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#1
12-06-2009, 02:18 PM
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good tubes gone bad?
I had a thread about my CBS-Columbia set a while ago. I did get that problem taken care of . I was still left with a small issue, The left side of the screen was a little brighter than the right. I rechecked every tube, cap, resistor and adjustment in the horiz. Everything was good. I got bored yesterday and took out the 6SN7 and the 12CU6 and put in different ones. Of course both were in the Horiz. and now the problem is gone. The tubes I took out and the ones I put in tested pretty much the same. Can anyone explain this?
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#2
12-06-2009, 04:58 PM
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What type of tube tester did you use? I have observed that a low-cost "cathode emission" checker may not always detect tubes that will perform poorly in some critical circuits, such as sweep and RF/IF amp.
When in doubt, it is always a good idea to try several tubes.  jr
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#4
12-06-2009, 10:11 PM
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I didn't think that any tube tester could effectively check horiz. sweep tubes. It is my understanding that if those tubes are suspect on the set, change the horiz. output and damper and see if the problem goes away. The tv-7 is a great tester, though.
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#5
12-06-2009, 10:18 PM
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My emission tester can't test a horizontal oscillator tube effectively. In one case I went through a handful of equally "good" tubes before I found one that the TV liked.
Phil
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#6
12-07-2009, 10:40 AM
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Agreeing with the previous two posts, you can't test a horizontal oscillator accurately on a tube tester. They can have perfectly good emission & no gas, and still not be willing to run at 15.750 Kc. You pretty much have to have 2 or 3 to try, and pick the most stable. Same goes for RF oscillator and IF tubes.
Charles
__________________
Collecting & restoring TVs in Los Angeles since age 10
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#7
12-07-2009, 02:49 PM
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One thing that helps is having dead nuts accurate values for caps and resistors in those particular circuits. I've found that the horizontal section is especially picky about what you put in it, even a small change from what's called for in the schematic will leave the set completely dead. And even if the circuit does work correctly, you have to be especially mindful of the horizontal output tube. When I was getting my Halolight up and running, I never thought to check the 6BQ6GT tube because it was working perfectly and drawing the correct cathode current. Big mistake to assume it was OK! I happened to look over at it while doing some work on the chassis with the lights dim, and the plate was glowing bright red! Only thing that fixed that was a new tube, which doesn't glow at all. Strange though, that same tube in my Philco works just fine...
Ah well, I don't pretend to want to understand these things. It's enough that I enjoy watching them while they are around. 
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#8
12-07-2009, 02:55 PM
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mimiman82, you may want to check the drive voltage to the Horizontal output tube in your Halolight. Even though, it is not glowing red, the Horizontal output tube may be overloaded if the horizontal drive voltage is to low. This could greatly reduce the life of the tube and also cause early flyback transformer failure.
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#9
12-09-2009, 06:58 AM
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Drive is fine, trust me I checked!
There is a sweet spot for it too: too long, the HOT draws too much current and replates, too much and the HV falls. It's also very sensitive to the values of the coupling caps in that section, a 100 Pf is enough to make it throw a temper tantrum. I think it has a weak pix tube though, no matter what I've done it's kinda dim. Still OK picture with lights dimmed, but nowhere close to the picture quality my Philco has.
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