Admiral 20B1

[Adam]

This is the last to restore of 2 carloads of sets I picked up from captainmoody in the past year or so. Actually now I have all but a few of my sets working. My ratio is something like 25/30 restored and working.

Anyway, the Admiral came up with picture and buzzing, but no sound after just a recap and replacement of bad tubes. The only unusual problem was a broken 5U4 socket. I turned the discriminator transformer until sound came up with minimal buzzing. I'll go through the proper sound IF alignment tomorrow.

For a while I couldn't figure out why I had no picture on the right edge of the screen. I tried moving the ion trap and the focusing coil, but I couldn't quite get it. It turned out the whole assembly with the yoke/focus coil was actually a little off center.

In many scenes the retrace lines become visible, I might also try and add some retrace blanking to this.

I also have to clean some minor rust spots off the top of the chassis, and do some minor touch up to the cabinet. I have the knobs, but I'm missing the back, like most of these sets seem to be.

[Adam]

I got the sound all fixed up, left it on for a few minutes, and then ... no more HV

[bandersen]

Bummer Did the 250mA fuse inside the HV cage blow ?

[Adam]

I lost the damper tube and that big 25w 8.5k damper resistor went open. I got plenty of 6W4s, but anyone know where I can get one of those resistors?

[bandersen]

I need one for a 20A1 chassis. I found a 7500 50W from Mouser that's almost the same size as the original.

http://www.mouser.com/Search/Product...ey588-L50J7K5E

Nuts, I see you need an 8500 for the 20B1 chassis. I checked Mouse, Digi-key, Surplus Sales of Nebraska and Allied. Nobody has 8500 Maybe you can string together a couple in series ?

[Adam]

Well I got an 8.5k 25w resistor, but it's not that nice chassis mount tubular type, but it works. Also replaced a weak AGC tube and messed about a bit with retrace blanking. Now I just have to clean the chassis and cabinet a bit more, then on to the next set...

New pics...

[Phil Nelson]

Nice. That was pretty fast work.

You could use that first screen for adjusting screen geometry -- built-in crosshatch pattern!

Phil Nelson

[bandersen]

Great job! What modification did you make for the retrace blanking ? I have to keep the brightness a bit lower than I'd like on my 20A1 chassis or they appear.

[Adam]

SPAM

[Adam]

For the retrace, I connected a .047mfd cap to the output of the VOT, followed by a 39Kohm resistor connected to the CRT cathode.

Now a 27k resistor blanks the retrace completely - even at high brightness low contrast, but it lowers the contrast overall a bit. 47k doesn't really blank the retrace at all. 33 or 39k don't completely blank the retrace in all circumstances, but don't affect the picture so much.

I also tried a .01 and .1mfd capacitor, didn't seem to make much difference. I suppose I could try a .001 or a 100pf and see what happens.

Those pictures above were taken with the .047mfd and 39Kohm as described.

[bandersen]

Thanks. I just happen to have mine on the bench and will give that a try real soon.

[bandersen]

I tried connection a 0.047 to the plate of the 6K6 VOT then a 39K to the CRT cathode. It really killed the vertical output - the display lost half it's height!
I did some experimenting and found 0.0022 and 1M did a pretty good job suppressing the retrace but I got some twisting at the top of the picture and hum in the audio.

[Adam]

I was connecting the cap to the yoke side of the VOT, rather than the plate of the 6K6.

[bandersen]

OK, I was wondering about that. I'll give it a shot.

[wa2ise]

Originally Posted by bandersen

I did some experimenting and found 0.0022 and 1M did a pretty good job suppressing the retrace but I got some twisting at the top of the picture and hum in the audio.

Does the video signal feed into the cathode of the CRT? And if so, often in these sets the same video signal also feeds the sync circuits and the sound IF. This can confuse the vertical sync circuit, as it starts to lock onto its own output rather than the TV channel's. Many later sets would inject the video into the CRT cathode, and the vertical blanking pulse on the CRT grid 1. This avoids the above interaction. Only thing is that the blanking pulse needs to be inverted or not inverted correctly. Get it backwards and all you'll see are retrace lines, and no or very dark picture. Oh, and be careful of the vertical output tube's plate circuit, the retrace pulse can be a kilovolt or 2 tall. Usually most later TVs grabbed this pulse off the yoke tap. There it's about a hundred volts tall. If the blanking pulse turns out to be backwards to what you need, you could try moving the B+ or ground connection going to both the vertical output transformer and yoke from one end of the yoke winding to the other end, but I'd only recommend that to people who are experts. This may effect the height and maybe centering. And then get the blanking pulse from the end of that winding not connected to B+ or ground. Thru a cap. I have an Admiral that could use it, but I haven't tried this yet. YMMV.