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#111
02-12-2015, 09:42 AM
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ok i changed the .22 cap had no effect so now i am really trying to understand what is happening or not happening here with this set. i took vlotages again with the bias at min and the g1 164v 163v 164v g2 1145v 1142v 831v cathodes 338v 310v 338v and these i guess would vary depending on where the pots are. the green drive with bias at min comes right up the way all 3 colors should but there is no blue or red that comes up unless the bias is at almost max and this is with 2 different crts.. these voltages look to be somewhat where they should be with the bias at min so the question is what is preventing the screen from being brighter then it is and or what is preventing the red and blue from coming up the way the green does with the bias down . the crt bias, this works with the blanker circuit so maybe there is other issues within that circuit and would it affect the drives ability to show lines needed for setup and ultimately help in the brightness area via the drives. however i do lack some understanding in some areas of this type of electronics but i do try.
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#113
02-12-2015, 12:00 PM
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i tried 3 pairs of 6gu7s, an audio tube , video out tube, and did not replace any out of spec resistors. the 3.3 and the 330k i pulled from circuit and checked again and they are good. i went over every high wattage resistors both in the power supply and where the RGB drives are on that panel.
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#114
02-12-2015, 12:01 PM
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the problem looks to be a bias issue for the CRT assuming the CRT test ok.
the G1 to cathode voltages have a lot to do with the cut off of the electron flow from the electron gun, the G2s look ok, not sure about why one was a lot lower, I presume that was the green. The CRT cathodes are high, indicating a weak video out tube or a bias issue (brightness circuit applies a bias voltage, the more neg the control grid of the video out, the less current flow and the less of a drop on the plate load resistor, the less drop (higher plate voltage) the higher the directly coupled cathodes of the CRT. Since the voltages there seem high in relation to the G1 voltages you need to determine the cause (video out tube/bias too low/screen voltage too low/emissions too low all reducing the current flow of the tube resulting in plate voltage too high). if the tube is ok, and the bias voltage on the control grid is ok, then about the only thing left is the screen grid of the video out. this is supplied by the 140v supply which comes from a stacked b+ derived by the voltage drop across the audio out tube. The other possibility is the CRT G1 is too low. this comes from the plates of the 6Gu7 difference amps. again you need to look into bad tubes but this time it would not be weak but rather gassy or grid emission problems upsetting the tubes and causing too much conduction dropping the plate voltage (the opposite prob of the video out tube), and direct coupling to the G1 of the CRT too low a voltage (160ish)
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#116
02-12-2015, 12:18 PM
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I was a bit concerned by the screen voltage of the video out tube. Has this set been recapped, specifically the caps around the 140v source? there is a 20uf and a 100uf cap in that circuit. you would not want to have leakage there.
check the voltages of the 6GC5 again.
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#117
02-12-2015, 12:27 PM
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Quote:
the blanker tube pin 1 243v 2 75.7v 3 4.67v 4 0v 5 0v 6 172v 7 1.582v 8 7.50v 9 0v
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#119
02-12-2015, 12:33 PM
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Quote:
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Linear Mode
