Amazing how much damage one bad cap can do - Page 2 - Videokarma.org TV - Video

jr_tech · #16 09-13-2013, 07:02 PM

Barkhausen oscillation? Perhaps try holding a strong magnet close to the horizontal output tube to see if the pattern changes. Just a WAG, I have never observed BO.

jr

bandersen · #17 09-13-2013, 09:46 PM

Swapping out tubes and the drive controls had little if any effect. What did help a bit has to adjust the horizontal oscillator coil on the back. The problem isn't entirely gone now, but it's greatly reduced.

Kevin Kuehn · #18 09-13-2013, 10:07 PM

Was the waviness noticeable before the total recap?

bandersen · #19 09-13-2013, 10:20 PM

No, I don't recall it being an issue, but the set did have others problems like poor linearity. Seems like fixing some problems caused others

bandersen · #20 09-13-2013, 10:31 PM

Here's where I'm currently at. With careful adjustments I can reduce the waviness to just a bright band on the leftmost edge. I think that's good enough for now. I'm going to pop it in a cabinet and let it run for a while and see how it goes.

P.S. There's no keystoning - I just had the camera at an odd angle.

Dude111 · #21 09-14-2013, 12:29 AM

Quote:

Originally Posted by ISawItOnTV

You sure know your stuff on these things.

Yes he does and its good seeing people HAVE THE WILL to make it work again

GOOD FOR YOU MY FRIEND

kramden66 · #22 09-14-2013, 01:30 AM

those wavy lines are the same ones I was getting on my Dumont ra-109 , what made them go away was when I adjusted the width and drive and all that , they may or may not still be slightly there , I know when they were there tuning would make them lighter or bolder , I hadn't noticed them lately so when I run it again I will look closely and if they are there even faintly I will report it.

mike

bandersen · #23 09-14-2013, 01:42 AM

Quote:

Originally Posted by Dude111

Yes he does and its good seeing people HAVE THE WILL to make it work again

GOOD FOR YOU MY FRIEND

Thanks! Once I get going on a set, I'm pretty tenacious. Especially with these Admirals now that I have some experience with them.

Quote:

Originally Posted by kramden66

those wavy lines are the same ones I was getting on my Dumont ra-109 , what made them go away was when I adjusted the width and drive and all that , they may or may not still be slightly there , I know when they were there tuning would make them lighter or bolder , I hadn't noticed them lately so when I run it again I will look closely and if they are there even faintly I will report it.

mike

Adjusting the width and drive did help. The lines vary a lot depend on the programming. It's especially bad in dim scenes.

I put the chassis into my blonde 30A16 Admiral cabinet and retired the unrestored green cadmium monster that had been in there to the attic.
The sound really surprised me. It was much louder with my cable hookup than on the workbench.

All was well for about 45 minutes then the vertical went haywire

I guess I'll be pulling it back out sooner than later. Oh the joys of running vintage sets

miniman82 · #24 09-14-2013, 09:42 AM

Looks like the vertical lin needed some tweaking, I suppose there's some more caps that need replacing in that circuit.

Kevin Kuehn · #25 09-14-2013, 02:00 PM

It might be interesting to put the original caps back in the horizontal oscillator and output section. Sometimes I wonder if our modern caps don't shield these circuits properly from outside interference. One end of the old paper caps were marked with the outside foil shield. That end typically went towards the lower potential voltage source.

Phil Nelson · #26 09-14-2013, 02:45 PM

Quote:

Originally Posted by jr_tech

Barkhausen oscillation? Perhaps try holding a strong magnet close to the horizontal output tube to see if the pattern changes. Just a WAG, I have never observed BO.

I believe that I observed it in my Hallicrafters T-67:

The interference is seen as squiggly vertical lines running down through the reporter's face. It disappeared after I replaced the 6BG6T horizontal output tube, just as suggested in an old service book. The book noted that Barkhausen's is more prominent with weak signals than strong ones.

Another solution is to place a small circular magnet around the tube. The DuMont RA-103 has a little magnet mounted inside the HV cage to suppress Barkhausen's.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

bandersen · #27 09-14-2013, 06:45 PM

Quote:

Originally Posted by miniman82

Looks like the vertical lin needed some tweaking, I suppose there's some more caps that need replacing in that circuit.

The set has been fully recapped, but I used the cheap generic yellow caps. I wonder if one failed ?

Quote:

Originally Posted by Kevin Kuehn

It might be interesting to put the original caps back in the horizontal oscillator and output section. Sometimes I wonder if our modern caps don't shield these circuits properly from outside interference. One end of the old paper caps were marked with the outside foil shield. That end typically went towards the lower potential voltage source.

I suppose I could dig them out of the trash. I've heard there are tricks for determining the outer layer in modern caps using an RF generator and oscilloscope. I think the idea is to put the probe tip against the body of the cap, apply RF to either end and observe the signal strength on the scope. The stronger end will be the outer layer.

Quote:

Originally Posted by Phil Nelson

I believe that I observed it in my Hallicrafters T-67:

The interference is seen as squiggly vertical lines running down through the reporter's face. It disappeared after I replaced the 6BG6T horizontal output tube, just as suggested in an old service book. The book noted that Barkhausen's is more prominent with weak signals than strong ones.

Another solution is to place a small circular magnet around the tube. The DuMont RA-103 has a little magnet mounted inside the HV cage to suppress Barkhausen's.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html

I swapped out several 6BG6s and it had zero effect so I think the problem lies elsewhere. My suspicion is the mica cap inside the yoke across the horizontal windings. It's not so easy to get at though.