Question about how high voltage works

[Aperna1985]

Hey guys a while back I was working on an RCA black and white set, and for whatever reason my high voltage was always weak I only got about 3000v. Eventually I got frustrated and put it to the side. My question is how do you get the high voltage. I'm having a hard time explaining my question but basically I know there's a series of stages that create the high voltage. Does the vertical have to be running to get a horizontal output to work? Or is it the horizontal output that has to be running to get the vertical to work. Looking at the schematic it seems like I need the horizontal oscillator running to get the negative voltage to turn on the horizontal output tube, which powers both the focus rectifier and the power for the flyback, which feeds the high voltage rectifier and I should get 14.25kv. I've ohmed out the fly back and it's good, I've also completely recapped the set, all the tubes test good. So I'm trying to understand the circuit a little better so I can find my problem.

I can't make the Sam's PDF small enough that I can upload it on here so here's a link to the file on my Google Drive. Page 4 has the schematic.

https://drive.google.com/file/d/1Wse...w?usp=drivesdk

[timmy]

Horizontal is needed for hv check the horizontal output tube also the hv rectifier tube and maybe double check your work or maybe a defective cap in the horizontal section new don’t always mean good. Do you get a picture even with 3 kv.what kind of set is it.

[old_tv_nut]

As Timmy says, describe the symptoms more completely please. Can you measure some voltages in the horizontal oscillator section? Do you have any raster at all? Even better, do you have a scope to look at some of the waveforms? Partial operation may be harder to diagnose than full failure.

As far as the way it's supposed to work, development of high voltage is not related to vertical. It is generated by the horizontal retrace pulse acting through the flyback transformer. (In some sets, there is a "boost voltage" of a few hundred volts generated by the horizontal sweep that may supply the vertical sweep circuit, but that doesn't appear to be the case in this set.)

[Electronic M]

Can you tell if you have horizontal sync or at least that it's close? If horizontal is badly off frequency it may not be reaching the resonance point of the flyback needed to create proper HV levels. Another thing is it looks like there is a doorknob HV filter cap in that set... Sometimes those leak and or short dragging HV down.

Some sets use boost voltage derived from the flyback to power the vertical and other circuits, on those sets if one of the driven circuits has failed in a way that loads the boost line (and thus the flyback) excessively that can suppress the HV.

[Yamamaya42]

Sort of right, when the horizontal oscillator starts, the horizontal output will go negative, and HV should start, if it does not, a dangerous state can happen and a free run overload with the output and it may start to red-plate and self destruct. Thus the need for the fuse M1 for protection.

The dependency on the vertical sweep to start after horizontal start depends on how the set is made, and the above mentioned boost voltage needed for vertical, which as mentioned this set does not seem to have, but it does seem to have a sync link via C120 R118 C4 from the plate of the horizontal output to the cathode of the vertical output.
So it looks like they are independent of each other, but if there was a problem in that above area, the vertical part linked to the horizontal might cause a problem?

But it more likely what EM mentioned, horizontal may be badly off, or even the waveform, meaning a problem with L38 B3 B1.or even B4.. YOU SUNK MY BATTLESHIP!!! :P

[Aperna1985]

Thank you guys for all the info. I will try and get some measurements. I'm starting to think what Yamamaya42 said is happening. I've blown the fuse M1 twice. The TV doesn't produce a raster at all. This TV has a phono input, and a position to switch from TV to Phono. What's interesting is if I flip between audio and TV quickly or if I turn the set off I'll get a solid line similar to a tv with a collapsed vertical for a split second.

To answer the other questions, I considered the Door Knob cap but my Heathkit C3 doesn't generate voltage high enough to check for leakage.

I have a Tektronix 531 scope, I literally just got it working, I just don't know how to use it well enough. Can I blow it up using it on certain voltages, like you could with a meter? Also it came with a 10x probe will that be good enough.


The TV is a RCA 7T11B with a KCS 47-GF2 chassis.

[zeno]

You need horz osc, horz out, damper, HV rect & focus rect.
Blowing fuse probably from too much cathode current on the
hoz out. That will be either bad drive or a bad load. Start
at the hoz out G-1 to look for neg drive V. Keep in mind the horz
out tube is probably damaged !
You can also pull the caps off the focus & HV tubes, & see if you
can pull a healthy arc from the flyback into the air.
FBT's & yokes will short most the time but ohm out OK.An anylist
with a ringer is the best test unless they are burning !
Look on scope probes for max DC rating.

73 Zeno
LFOD !

[Electronic M]

If you haven't compared the negative voltage at G1 (control grid) of the H output tube to the value listed in the schematic then that should be your first mission. Screen voltage on that tube is also informative if G1 is right. Screen too high, something is shorted in the flyback, yoke, HV damper...Too low screen and something is open in those stages.

There's no good equipment to test a doorknob cap other than the TV. What I do is disconnect one side of the doorknob and stick it in a small glass bottle (a shot glass would also work) so it can't arc. And run the set and see if I get more HV. If the set has a glass CRT with dag you don't need the cap, and if it doesn't you can sub another HV cap or try it without it (HV may vary across the screen but it should somewhat function).