Working on an Admiral 20X12

[Kamakiri]

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?

[Kevin Kuehn]

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.

[Kamakiri]

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+.

[Kevin Kuehn]

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

[Kamakiri]

Ayup, it is indeed.

[Kevin Kuehn]

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?

[old_coot88]

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?)

[Kevin Kuehn]

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.

[Kevin Kuehn]

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

[Kamakiri]

Originally Posted by Kevin Kuehn

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.

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

[Electronic M]

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

[Kevin Kuehn]

Originally Posted by Electronic M

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.

[Kamakiri]

Originally Posted by Kevin Kuehn

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?

Roger on both counts.

[Kevin Kuehn]

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?

[Kamakiri]

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