View Full Version : High Voltage problem


VA561
03-04-2017, 04:40 PM
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.

teevee
03-04-2017, 05:10 PM
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.

N2IXK
03-04-2017, 05:10 PM
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.

Findm-Keepm
03-04-2017, 05:15 PM
.....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.

andy
03-04-2017, 06:37 PM
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.

VA561
03-04-2017, 07:10 PM
[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.

andy
03-04-2017, 07:23 PM
[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.

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.

VA561
03-04-2017, 07:45 PM
Just ordered n.o.s. 6BK4A-B and 3AT2 tubes. Is it ok to use a 6BK4A-B in place of the 6BK4?

N2IXK
03-04-2017, 07:51 PM
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.

VA561
03-04-2017, 08:03 PM
Tried pulling the crt cap off as suggested by Andy. Voltage at the crt anode with cap off, 25K.

timmy
03-04-2017, 08:24 PM
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.

miniman82
03-04-2017, 09:43 PM
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.

VA561
03-04-2017, 11:04 PM
Nick don't you miss it here in Norfolk? You could drop by here and perform surgery on the Zenith!

Username1
03-05-2017, 06:25 AM
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.

.

VA561
03-05-2017, 10:06 AM
What does it mean when I am getting a full 25Kv at the crt anode when I unplug the crt cap first?

Username1
03-05-2017, 10:23 AM
To be more clear: Take the cap off the 6BK4, then run the set.
This will take the shunt regulator out of the circuit.
By definition, the shunt regulator puts an additional load
on a power supply of limited current availability. It relies on
additional loading to provide voltage regulation.

Think of the HV supply as providing enough voltage and current
to run the picture tube. When the picture gets dark, beam current
would go down, reducing load on the HV supply. Voltage would
naturally go up. The 6BK4 should then conduct more, and load the
HV supply source, and bring down the voltage level.

Removing the HV lead, or plug at the end of the tube takes out the
main current user of the HV supply, HV should go up as it has a very
limited load at this time.

Normally, the set should supply 20 - 25KV to your set with a mid brightness
picture on the screen. You have to determine if the supply has a problem
or if there is a problem with the tube, or the reg.

It is fairly easy to disable the regulator. Then move on from there.

Your tv sure looks like it's producing a nice bright picture.


.

miniman82
03-05-2017, 02:53 PM
Nick don't you miss it here in Norfolk? You could drop by here and perform surgery on the Zenith!

I tried to stay Larry, but they wouldn't let me! lol

Check the value of the resistors around the 6BK4 grid, I'm sure you'll find that one of them has drifted out of spec. Could also be the HV pot itself, use your DMM. Once you've replaced the bad parts, you should be able to set HV correctly with the pot.

dieseljeep
03-05-2017, 03:08 PM
Just ordered n.o.s. 6BK4A-B and 3AT2 tubes. Is it ok to use a 6BK4A-B in place of the 6BK4?

Did you remove the HV rect tube socket and check the resistor in there?
Is it jumpered out, like it was recommended in another thread. :scratch2:

VA561
03-05-2017, 05:44 PM
Going to tackle it next sat. Excellent advice from everyone. Much appreciated. I want to keep this set for a long time so I want to get it fixed.

zeno
03-06-2017, 10:28 AM
OK this was missed by all. If you turn the brite down on any
set the pix should go black or nearly so. Fix this first as it causes
an abnormal load on the HV. Start with a by the book set-up especially
the G-2's & bright limit. Could be just someone set them too
high to get a brighter pix. If not adjustments move to the
vid amp & blanking tubes. I remember NO scenario where the
HV will cause too much brite........
Normally I would start at the sweep then move on BUT the video
is so far off it could be the root of the problem.

73 Zeno:smoke:
LFOD !

[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.

VA561
03-06-2017, 10:25 PM
Zeno You nailed it! I had previously mentioned that I couldn't get a dark screen. Not even close and the best I could get was 16Kv. As per your suggestion, After turning the brightness knob down as far as it would go, I adjusted the three G2 controls and got a dark screen and most importantly........25Kv! I then brought up the G2 knobs up slightly to where I thought I had a decent picture. I guess the the fact that I couldn't get a dark screen caused you to consider the G2 settings. Thank you so much. I now have a full screen and a pretty good picture. After a convergence is done, that should do it.