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  #16  
Old 09-12-2018, 11:55 PM
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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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  #17  
Old 09-13-2018, 12:58 AM
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Quote:
Originally Posted by Zsuttle View Post
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?
I don't see how you can calculate a "should be" resistance value without knowing the nominal average screen grid current.
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?).
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Old 09-13-2018, 11:12 AM
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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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  #19  
Old 09-13-2018, 01:46 PM
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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  
Old 09-13-2018, 02:43 PM
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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.
Yeah, the Pot can go to zero and I had replaced the 4700 ohm resistor as it had gone bad.
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Old 09-13-2018, 04:09 PM
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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 04:12 PM.
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  #22  
Old 09-13-2018, 04:20 PM
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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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  #23  
Old 09-13-2018, 04:30 PM
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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)
Leakage current to ground internally in the CRT or possibly something interfering with correct bias on the grids.

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.
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Old 09-13-2018, 04:36 PM
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Leakage current to ground internally in the CRT or possibly something interfering with correct bias on the grids.

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.
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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  #25  
Old 09-13-2018, 05:39 PM
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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.
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  #26  
Old 09-13-2018, 06:05 PM
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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  
Old 09-13-2018, 07:01 PM
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Originally Posted by Electronic M View Post
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.
That explains a few things, the horizontal oscillator was at 15.35khz instead of 15.75khz listed on the service manual. This might be a stupid question but what exactly gets adjusted? Do I adjust the master oscillator? Or is there an individual adjustment for the horizontal oscillator?

Sorry for all of the questions, still a newbie at this
~Zach
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  #28  
Old 09-13-2018, 09:45 PM
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Originally Posted by Zsuttle View Post
That explains a few things, the horizontal oscillator was at 15.35khz instead of 15.75khz listed on the service manual. This might be a stupid question but what exactly gets adjusted? Do I adjust the master oscillator? Or is there an individual adjustment for the horizontal oscillator?

Sorry for all of the questions, still a newbie at this
~Zach
The only horizontal frequency adjustment you have on this set is the horizontal hold control. Can you see the horizontal frequency change when you move that control to its extremes? I agree with Electronic M that you should see a raster even with 7.5kv. I'd try rotating the Ion trap around the neck and also forward and back in a dimly lite room, in hope that a hint of light shows up. Then the idea is to get it to the position where the screen is at it's brightest. Once you see any light, turn down the brightness and keep trying to bring it back with movement of the trap. Since you've had the CRT out there's no telling how far it's been moved from it's former working location.

Last edited by Kevin Kuehn; 09-13-2018 at 09:53 PM.
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  #29  
Old 09-13-2018, 11:23 PM
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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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  #30  
Old 09-14-2018, 12:42 AM
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Originally Posted by Zsuttle View Post
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.
Attachment 197676
Attachment 197677
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.
Good job!

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; 09-14-2018 at 12:52 AM.
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