#31
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From the youtube video it looks like a low emission CRT to me. It has that silvery look on highlights. If the CRT tester you are getting has a rejuvenate function it might be worth a go. I've found that older B&W tubes rejuvenate quite well and usually last for a good amount of time. Colour tubes not so good.
Cheers. |
#32
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Still absolutely no effect from the AGC control on the picture. The schematic says the control is 750K, but it tests closer to 475K. However the wiper travel is smooth and resistance is correct all the way around and I'm not finding anything out of the ordinary at all. Now for the good news. I tested the CRT on the CR70 and sure enough, no cutoff and tested at about 2 in the bad range. Irext, I took your advice and did a rejuvenation, and just like that I have cutoff and emissions in the 10-11 in the green. And now the picture is indeed brighter, though the life test drops like a stone. Is this effect typically permanent or have I just given it an adrenaline shot that will wear off? This was how it tested after the rejuv, last night. I left it connected to the CR70 and tried it again this morning and while cold it started at about 2 in the bad and slowly creeped up to about 9-10 in the good range over about 3-4 minutes. Still had cutoff control, but not as good as in the video. Is there ever any benefit to hitting it again with rejuv/auto restore or is that just asking for trouble? https://youtu.be/tGNYEsj6CGw Not sure what's going on now with this pulsing/noise. Here's the new, brighter picture with that annoying new problem. https://youtu.be/GSc_GAQxrsI FK Last edited by FrankieKat; 08-23-2017 at 11:33 AM. |
#33
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That noise could be HV arcing. One of my sets did a milder version of that for a few weeks before the HV rect. died.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#34
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I did notice that rotating the AGC control does have an audible effect on the normal HV sync pulsing/buzzing sound (whatever you call it), mostly from a volume perspective. So it's doing something, just absolutely no change on the picture. If the 15M resistor between B+ and the AGC control was bad or flakey (it tested at about 35M on my VTVM, and was beyond my DVM's measurement) could that cause the AGC issue and perhaps this new noise issue? The vertical seems to be affected most by this and occasionally the screen will go bright and saturated for a second. Just a thought... FWIW tried injecting IF signal from the VA62 and still had same "arcing" and AGC problem. So at least we can rule out the tuner contributing to any of the problems... not like that was really a culprit, but oh well. Thanks! FK Last edited by FrankieKat; 08-23-2017 at 11:27 AM. |
#35
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That 15 meg resistor might be the cause of your problems, causing the noise and poor agc control.
15 meg sounds high, are you sure that is the correct part? Have you cleaned the pins of the tubes in the IF amp? jr |
Audiokarma |
#36
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I've swapped around the IF tubes several times and wiggled them and doesn't affect the noise. I wiggled all of the tubes too and even tapping on the chassis does nothing so really doesn't seem like something loose. Going to hit an electronics store on the way home and if I have to stick six 2.2M's together I will, lol! FK |
#37
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Since the gold band indicates a 5 % tolerance, I would think that it is fairly critical. It is part of the voltage divider network that sets the AGC voltage. Likely 3-4.7 or 5.1meg resistors in series would be close enough (since the pot gives you some adjustment range).
jr |
#38
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Too little of the schematic is shown to fully evaluate it's purpose, but it's value is likely critical. See that gold band on the end?...That signifies a 5% tolerance resistor. Most resistors in most TVs were 10%, or 20% tolerance. %5 resistors usually cost more and typically were not specified by engineering unless needed. You can make a series set of resistors if you need, but you probably want to check that it measures within 5% of the design value. If it is off a bit more it should not kill the set, but AGC may not set or react right...If you have signal and no more flicker that would confirm the resistor.
Speaking of resistors I've seen ones in series with the HV lead develop internal arcing...They may or may not test fine with 9V from your multi-meter, but at 10-30KV will arc and make RF noise that the set can tune and feed back to the picture and sound. AGC stages especially non-keyed ones often had long time constant RC circuits with large value paper caps and or resistors.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#39
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Attaching a larger version of the relevant schematic (really hard to get these legible and stay under the upload file size limit!) FK Last edited by FrankieKat; 08-24-2017 at 11:35 AM. |
#40
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Just 6 of these together and I've got myself one 15 meg 1/2 watt resistor... |
Audiokarma |
#41
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Why don't you try three 10 Meg resistors? One 10 Meg (10 Meg) in series with two 10 Meg resistors in parallel (5 Meg). That gives you your needed 15 Meg total resistance.
I take it you still have a Radio Shack store in operation? They pulled out of Atlanta awhile ago. |
#42
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FK Last edited by FrankieKat; 08-24-2017 at 11:35 AM. |
#43
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Manufacturers didn't use 1/4 watt resistors back then. 1/2 watt was the smallest. Using the formula P=Vsquared/R. P=250*250/15,000,000= .00384 watts. 1/4 watt resistors should do just fine.
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#44
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Well, I replaced the 15M resistor (R20) and made no difference -- the picture still has that noise and AGC control has no effect. I've checked and replaced all resistors and capacitors between the 1st IF grid, through the AGC control and to the plate of the AGC keying tube. I bypassed the AGC control with a low fixed value resistor and nothing different.
The DC voltages on the AGC tube and IF tubes are close enough to spec. Also verified all of the scope traces drawn on the Sams and frequencies are all correct, though P-P voltages vary a bit from the schematic but are within the right ballpark. I don't see any visible HV arcing, and while there's an audible "pulse" that occurs with the noise, it sounds softer, more like a hold/sync issue than a sharper HV noise. I swapped out every tube in the set last night including the HV rectifier and still no change. I can try to order up some more tubes to try (though I have at this point at least three of every one) if it's still possible it might be tube-related. I injected an IF video signal from the VA62 after the third IF tube (with it removed) and still has noise and no AGC control. I also saw no visible AGC control on the first power up of the set too, so I'm pretty confident that it's not related to something I did. The only thing I've noticed is that the noise seems to go away when it's tuned a bit away from the station or the signal level from the VA62 is low. Where could I be looking next? Thanks! FK (schematic attached) Last edited by FrankieKat; 08-24-2017 at 12:35 PM. |
#45
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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.
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. 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.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
Audiokarma |
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