Horizontal Drive waveforms on my 1950 Zenith porthole

[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?