Working on an Admiral 20X12

[old_coot88]

The B+ is 'waay too high, suggesting there's insufficient load to pull it down to spec. Likely the H.output stage is not conducting.
There's B+ on both sides of the fuse, right?
Since The damper is in series in the B+ pathway to the H.out stage, an open damper would cause your problem. Have you subbed the damper?

If so, with the set off, check for continuity from the fuse to pin 5 (plate) of the damper. Should be about 11 ohms.
Then check from pin 3 (cathode) of the damper to the plate of the H.out tube. Should be around 165 ohms.

[Kamakiri]

Originally Posted by Kevin Kuehn

So what voltage did you measure on C1, section C? What dc voltages do you measure on pins 2 and 5 of the 6SN7 horizontal oscillator tube?

First, I screwed up here and somehow measured voltages on the correct cap, but the wrong tube

Let's revise this one. Pin 2 has -30VDC, pin 5 has nothing.

[Kamakiri]

Originally Posted by old_coot88

The B+ is 'waay too high, suggesting there's insufficient load to pull it down to spec. Likely the H.output stage is not conducting.
There's B+ on both sides of the fuse, right?
Since The damper is in series in the B+ pathway to the H.out stage, an open damper would cause your problem. Have you subbed the damper?

If so, with the set off, check for continuity from the fuse to pin 5 (plate) of the damper. Should be about 11 ohms.
Then check from pin 3 (cathode) of the damper to the plate of the H.out tube. Should be around 165 ohms.

B+ on both sides of the fuse, yes. Subbed the damper, both tubes tested good (both original and replacement), no changes.

continuity from the fuse to pin 5 (plate) of the damper. Should be about 11 ohms. Spot on.

Then check from pin 3 (cathode) of the damper to the plate of the H.out tube. Should be around 165 ohms. Zero.

[Kevin Kuehn]

Originally Posted by Kamakiri

Then check from pin 3 (cathode) of the damper to the plate of the H.out tube. Should be around 165 ohms. Zero.

Zero is very bad. Sounds like your flyback may have a shorted primary, although it seems unlikely that it would form a dead short? Possibly there's a wiring error? Try measuring directly across pins 1 and 2 of the fly transformer, there should be like 185 ohms.

Also you need to find out why there's no B+ getting to pin 5 of the horz osc tube. You should measure something like 75,070 ohms from the fuse to the 6SN7plate(pin 5).

Here's the horizontal section from my Admiral schematic. Easier to follow than the Sams, plus it has the voltages and resistances marked on it.

http://i452.photobucket.com/albums/q...pstvpvazop.jpg

[old_coot88]

Originally Posted by Kevin Kuehn

Zero is very bad. Sounds like your flyback may have a shorted primary, although it seems unlikely that it would form a dead short?

By "zero" he probably means 'open'. There's also the H.linearity coil in series between the damper cathode and fly primary. The H.lin coil could be open.

[Kamakiri]

Originally Posted by old_coot88

By "zero" he probably means 'open'. There's also the H.linearity coil in series between the damper cathode and fly primary. The H.lin coil could be open.

You nailed that one. Sams shows it should read 36 ohms. Totally open. I think I have one from a parts chassis I can scrounge later.

[wa2ise]

Originally Posted by old_coot88

The 6AS5 is used as a B+ dropper in lieu of a big fat resistor.

It's a sensible way to drop the B+, putting the otherwise wasted power to work. Cuts waste heat, and lessens the load on the power supply.

[Kevin Kuehn]

Originally Posted by old_coot88

By "zero" he probably means 'open'. There's also the H.linearity coil in series between the damper cathode and fly primary. The H.lin coil could be open.

Oops, my bad. I always think infinity, but I should have read it in context of too high of B+.

[Kevin Kuehn]

Originally Posted by wa2ise

It's a sensible way to drop the B+, putting the otherwise wasted power to work. Cuts waste heat, and lessens the load on the power supply.

Apparently it regulates well too.

[Kamakiri]

Well, I nabbed a linearity coil out of a GE 810 chassis that I had here. It read 30 ohms versus the 36 in the Sams, but I figured that would get me a lot closer than an open coil would.

Sure enough, the horizontal oscillator started to run, and I got nice HV to the plate cap of the 1X2

No HV to the picture tube yet, but all that's left in that chain is a 1 meg resistor and a door knob cap.....both of which I should have in stock. Time for some more testing!!

[old_coot88]

Good goin' bro. Still kinda curious why there was no plate voltage on the H.osc before.Maybe there was a production change so it runs offa Boost instead of B+ (?)

[Kevin Kuehn]

Equally curious how the vertical Osc. was running before, with no boost.

[Kamakiri]

Not sure, but I could hear it, and the frequency did change with the vertical hold.

Doorknob cap is fine, as is the 470 MOhm resistor on the 1X2 (Sams shows it to be a 1 Meg, but....anyway). Next, I'll clean the snot out of the tube pins and tube socket.

[Kevin Kuehn]

Originally Posted by Kamakiri

Not sure, but I could hear it, and the frequency did change with the vertical hold.

I guess Sams does show that some version had the V Osc. running off of B+. They show the alternative path with a dotted line.

Originally Posted by Kamakiri

Doorknob cap is fine, as is the 470 MOhm resistor on the 1X2 (Sams shows it to be a 1 Meg, but....anyway). Next, I'll clean the snot out of the tube pins and tube socket.

470 M, as in mega ohms? Or is that the really really ancient abbreviation for one thousand?

[Kamakiri]

470K. I typed it too fast

But hey! I gots a raster!!!!

I can barely get any sound at all on frequency and it looks like the IF's gonna need work, but this is a major leap forward, thanks to you guys!!!

Now, next question....source an Admiral replacement for that linearity coil, I assume?