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  #46  
Old 08-24-2017, 03:24 PM
FrankieKat FrankieKat is offline
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Originally Posted by Electronic M View Post
Is the 100K resistor in series with the CRT HV lead still original? If so try replacing it. I mentioned earlier that resistors in the HV can develop internal arcing, the arcing acts like a spark gap transmitter and feeds back through the RF chain as video and audio interference.
No, that one is original, though I tested it and value was good. I'll replace it next time I pull out the HV box (it's not a lot of fun to do on this).

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Originally Posted by Electronic M View Post
Was the 3RD IF tube installed when you injected after it? If so large RF noise spikes could still be getting through the IF and mix with the injected signal resulting in the noise remaining present. If you pulled the 3RD IF for the signal injection then it suggests a noisy component, B+ line, or connection in the stages between the injection point and the element of the CRT being driven with video.
It was removed, and I also removed the 1st IF and injected signal right after it. I will repeat the experiment again tonight and make detailed notes about what happens.

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Originally Posted by Electronic M View Post
Do you have a working o-scope? If so it may be good to ground out the signal going to the detector (a large cap ~.1-10uF from it to ground should do the trick) and if the noise persists search for it in the stages between there and the CRT. Once you find it trace it in in the signal chain to the strongest point, then look for it on any power supply lines in the area to see if it is stronger in those lines.
I do, and I'll give that a try. It's tricky because the noise either goes away or becomes imperceptible with low or no signal. With the antenna leads disconnected or IF tube pulled the screen is solid white/blank.

Do you think this is related to the AGC or do I have two unrelated problems?

Thanks for the suggestions -- will get to work on those next and let you know.

FK
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  #47  
Old 08-24-2017, 03:54 PM
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If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.

If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.

One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.
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  #48  
Old 08-25-2017, 10:56 AM
FrankieKat FrankieKat is offline
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Originally Posted by Electronic M View Post
If the noise goes away completely with the IF tubes pulled then the problem is either up stream of the 1st IF tube you can pull to kill the noise or it is in the AGC.
With all three IF tubes removed and the IF signal injected afterwards, same issue.

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If the noise goes away with the 1st IF tube pulled, then odds are it is in the tuner, or it is local interference. If you have a signal source that operates on other channels than the one your currently using it may be wise to try different channels. There is a large gap (FM band + other stuff) between channels 2-6 and channels 7-13. So switching from low band VHF to high band or vice-versa may be a wise test (to get away from wide band noise). Also if a local DTV channel is using the same RF carrier freq as your signal source, you will likely get noise from that DTV carrier leaking into your set.
I used the VA62 to try on channels 7+ and still there. The DTV box is only 3 and 4, but same on both.

Here's what I'm seeing now with VA62 feeding channel 3 RF: https://youtu.be/q8nD70vokkE

It's odd that the audio takes hits too because it's picked off immediately after the detector which is basically right after the IF injection point that I used. Would this seem to suggest the issue is in fact HV-related since that would be about the only thing remaining that could cause this effect on both audio and video?

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Originally Posted by Electronic M View Post
One way you can eliminate AGC noise is to connect the AGC buss to an external adjustable DC supply 0-25V and use that supply to clamp the AGC line to a proper fixed voltage.
I used the VA62's DC output to vary the AGC voltage and that actually did what the AGC control is supposed to do. Also, with at least -5V applied, the AGC control does actually have an effect on the picture( for the first time) since the injection point is at the wiper of the AGC control. Without it, at that point, I measure a small negative voltage only in mV that varies maybe 8mV with the rotation of the AGC control. With just the VA62 connected with the voltage set all the way down to nearly nothing, the picture does become slightly more saturated - like the AGC control being turned slightly up.

There are otherwise no voltage readings called out on the schematic for that portion of the circuit so I'm confused as to where this negative voltage would be normally developed? I should expect to see that same between -9 (or so) and 0V at that point if everything was working correctly yes?

Thanks!

FK
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  #49  
Old 08-25-2017, 11:45 AM
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Are the power supplies to the video output and audio output stable and quiet?
Is the external dag coating on the crt in good shape and grounded well?

jr
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  #50  
Old 08-25-2017, 12:59 PM
old_coot88 old_coot88 is offline
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Originally Posted by FrankieKat View Post
...There are otherwise no voltage readings called out on the schematic for that portion of the circuit so I'm confused as to where this negative voltage would be normally developed? I should expect to see that same between -9 (or so) and 0V at that point if everything was working correctly yes?

Thanks!

FK
Excellent discussion here on how keyed (or gated) AGC works. Especially the 4th post down..

http://antiqueradios.com/forums/view...p?f=3&t=201711
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  #51  
Old 08-25-2017, 01:06 PM
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I second what JR said.^

Given that with the 3RD IF pulled and the signal injection after that, that the interference remains. The problem is most likely noise in the B+ or HV arcing. There could be issues in the stages between the video detector and the CRT, but it is not as likely.

Since you are getting sound interference too, here is an idea to save you some time....Pull the H output tube or damper (to stop supply of HV) and see if the sound interference goes away after it has had a couple of minutes to run without HV (you obviously can't check picture with no HV).
If disabling the HV stops the interference then the issue should be in the HV or CRT grounding. If the noise remains without HV then check the B+ lines with a scope, and if those are clean use the scope to look at the stages between the injection point you were using and the CRT video feed.

You may have AGC issues, but if as you say the IF is passing signals properly (with the exception of the mystery noise), then I'd ignore the AGC until the noise issue is fixed. The AGC appears to only affect the tuner, 1ST and 2ND IF...So given you can inject after the 3RD IF with the 3RD IF tube pulled and still get noise, I think it is safe to say the AGC is not your noise source, and can be ignored for now.
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  #52  
Old 08-25-2017, 01:08 PM
FrankieKat FrankieKat is offline
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Originally Posted by jr_tech View Post
Are the power supplies to the video output and audio output stable and quiet?
Yeah, all new electrolytics - Nichicon 105 deg, 500V caps replacing the original 450 and 350 volt can sections. Replaced the original candohm with two 10W power resistors as well and the B+ lines are all within 3 volts of schematic. My DVM locks right on and they are stable, but I'll scope them all and look for any ripple or odd noise too.

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Is the external dag coating on the crt in good shape and grounded well?
Interesting question! The grounding spring is tightly in place and overall the coating looks in good shape, however it is a little scratched where it makes contact with the tube - see attached pic. Would that be enough to cause a problem? Recommend trying to bend the spring so that it contacts in a slightly different place?

FK
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  #53  
Old 08-25-2017, 01:22 PM
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Originally Posted by FrankieKat View Post
Interesting question! The grounding spring is tightly in place and overall the coating looks in good shape, however it is a little scratched where it makes contact with the tube - see attached pic. Would that be enough to cause a problem? Recommend trying to bend the spring so that it contacts in a slightly different place?

FK
I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.
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  #54  
Old 08-25-2017, 01:47 PM
old_coot88 old_coot88 is offline
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One way to test for questionable dag grounding is tape on a grounding wire, and see if the noise disappears. Strip several inches of insulation off a piece of hookup wire and tape it to the dag, and the other end to ground.

That's assuming there's no isolated patches or 'islands' of dag lurking around, which happens occasionally.

Last edited by old_coot88; 08-25-2017 at 01:54 PM.
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  #55  
Old 08-25-2017, 02:57 PM
Tom9589 Tom9589 is offline
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You might want to be careful using tape to hold a wire against the dag. The dag is not known to have very good adhesive characteristics and you might pull off a big amount of dag when you remove the tape.
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  #56  
Old 08-25-2017, 04:10 PM
FrankieKat FrankieKat is offline
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Originally Posted by Electronic M View Post
I'd try changing the contact point. I f the dag is missing (even just a whisker) under the point of contact or making poor electrical contact with the spring then you will get HV arcing (and spark gap transmitter action) at that contact.
Okay, so pulled away the metal contact and cleaned it up a bit and then bent it to another place on the tube, and looks to be making good contact. I don't see any "islands" or places where the dag is separated from the rest. The coating looks pretty complete... just a little scratched off where the metal has been resting for the last 60 years.

Now here's the thing... before that, after playing a bit with the suggestion of grounding out the 3rd IF with a cap, and working with DC on the AGC circuit, the noise seems to have gone away... along with decent picture contrast. It's possible that it could have been poor contact with that rubbed-off dag and in working on the chassis the contact was intermittent.

So without a clear explanation, I seem to have a more or less clean but poorly contrasted image. As with before, it looks brighter in the video than in person.
https://youtu.be/xWOoI252aZY

There is definitely some wobble to the picture as well, which you can see with the DTV but can see it much better with the test pattern. I'm adjusting first the fine tuning then the brightness then the contrast and you can see there's not really a sweet spot.
https://youtu.be/XO5ciFJxHiM

Since the CR70 rejuv, the tube now produces a lot of light with brightness all the way up. I re-checked it again and still has responsive cutoff and gets up to about a 10.5 on the CR70 emissions when it's warm.

Still nothing on the AGC control, but injecting negative or positive DC doesn't make any large improvement to the picture's contrast so maybe that's just not going to be a big help.

I would say contrast and vertical hold tends to get worse as the set warms up. The sound (as can be heard in the test pattern video above) get very soft when the channel is fine tuned in for best picture. I know this is typical with old B&W sets and cheap DTV boxes though.

Just got the HV probe and it's measuring 15kV, while the schematic calls for 17.5k. I'll try a few different horiz damper and output tubes and see if that makes any difference.

FK
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  #57  
Old 08-25-2017, 05:01 PM
tom.j.fla tom.j.fla is offline
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Remember, 17.5kv is at zero brightness set brite control all the way off. HV will be lower as the screen gets brighter. All the best,Tom.J
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  #58  
Old 08-25-2017, 07:08 PM
FrankieKat FrankieKat is offline
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Remember, 17.5kv is at zero brightness set brite control all the way off. HV will be lower as the screen gets brighter.
Ah, got it. Okay, that brought it up to 15.35kV. I tried all three output tubes and the one that was in there is the best HV. Changing damper had no effect.

Edit: Turning width control all the way up and brightness/contrast all the way down, I've got 17.6kV so HV seems solid.

FK

Last edited by FrankieKat; 08-26-2017 at 04:13 PM.
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  #59  
Old 08-25-2017, 07:23 PM
old_coot88 old_coot88 is offline
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Originally Posted by Tom9589 View Post
You might want to be careful using tape to hold a wire against the dag. The dag is not known to have very good adhesive characteristics and you might pull off a big amount of dag when you remove the tape.
Good point. But we used to leave the grounding wire in place as a "belt & suspenders" supplementation when there was a sketchy spring contact.
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  #60  
Old 08-26-2017, 05:05 PM
FrankieKat FrankieKat is offline
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So here's the update in quick bullet points.

What's working:
  • Horizontal sync - strong, stays locked in at center of control rotation
  • Audio - full and loud (when using BT Agile modulator)
  • High Voltage (measures 17.5kV)
  • CRT has responsive cutoff and good emissions as tested by CR70. Output is bright
  • Odd video/audio "noise" has stopped (unclear why)
What's not working:
  • AGC control non-responsive
  • Contrast very poor
  • Vertical hold poor -- worse as set warms up
  • Picture has a "wobble" -- especially visible with test pattern (https://youtu.be/XO5ciFJxHiM)
The Sams troubleshooting notes keep pointing at V7-6AN8 (Sync Amp), V8-6AN8 (AGC keying/Vert Mult) and V6-12BY7 (video output). Have tried four different 6AN8 tubes, and two 12BY7's -- however I've just ordered some more of both.

At this point, is there anything else this could be other than those tubes or anything else I could try?

Thanks again everyone, for all of your help and great suggestions!

FK
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