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Old 03-04-2017, 03:40 PM
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High Voltage problem

This thread relates to the previous vertical centering thread. A'64 zenith console, chassis 25LC20QS. The hv should be 25K. With the brightness all the way down, I am getting 16K at the crt anode. When I turn the brightness up to a normal viewing level it drops down to 12K. The voltage regulator is a 6BK4 and the h.v. rectifier is a 3AT2. I am not able to check either tube on my tube tester. The hv adjustment on the back of the chassis is turned up as far as it will go. I currently have about a 1/2" gap at the bottom of the screen. When the brightness control is turned up to a certain point, the picture gets out of focus. Advice needed as to what steps need to be taken and in what order. Thanks in advance.
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Old 03-04-2017, 04:10 PM
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The oldtimers would recommend checking the cathode current on the 6BK4, there may be provision to do this. I have seen the recommendation of taking the plate connection off of the 6BK4 and (briefly) trying the set to see if the HV comes up. Bias on the regulator tube should be about 15V cathode to grid.
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Old 03-04-2017, 04:10 PM
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A few checks that you can make to eliminate the most common causes of low HV:

Check the horizontal output, damper, and oscillator tubes.

Check the HV rectifier and regulator tubes by substitution.

Verify that you have proper drive to the HO tube grid, proper screen voltage, and proper cathode current.

Check the DC resistance of the flyback HV winding.

Look for leaky caps, deteriorated coils or off-value resistors in the horizontal sweep stages.
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Old 03-04-2017, 04:15 PM
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.....and check the B+ sources to the Horiz oscillator, and to the fly. So many times it's not a tube, but the power supply to the tube....

The 1K ohm resistor in the cathode circuit of the 6BK4 will allow you to measure the regulator current - Sams has the procedure.
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Old 03-04-2017, 05:37 PM
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Does the HV adjustment have any effect? There are two general causes of low HV. Either the flyback isn't outputting enough voltage (a bad HV rectifier could also cause this), or it's being loaded down by the regulator.

The cathode current of the 6BK4 will tell you which is the problem. If it's low, it's not producing enough HV. If it's high, it's a problem with the regulator circuit. Normally, it should be about 1mA on a black screen and will increase as you turn down the HV.
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Old 03-04-2017, 06:10 PM
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[QUOTE=andy;3179849]Does the HV adjustment have any effect? There are two general causes of low HV. Either the flyback isn't outputting enough voltage (a bad HV rectifier could also cause this), or it's being loaded down by the regulator.

The HV adjustment has little to no effect. A strange thing, I can't get the screen to go black. The picture I posted of a blank screen was with the brightness turned all the way down.
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Old 03-04-2017, 06:23 PM
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[QUOTE=VA561;3179850]
Quote:
Originally Posted by andy View Post
Does the HV adjustment have any effect? There are two general causes of low HV. Either the flyback isn't outputting enough voltage (a bad HV rectifier could also cause this), or it's being loaded down by the regulator.

The HV adjustment has little to no effect. A strange thing, I can't get the screen to go black. The picture I posted of a blank screen was with the brightness turned all the way down.
What about if you turn the screen controls all the way down? For this test, unplugging the CRT socket will work. It will make sure the CRT isn't drawing any HV current.

If the raster is very bright, the CRT could be pulling down the HV. It doesn't look that bright, but it's hard to tell from a picture.
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Old 03-04-2017, 06:45 PM
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Just ordered n.o.s. 6BK4A-B and 3AT2 tubes. Is it ok to use a 6BK4A-B in place of the 6BK4?
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Old 03-04-2017, 06:51 PM
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Quote:
Originally Posted by VA561 View Post
Just ordered n.o.s. 6BK4A-B and 3AT2 tubes. Is it ok to use a 6BK4A-B in place of the 6BK4?
Yes, but the 6BK4C is the preferred type, with the best X-ray shielding.
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Old 03-04-2017, 07:03 PM
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Tried pulling the crt cap off as suggested by Andy. Voltage at the crt anode with cap off, 25K.
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Old 03-04-2017, 07:24 PM
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Also check the 2- I think they are 1.5 meg resistors off the hv adjust pot they should be matched or at least close. I had the same problem and it wound up being a dogbone cap in the IF strip that had carbonized and rendered the 6bk4 tube useless because the heater for the hv reg tube and the crt and the tube where the bad cap was was bleeding in because it was shorted and the 6bk4 would not regulate. Took along time to find this unusual problem.
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Old 03-04-2017, 08:43 PM
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You have 6BK4 grid issues, probably caused by the bleeder resistors like Timmy said. My Silvertone had a similar issue, the anode got dragged all the way down to 9kv.
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Old 03-04-2017, 10:04 PM
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Smile

Nick don't you miss it here in Norfolk? You could drop by here and perform surgery on the Zenith!
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Old 03-05-2017, 05:25 AM
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before you replace stuff, remove the 6BK4, run the set, if HV
goes up, the problem is in the HV Reg. circuit. And most
likely not the tube....

You might have to just remove the cap off the tube.
Be sure to keep it away from arc to ground
potential.

.
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Last edited by Username1; 03-05-2017 at 06:19 AM.
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Old 03-05-2017, 09:06 AM
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What does it mean when I am getting a full 25Kv at the crt anode when I unplug the crt cap first?
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