Issue with Sparton Dexter Model 5014

[Chris K]

Hoping the experts here can point me to something obvious I might be missing. After a recap, the TV was working but there were some issues with the horizontal circuit resulting in a distorted picture. I went through the 5% resistors in and around the horizontal circuit and replaced about 4-5 that were out of spec. The next powerup, the original 0.25 amp 250V fuse in the schematic below blew open. I replaced it with a new slow blow fuse of the same value. Next powerup, the horizontal output cathode 68ohm (R90) resistor to ground physically blew apart. The voltage on the cathode pin 8 is supposed to be 5V DC. It was over 200V when I checked it. I went through the circuits from top to bottom and I can't find a thing wrong with how the circuit is wired, the resistors I replaced and the wiring of the replaced electrolytic caps. This is the very start of the B+ circuitry and I'm thinking I'm missing something obvious. The speaker voice coil is good, the 5U4 is hooked up correctly, the focus coil and the focus control are fine, the power transformer ohms out as stated etc. Are there some things in the flyback I should be concerned with? Anyway, excessive voltage on the horizontal output cathode I guess is the issue. Any thoughts?

[bandersen]

Horizontal oscillator probably stopped running. Should always have a negative voltage on the grid (pin 5).

[Chris K]

Originally Posted by bandersen

Horizontal oscillator probably stopped running. Should always have a negative voltage on the grid (pin 5).

I'll check it when I get home. Thanks Bob. Looks like it's the half of the 6SN7GT, V15 that's just outside the schematic snip. I put new tubes in it but I suppose something may have happened to it.

[Chris K]

Is this what Shango talks about when he brings his TVs up on a dim bulb and pulls the horizontal output tube? I remember him saying it could be damaged if you are too low in voltage to start the oscillator.

[Yamamaya42]

Yeah you can damage the horizontal output tube w/o the oscillator running, but it takes some time, normally you see some signs of distress in the output before any REAL harm is done like red plating, in other words, it just won't go FOOM in 20 seconds or so!

[Chris K]

Is there something other than a faulty tube or low voltage that would keep the 6SN7GT multi-vibrator from oscillating? What is the function of the other half of the tube...horizontal discharge?

[Chris K]

Here is the rest of the circuit

[old_tv_nut]

I believe the multivibrator circuit consists of both halves of the 6SN7GT. They just named the 2nd half for the function it has in the multivibrator circuit to differentiate it from the first half.

here's a page about it:
https://mysite.du.edu/~etuttle/electron/elect36.htm

[Yamamaya42]

From what I understand, in these early sets, “horizontal discharge” is the stage to take the signal from the horizontal oscillator and shape it into the correct sawtooth wave for the output, they found better ways to do this in later years with out the need of a tube stage using just capacitive coupling, and resistors. ref -RCA630TSService manual

[Chris K]

Originally Posted by Yamamaya42

From what I understand, in these early sets, “horizontal discharge” is the stage to take the signal from the horizontal oscillator and shape it into the correct sawtooth wave for the output, they found better ways to do this in later years with out the need of a tube stage using just capacitive coupling, and resistors. ref -RCA630TSService manual

Very interesting. I'm amazed at the knowledge and experience available here and I'm so grateful for the opportunity to learn and grow. I thank you all so much!