## in ETG, NTE or other sub book.
Most likely its there. A paper copy is best but....

73 Zeno 8)
LFOD !


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Sun 04, 2022 5:13 am 
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Location: Queensland, AU
It may well be exactly as irob2345 suggests. And the problem relates to signals modulating the beam current during H scan. And as @irob points out one good way is to use a capacitor to ground AC signals in the signal chain (CRT's grid and cathode circuits without altering the DC conditions) to see if it makes the vertical bars go away. Ultimately a capacitor placed between the grid & cathode of the CRT will kill any relative AC modulation voltage affecting the beam current and rule that in or out. Don't connect the capacitor with it powered, it is better if it slowly charges during warm up.. It is relative grid to cathode voltage that modulates the CRT's beam current.

But, I think the first thing is to try to decide if the brightness modulation you are seeing in the CRT's beam current or oscillations in the scanning raster current. In the latter case it occurs because of beam velocity modulation, when it slows, more energy is imparted to the phosphor. It represents minor oscillations superimposed on the yoke's H scanning current.Sometimes it is necessary to rule this out putting a current sensing resistor in series with the H yoke coils and using an isolated scope, I use a Tek 222ps for this sort of task.

When the diode in the H beam blanking circuit fails, the Jail bars are usually more dramatic. There were articles about this problem in Electronics Australia back in the 1960's.

There is yet a third way it can happen it is rarer. During scan time, the LOPT primary does not normally have any resonances because it is heavily damped, either by the damper diode on the L side of the raster or by the output tube's plate current on the right side of the raster. And the H yoke's current is normally free from minor oscillations superimposed on the sawtooth current wave. However, there is an oscillatory proccess happening. Many technicians do not know it is there, because it is in a location that is normally never measured with the scope, it is on the Anode side of the EHT rectifier and it requires a very special probe to measure it. It comes about because of the imperfect coupling between the primary and the EHT winding(overwind). These oscillations can be electromagnetically coupled directly to the CRT's beam in cases where the LOPT is close to the CRT and there is no shielding. It comes about due to the leakage inductance of the overwind.

If you want to see a recording of these with a high voltage Tek P6013A probe it is on page of this article it is discussed pages 12 to 14:

https://www.worldphaco.com/uploads/BUSH_TV22.pdf

Finally , in sets which have a linearity control, like yours, it is designed to resonate , during line scan time, with about a half cycle or more to bend the HT voltage applied to the LOPT. This is how the linearity can be modulated. If that L-C value was overall defective (wrong L or C value or both), and it was ringing at a much higher frequency, that would put ripples into the H scan current and vertical bars on the screen too.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Mon 05, 2022 11:57 pm 
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Greetings, All!

I am reminded of Winston Churchill's remark, something to the effect that: Americans and Britons are two peoples divided by a common language. Unexplained abbreviations make this worse. What the heck is an LOPT?

This phenomenon is often caused by Barkhausen oscillations in the horizontal output tube.

My suggestion would be to replace the tube. If there is a viable substitute of a different type number, try that. Also, try the same type number from a different manufacturer. You might also try bringing a magnet near the envelope of the horizontal output tube while it is in operation while observing the "jail bars". The remedy is to replace the tube or in extreme cases, add means to suppress oscillations above the horizontal sweep rate such as low value grid and screen bypasses or suitable ferrite beads. Sometimes the magnet trick works, sometimes not. The hard part is figuring out how to attach it to the tube in the correct spot without causing thermal problems to either the magnet or the tube envelope.

Good Luck,

_________________
Jim T.
KB6GM
Palus delenda est.


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 Post subject: Re: RCA 19"BW 1965 KCS 149AA VIDEO BARS??
PostPosted: Dec Tue 06, 2022 1:48 am 
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Jthorusen wrote:
I am reminded of Winston Churchill's remark, something to the effect that: Americans and Britons are two peoples divided by a common language. Unexplained abbreviations make this worse. What the heck is an LOPT?

Line OutPut Transformer. What we call the horizontal output transformer, some times abbreviated HOT. Most people in the USA call it the flyback after how it works.
This one took me a moment to get when I first saw it!

_________________
Tom


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