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#31
10-31-2006, 01:46 AM
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Quote:
http://audiokarma.org/forums/showthread.php?t=54696 John 
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#32
11-09-2006, 08:47 PM
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Still having a problem with the RED color fading in and out...
 I'm not sure where to start troubleshooting for this problem. The picture is great when the red is working. Then it just slowly fades out until there is no red at all. Sometimes it will eventually come back, other times it takes messing with the red, green, blue and bias pots on the back. The crt hasn't been tested. Should this be my next step? I'm not sure what the signs of a weak red gun are? Seems to me that if there was a problem with the red gun, that it would always be bad.... not fade in and out. Maybe something in the color demodulator section? 
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#33
11-09-2006, 09:01 PM
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A weak red gun would be weak all the time. Look at the crt filaments while it is happening, make sure one isn't going out. Look in the matrix tubes for the same thing. If it passes that, check the DC voltages at the CRT (avoid the focus pin), see what is changing when it goes bad.
Work your way back through the screen circuitry (if the screen voltage is screwed up, be careful, ~600-900v here). Work back through the bias circuitry if the cathode voltage is off. If it's the grid (most likely) work back through the color demod section. Anything bad enough to remove red should screw up the DC voltages horribly. Let us know what you find. John
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#34
11-11-2006, 10:52 AM
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John:
All of the voltages at the CRT are a near perfect match for the voltages on the schematic. The crt filaments all look good. The screen circuitry seems fine - the voltages all respond to adjustments of the screen pots. After studying everything a bit, the problem seems to be that the red gun is working fine, but the red information is not being modulated to it. The set has a great black and white picture. The adjustment procedure with the service switch and screen pots looks normal. I have a nice full screen of red when setting the purity. So, should I be focusing on the color demodulator section? BTW, the problem is not intermittent anymore. I can't get an red in the picture at all now. I may try swapping the 6CG7's around to see if that changes anything. Thanks for the help! This is my first attempt to repair a color set, so I'm learning as I go along... Wes
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#35
11-11-2006, 07:29 PM
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Isn't the green signal derived from both the R-Y and B-Y demods? I would think if it were a demod issue that the green would be bad too...I would check the R-Y output circuitry. It may be good to try a scope on the red CRT grid lead and try and trace the AC signal back through the output tube to the demod as this may not be a DC voltage issue.
Last edited by Chad Hauris; 11-11-2006 at 07:31 PM.
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#36
11-12-2006, 01:59 PM
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I had a CTC-7 that would flash green intermittently. I found that the wax from the -Y output tubes would drip on to the sockets. I don't think this is your trouble, as RCA started using "mylars" in subsequent chassis, from 7AA onward, but thought I would offer the hint.
Kevin G.
__________________
stromberg6
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#37
11-12-2006, 07:46 PM
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Spent a little time with the set today:
Quote:
The chroma section is not on a PC board and is all mounted under the chassis. So, I guess the chassis is coming back out of the cabinet.  BTW, switching the 6CG7's didn't help anything. Quote:
Thanks!
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#38
11-12-2006, 08:18 PM
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I guess it's a long shot, but do you have any way to check the DC voltages on the 6cg7 (v-706) without taking the chassis out? Maybe a tube extender? It would be a clue even if the voltages are right. I was looking at the schematic, and I think if either L705 or c731 were open, that would cut red without screwing any of the DC voltages up. If c731 was leaky, or r730 was open, it would show up as incorrect grid voltage on pin 2.
If you do find your scope probe, the waveforms usually given with RCA sets were taken with the set displaying a "gated rainbow" color bar display. John
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#39
11-13-2006, 09:00 PM
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John:
I used a mini-grabber test lead to clip onto pin 2 of v706 which is the grid of the R-Y amp. This allowed me to test without removing the chassis. The DC voltage here shows roughly 1.5 - 2.0 volts. For comparison, I checked the grids of the G-Y amp and the B-Y amp. Both of these show around 3.5 VDC. All of these voltages are lower than shown on the schematic which is 6 VDC. Perhaps the lower voltage on the red is right at the threshold of working / not working? Thanks, Wes
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#40
11-15-2006, 03:04 AM
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My schematic also shows 6 volts (red) 6.2v (grn) and 6.4v(blue). What really matters most is the difference between this and the cathode voltage. The cathode voltage on my ctc11 schematic is 11v, so the grid bias is really -5v (red).
The voltages on the schematic were probably taken either off-channel or with a gated rainbow generator, so the absolute values might not mean much. It is very interesting that one is a little different. I wonder what the cathode voltages are? I'm guessing, based on a cathode voltage of 11v , that -9 volts on red (versus -7.5 on blue, which is working) wont be enough difference to cut the tube off. I'll bet the peaking coil (L705) is open. It will be tough to catch it in the act if this intermittent, by the time you get the chassis out, it might be working again. If it doesn't test open, I might try bridging it. This will probably have some undesirable effect, but if it stopped the red from going away.... c731 cant be leaky, because that would make the grid voltage too high. It could be intermittently open, though. Is it a ceramic disc or mylar? You might also see if r730 is out of spec. Maybe even put a little heat on it with a soldering iron and see if the resistance reading goes wacky John
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#42
11-21-2006, 10:18 PM
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John, I think you may be right about the L705 coil. The chassis is back on the bench so I was able to more easily check the voltages at the R-Y amp. Here's what I find: The plate voltage is 217 volts (pin 1). The cathode measures right at 12 volts (pin 3). And the grid was first measured at ~ 6 volts. I decided to monitor the grid voltage for a while and after a few minutes I saw it ramp up to around 12 volts. That's not right! So with a pencil I poked around at capacitor C731 and the L705 coil. With the mechanical movement, I saw the voltage drop back to 6 volts. I was able to duplicate the voltage change again by moving L705 back and forth. Doesn't look like a bad solder joint, but I will check that carefully before trying again.
Is there a source for this peaking coil? The schematic shows "620 MU-H" as the value. Thanks! Wes
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#43
11-21-2006, 10:31 PM
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Contact Moyer's Electronic Supply in Sunbury Pennsylvania. 1-800-577-6001
Ask for Bill Moyer, he is the son of the man who started the business. http://www.moyerelectronics.com/ polaraman
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#44
11-21-2006, 10:57 PM
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I would try bridging across the coil. Solder a wire around it and see if the voltage still ramps up.
John
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#45
11-23-2006, 01:28 PM
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Well, I spoke too soon. Jumpering around the coil did not change anything. And, I suppose the mechanical movement thing was a coincidence. Guess I'm in too big a hurry to track down the problem!
 Did some more testing last night and I see that there are some other voltage changes going on:When the grid of the r-y amp jumps from 6 volts to 12 volts, I also see a decrease in the plate voltage (pin 1) from 217 volts (normal) to 167 volts. Also see a slight increase in the plate voltage of the 12AZ7 "X" demod. It normally runs 260 volts and jumps to 272 volts. These jumps in voltage are a gradual ramp-up or ramp-down over about 5 seconds. I checked most of the resistors in the area of the r-y amp and they are all within spec. Should I be thinking about recapping the chroma section as a start? (Remember, I have the troubleshooting skills of a beginner  )Thanks! Wes
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