#16
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Someone just check this and my logic. Assuming that the screen on the 6BQ6 doesn't provide any voltage, the resistor (R15) faces the whole 340v of the B++. Given this, we can determine the "should be" resistance of R15 based off of the given voltage drop from the Sams voltage chart. If I did my math correctly, the resistance should be about 8K ohms. Why isn't it 8K ohms then?
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"If it isn't broke, you aren't trying hard enough" |
#17
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The average screen grid current can vary with the peak current and the length of time that it draws current, which I would think is a function of the drive control. So, I would look at the drive control value and also the adjustment procedure in SAMS. I would expect the DC screen voltage measurement varies with the drive adjustment (check if it's so?). |
#18
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Well, I gave it a shot. I can't find the alignment describing how to adjust the horizontal drive in my Sams. Either way, I've messed with it and it moves the HV up some, but only to about 7.5kv. I noted earlier that the horizontal drive potentiometer tested a little high (34k vs 25k ohms) Not sure if that could be affecting anything.
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"If it isn't broke, you aren't trying hard enough" |
#19
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If the H drive pot can go to zero, then the max value of 34k is not a big deal, as long as the 4700 ohm is the right value.
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#20
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Yeah, the Pot can go to zero and I had replaced the 4700 ohm resistor as it had gone bad.
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"If it isn't broke, you aren't trying hard enough" |
Audiokarma |
#21
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Just for the heck of it unplug the HV lead from the CRT and take a reading on the HV lead connector. Just to verify that the CRT isn't somehow loading down your HV.
Last edited by Kevin Kuehn; 09-13-2018 at 03:12 PM. |
#22
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Actually, I've been doing most of the tests without the CRT, I hooked it up last night just to verify that the voltage wasn't high enough. Still couldn't get anything. I've also ordered a few more 6BQ6s just to verify it isn't the tube, though both of the ones I have tested NOS without any shorts. Just curious, what could possibly be loading down the voltage on the screen of the HV output? and what is the plate voltage of the 6BQ6 supposed to be? (Listed as do not measure)
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"If it isn't broke, you aren't trying hard enough" |
#23
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The plate of HO tube has high voltage spikes that can easily take a meter out. It's generally not recommended to try measuring there. |
#24
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I meant using a HV meter to measure it. Either way I'm still not sure why I'm not getting the proper voltage on the screen of the Horizontal output, is there anything I can do to isolate the problem?
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"If it isn't broke, you aren't trying hard enough" |
#25
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Measuring the DC at the HO plate, even with a high voltage probe, is useless.
If you have service info that tells what the retrace time should be, you can compare that to the actual pulse width, using a scope. Just bring the probe somewhat near the HO plate lead, but not close enough to touch or draw an arc, and view the capacitively coupled pulse. If the pulse width is notably different from nominal, this could affect the HV. This would lead to checking components around the flyback and yoke. |
Audiokarma |
#26
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7.5KV should be enough to get some kind of light on a healthy 12" CRT...Try pulling the Vert output tube and see if you can get a horizontal line on screen. Is the ion trap of the CRT adjusted correctly, and do the electron gun bias voltages look reasonable?
One thing to be wary of is that if the horizontal oscillator is not running very close to the correct frequency the HV can become low (yet all the DC voltages on the output tube can be about right). If you haven't adjusted the horizontal osc with a scope and a frequency reference (the sync pulses on the composite video output of your signal source is a good reference), and you have never got video on the screen then the osc is likely to be off freq a ways.
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#27
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Quote:
Sorry for all of the questions, still a newbie at this ~Zach
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"If it isn't broke, you aren't trying hard enough" |
#28
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Quote:
Last edited by Kevin Kuehn; 09-13-2018 at 08:53 PM. |
#29
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Alright, I guess it worked because I'm getting some sort of picture. It's not the greatest or clearest picture I've ever seen and actually, the voltage worked itself out. It's at around 9kv which is better than it was before. Here's a picture of it displaying the Indian head test pattern.
20180913_230010.jpg 20180913_231327.jpg I'm still pretty new to the TV category, what else needs to be done to improve the image? I'll center it completely once I move it back into the cabinet, and looking back, I think the focus could be adjusted to be way better.
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"If it isn't broke, you aren't trying hard enough" |
#30
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Now try running though this horizontal setup procedure from your Sams folder 114-13 page 10. Hopefully you can get the pattern a little less egg shaped. Just remember these things were never perfect even back in the day when folks used them for their daily drivers. Besides focus adjusting your fine tuning control may help clear up your image some too. https://c2.staticflickr.com/2/1881/4...9748b076_b.jpg Last edited by Kevin Kuehn; 09-13-2018 at 11:52 PM. |
Audiokarma |
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