Horizontal Drive waveforms on my 1950 Zenith porthole

[Mike Stamper]

I'm confused as to why the high voltage fades out as I increase the horizontal drive setting. I took pictures of the waveforms at three different settings: minimum, halfway, and maximum rotation of the horizontal drive control. Here is the picture at minimum:

http://img02.picoodle.com/img/img02/...dm_600e144.jpg

Here it is at midpoint:

http://img01.picoodle.com/img/img01/...dm_ba3d48d.jpg

At at maximum:

http://img03.picoodle.com/img/img03/...dm_26c47b0.jpg

Notice that the sawtooth wave form becomes more and more distorted as the control is advanced. The voltage (at pin 5 of the 6BG6G where these pictures were taken) changes from 13VAC to 21VAC as the control is advanced to maximum. I don't think this is right. Can anyone tell me what SHOULD happen as the horizontal drive is advanced? Should the voltage go up? Should the wave form change? And finally, if this indicates a problem, where should I look? Thanks!

[Old1625]

Have you recapped that set? Checked the tubes?

It would seem that you might have a leaky capacitor, or perhaps a resistor or two that have drifted in value.

Does the HOT plate get red?

I laid my hands on a Sams for a slightly later model Zenith using a 6SN7 for horiz osc/disch and a 6BQ6 for HOT. THe waveform they show is a reasonably pure sawtooth, and not one where the top is clipped the way yours is.

Checking the operating voltages against design figures may uncover the culprit. The screen voltage at the HOT could be low. Also the plate resistor in the preceding stage could have changed in value. Also check the amount of developed negative voltage there is on the HOT control grid. The Sams I have shows -15 volts for control and 150V for screen. I feel fairly certain that your 6BG6 should operate at voltages not far from these....

[Mike Stamper]

Every cap was replaced, as well as every out-of-tolerance resistor. I'll take some voltage measurements and post them tomorrow. Thanks for commenting on my post.

[Old1625]

Does the raster show adequate width before the HV starts going down? Bear in mind that as the accelerating voltage goes down the raster size will lose brightness, focus--and become larger.

If your voltages all jibe then my gut is telling me that the HOT is played out, or that perhaps the flyback has issues....

[Mike Stamper]

Flyback is a brand new old stock FLY-51. 6BG6GA is NOS. Here are some voltage readings:

Horizontal discharge 6SN7: Pin 2 anode: expected +65v, actual +37v; Pin 1 grid: expected -33v, actual -30v

Horizontal output NOS 6BG6GA (tube plugged in, anode NOT connected to flyback): Pin 8 screen grid: expected 255v actual 267v; pin 5 control grid: -7v expected -27v actual; pin 3 cathode: +14v expected +2v actual

All caps replaced, and I have triple-checked accuracy. All resistors in that section replaced, again, have rechecked accuracy. The biggest variations seem to be the anode of the horizontal discharge, and the control grid of the horizontal output. Does all of this suggest anything? Thanks!

[Mike Stamper]

Re: "Bear in mind that as the accelerating voltage goes down the raster size will lose brightness, focus--and become larger." Yes, that is what happens as I turn the horizontal drive clockwise from start.

[Old1625]

The voltage readings on the HOT tend to point the finger at excessive drive. This would jibe with the waveforms you show the more I think about it. The clipping at the top suggests diode action of the control grid caused by its being driven well into conduction. There is some of this happening to a degree in normal operation, as that is how the HOT develops the negative bias that keeps the cathode current in check. In cases of horizontal oscillator failure the HOT would melt down completely. But it would seem that the reverse is happening here, as your cathode voltage reading and the control grid negative reading reflects little current flow, as if the tube is conducting only during a very small part of the waveform cycle. If that be the case then one would think that the width of the raster would be affected, and that one side of the raster would actually show a brighter edge, reflecting the beam lingering there during the HOT's cutoff. Just the same such condition would cause lower output from the HOT, as it is only conducting for short bursts. Attempts to further increase drive will only make matters worse.

It seems to me that you need to be going in the other direction, if possible.

Do you actually have a video program source or a test pattern in your testing? Or are you just working with a white raster or snow?

[Mike Stamper]

Tonight I will input a test pattern and re-do the voltage measurements. Is the low voltage on pin 2 of the 6sn7 a problem? Should I reduce the value of the 560k plate resistor to increase the voltage? Would it be helpful if I sent the schematic? If so, can you send me a personal message with your email address? Thanks!

[Old1625]

PM'd you.

That 560K does seem kinda high for plate resistor. The schematic I have is for a Zenith J2027, which has a different HOT but uses a 6SN7 that employs triode on pins 4,5 &6 as a Hartley osc and pins 1,2 &3 triode driving the HOT. It receives a B+ of 310V through a plate resistor of 150K. The plate op voltage is 50. There is a little difference, so seeing your schematic will be helpful. Lowering the value of the plate resistor may bring your gain down, but will also help with loading issues involving the network at the HOT control grid. This may bring things to where they belong, restoring your HT. That yours calls for a 560K is a head scratcher.

[sean]

Mike,

Here are photos of the waveforms I get on my set. These were taken at the grid of the 6BG6:

Drive full CCW

Drive midpoint

Drive full CW

Zoom in of the waveform

The vertical scale is 20V/div. With the drive pot in full CCW I also measured -5Vdc on the 6BG6 grid and 34Vdc on the Hor. discharge plate, full CW I got -7.4Vdc on the 6BG6 grid and 66Vdc on the discharge plate.

And here is a schematic of this set: Zenith 23G24

[Old1625]

Many thanks for the input, Sean! The clipping on top I'm guessing is where the HOT ctrl grid is being driven into conduction. There's something here we're not seeing.

I've got the scams from him, and I'm giving it some thought. Lately I've been out straight, but let's see what we can do.

Thanks again for your input!

[Old1625]

Many thanks for the input, Sean! The clipping on top I'm guessing is where the HOT ctrl grid is being driven into conduction. There's something here we're not seeing. If the HOT gets insufficient drive there is no HT. And the HOT goes to pieces with all that B+ and no developed grid bias. At the other extreme excessive drive can cause the output tube to be biased stone-cold, and thus not deliver the necessary output to the flyback in the doghouse--again: Insufficient HV. Just my humble theory...

This is my current thought on this matter: He's got a resistance value for plate res on his disch stage than I see called for in my scam of a similar model that shares a lot of the same stuff. Are we dealing p'raps with a typo here...? I'm still puzzling over the call for a 560K resistor where I am not used to seeing such plate res so high. And the scam I've been working from for a similar model shows a value of res of 150K. If he changes to that value I think his problems will be over. IMHO....

I've got the scams from him, and I'm giving it some thought. Lately I've been out straight, but let's see what we can do. Man! It has been so long since I've enjoyed the glory of troubleshooting an old set!

Thanks again for your input!

[sean]

The 560k value for the plate resistor is correct. That is what was originally in my set. I also had problems with the HV being low like Mike. This was due to low HOT drive caused by the 560k resistor having doubled in value.

[Mike Stamper]

Here are some fresh voltage readings. All tubes connected, including the HOT plate:

Pin 2 V14/6sn7 (plate of horizontal discharge; 65v expected):
-Drive at minimum: 34v
-Drive at midpoint: 48v
-Drive at maximum: 62v

Pin 3 of 6bg6 (14v expected):
-Drive at minimum: 13v
-Drive at midpoint: 14v
-Drive at maximum: 15.4v

Pin 5 of 6bg6 (-7v expected):
-Drive at minimum: -0.9v
-Drive at midpoint: -0.7v
-Drive at maximum: -0.5v

Pin 8 of 6bg6 (255v expected):
-Drive at minimum: 289v
-Drive at midpoint: 282v
-Drive at maximum: 275v

The above readings were made with no signal. Introducing a test pattern made little difference. Changing the brightness made no difference. HV output (as measured at the HT lead) was about 7,500 volts (10,000 expected). I replaced the 560k resistor with another 560K. No change. All caps replaced. All out of tolerance resistors replaced. Flyback is a new FLY-51. 6bg6g is new. Does any of this help? Thanks.

[Old1625]

Did your recapping schedule include the one across the HT?