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#76
02-06-2015, 07:24 AM
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the only 3.3 meg resistor i found in the chroma board near the video output tube has 124.4 volts on one side and is the same as pin 8 of the video tube. the other side of that resistor is 4.48 volts. i didnt pull it to check yet but the voltage is the same as pin 8 where its supposed to be 135 volts.
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#77
02-07-2015, 12:03 PM
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Since brightness is the issue, I would focus on the brightness circuit.
you tried a new video out tube (weak would reduce brightness) so next look at the brightness control which sets the bias of the video out tube. this is done via a voltage divider of 3.3meg and 330k with a source voltage of 135v The next logical things to check would be: 135 source (stacked B+ supplied by the audio out tube acting as a voltage divider). that's a little low but prob not enough to cause a problem. A sub of the audio out tube may increase the 135v source. The 3.3meg/330k voltage divider is where I would look. Since its unlikely the 330k has dropped (which would need to be the case for it to cause the low brightness) I would test the 3.3 meg (OUT OF CIRCUIT). If it has drifted high significantly that would reduce the voltage available to the high side of the brightness circuit. The trouble with going strictly with the sams for voltages is there are a lot of variables so they can only be a guide. After that you need to understand how things work and use reasoning to try and narrow down the problem. Like if brightness is low look in the brightness circuit.
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#78
02-07-2015, 12:11 PM
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the 3.3meg i found in the chroma board is probably the one you are talking about so ill lift a leg and check it but this resistor is the one with 125v on one side. the audio out tube in this set is not called that its a A F output i think thats the one your refering to. the onthers are sound IF and sound demod. this resistor dont have high voltage on both so shouldnt it have 135v on one side already if that voltage was there ?
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#80
02-07-2015, 12:28 PM
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6GC5 the big tube on the audio board.
The 3.3meg resistor has one side to the 135v source (provided by the 6GC5 acting as a voltage divider in a stacked B+ supply). The other side of the 3.3 meg connects to the 330k resistor to ground. the brightness control gets it voltage from the junction of these two resistors (a 10/1 voltage divider). If the 3.3meg drifts high the junction of the two (330k/3.3meg) will have a lower voltage for the brightness control to max out at, resulting in a dimmer pic. If the 330k drifts high the reverse is true the junction will have a higher voltage, brighter pic. if the 330k goes low, the junction voltage will be lower, if the 3.3 goes low the junction voltage goes higher. Look up voltage divider for how this works if the above is not clear.
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#81
02-07-2015, 12:31 PM
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a quick test to make sure you have the right resistor, jump another 3.3 meg in P with the existing one, that should brighten it up a lot. if it does then you know you are on the right resistor.
take it out of circuit and check.
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#84
02-07-2015, 12:37 PM
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the audio tube supply's the 135v to the rest of the set.
it is determined by the voltage drop in the tube from the plate to the cathode, this is largely determined by the bias of that tube (the 6GC5). there is grid bias set by a two resistors, again as voltage dividers, a 1.2 and a 1.0 meg resistor pair. The mid point of those two supply the grid bias.
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#86
02-07-2015, 01:07 PM
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one side should be what ever the 135v source is, this is determined by the audio out tube.
The other side I don't know, it depends on a lot of things. If the brightness control was not hooked up it would be what ever the ratio of the voltage divider is (1/10th if all was correct) but since the brightness control is in the circuit its not easy to determine.
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#87
02-07-2015, 01:17 PM
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ok so there is a problem in that loop since there is only 125v on one side of that resistor when there should be 135v and im sure that is the correct resistor since its near the video output tube. so being another video tube changed nothing then its safe to say either the 135v source coming from the audio tube could very well be the bias setting resistors at or near that tube or maybe what i read in that magnavox trouble shootlng guide that the 140 source is low because of a resistor in the power supply and its listed as low brightness.
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