Working on an Admiral 20X12
 

Just finished a recap of an Admiral 20X12. I'll start by saying that the chassis on this thing was REALLY rusty. It sat on a basement floor for God knows how many decades. The only reason I decided to tackle it is, well, I felt sorry for it. You know how it goes :)

Anyway, the Sams for this set leaves a lot to be desired, and here's what I've got.....

No reception, no static, no HV. The horizontal oscillator isn't running, but the vertical is. Okay, so time to start testing resistances. My problem is on pin 4 of the 6BG6. Totally open. It seems that pin 4 is simply a tie point going from the power switch to the transformer.

I checked resistances at the 5U4, and they're in the ballpark, but the moment I put my probe on the tube pin, the resistance will start climbing and climbing until I remove the probe, when the resistance starts to decrease, eventually back to the correct value.

I may be wrong, but I'm starting to put two and two together and think that I may have a bad winding in the transformer. The only other thing that is shown in the Sams that is NOT on the set is a 270K Ohm resistor between the AC line and the transformer.

Whatcha think?

It's possible the 5U4 pin 4 is broken between the pin socket and solder tab. Are you measuring from the top or bottom of chassis? First thing Id do is pull the 5U4 and measure all your power transformer AC secondary voltages referenced to ground, but from the bottom side of the chassis.

I measured topside with an octal test adapter, then confirmed the readings underneath.

I figured that the puzzling resistance measurements had to be an indicator of something which is way off base, but yep, gonna have to start mapping out the B+.

I just realized this is probably like a 20Z1 chassis where the 5U4 sits on top the power transformer cover?

Ayup, it is indeed.

With the 5U4 pulled are you getting around 370vac from pins 4 and 6, ref to chassis ground?

And about 5.3vac across pins 2 and 8?

Does the centertap of the 5U4 HV winding go directly to ground, or does it go thru a resistor chain to ground? If the latter, possibly an open resistor in the chain.
(Maybe post the schematic so everybody can have a look see?)

At any rate I don't understand how your vertical oscillator could be running if there were something wrong with your transformers high voltage winding.

You can verify your supplies by checking dc voltages on pins 1,6,7 of V11. Also pin 5 on V14

Yes the high voltage center tap goes to ground.

Schematic here.
http://www.earlytelevision.org/pdf/A...Sams_100-1.pdf

Neither can I, but I can hear it faintly running and changing in frequency with the control.

It is possible that climbing resistance is your meter's resistance test charging up a capacitor.

You're right. I guess I too often assume that people have that understanding when they're measuring across caps in circuit. And more than likely Sams assumes that you're measuring with something like a Simson 260, which charges caps faster than a modern DVM.

Roger on both counts.

Sounds like the power transformer is fine. How about voltages on that V11 audio output tube? Have any hum/ buzz when you touch it's grid, or the center tap of the volume control?

Well, interestingly enough, I now have static in the sound. No reception (no matter how I turned the contrast), but definitely static.

The 6AS5 is used as a B+ dropper in lieu of a big fat resistor. There should be well over a hundred volts at the cathode (pin 1).
The tube voltage chart says 0V. on pin 1, which is flat out wrong. Typical Sams screwup.

What's the voltage on the plate (pin 7) and screen grid (pin 6)? The chart voltages look 'waay too low here, too.

Sams has a note that shows pins 1,2,5,6,7 are all measured from pin 1 of V11. Which is why they show 0 volts on pin 1. Also why the the other voltages seem low. I'm not sure why they don't want to reference ground on those measurements.

FWIW, have you checked the 1/4 amp fuse in the fly cage?

I have a copy of what I think is a factory schematic, and they show the following for the 6AS5:

Pin 1 135v
Pin 2-5 120v
Pin 6 270v
pin 7 260v

Sound more reasonable?

First thing I did. I'll scope out the 6AS5 in the morning.

Hyup. I shoulda reddit closer.

:deal:

Everyone's starting to talk like Sandy around here. :D

And WHAT'S wrong w/THAT ?!? (grin) Tim, sorry yr having such TROUBLE w/this Bad Boy... I always thought these Admirals were sposed to be "Easy-Peasy" sets. Guess I was Wrong. By the Bye, I'm told that I DON'T have too bad of an accent. I DID "Cheat" a bit, & cranked it up to 11, when I went up to meet LA's family in Cape Cod.. THEY were Damyankees from the very Heart of Damyankeeland. I was rather scared they would chew me up, & spit me out..Surprise-They were LOVELY people, & couldn't have been any nicer. But I DID drop an awful lot of "Yes, Ma'ams/No Ma'ams/Yes, Sirs/No Sirs" just in case. 2 of LA's aunts/cousins said you NEVER hear that up there any more.. I told 'em that I DIDN'T want the ghosts of one or BOTH of my Southern grannies rising from the grave to jerk a Knot in my tail because I'd forgotten how to treat a lady.. They grinned at that... Little did they know I was about half SERIOUS...

Assuming Pin 2-5 means 2 and 5, not 2 through 5 :) , everything is spot on.

Once you get the Uv running if you get no reception check the rf chokes. Ive had a couple admirals and a choke was open. The fly is ok?

Correct

Lets check your dc voltage across C1, section C(20 MFD) - just to make sure your focus coil isn't open. Whatever that measures, you should have the same voltage on either side of the 1/4 amp fuse in the fly cage.

Yep, we're looking good there. What's next?

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?

Measuring 438V on C1 and on pin 5 of the 6SN7. Pin 2, I got nuthin. Ahaaaa.....that gives me some direction.

Did you mean 338v on C1 sec C? 438 seems too high based on your other measurements.

Positive. C1C is at 438V.

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.

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

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

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.

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

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.

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.

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

Apparently it regulates well too.

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!! :)