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  #31  
Old 10-11-2017, 07:43 PM
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Quote:
Originally Posted by dtvmcdonald View Post
The big problem with I-Q demod is that to get it really right required two
delay lines, one for Y, one for I. And to get them right and the filters with good phase response is not easy ... without digital (or bucket brigades).

My personal opinion is that best is to use any two axes and get the response
quite flat to about 700-800 kHz then rolling off to zero at 1.5 MHz. on both
axes. Few folks notice the resulting bleedthrough from I into Q. Using
equal bandwidths the results are identical independent of the axes used.

However, other people may seriously object to the bleedthrough.
This raises an interesting question. What exactly is "wide bandwidth" chroma demod? Where does one draw the line in the sand and say this set is narrow band whereas this other set is wide band? I guess it is more accurate to say that some sets are wider or narrower relative to one another...

Take for instance the CTC-2 vs the CTC-5N. Both are "wideband" but the CTC-2 demodulates along I/Q and has extended I response. The CTC-N uses difference demod and starts rolling off at about 600 kHz on both axes, yet both are described as having wide chroma bandwidth on Ed's site. The CTC-2 has wider bandwidth, than the 5, but compared to the CTC-4 they both would qualify as "wide".
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  #32  
Old 10-12-2017, 05:36 AM
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The actual terms are next to meaningless, but I recall someone telling me it’s actually more correct in most cases to call narrowband circuits ‘equiband’ since most difference circuits have about the same frequency response in both axis. Maybe it was wayne?
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  #33  
Old 10-12-2017, 11:32 AM
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I built a small Tesla coil yesterday using the self-tuning, and extremely efficient, "Slayer" exciter with junk box parts. This morning, I tested it with a 150 watt replacement CFL bulb. Lit up like a Christmas tree.

Retested the neck and face of the 15GP22 for gas with the new Tesla coil. Nadda.

If Dave and I get normal looking cutoff tonight, I'm calling the tube good.
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  #34  
Old 10-12-2017, 12:13 PM
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The ability of the RF from the Tesla coil to excite the gasses in a sealed low pressure tube is inversely proportional the pressure of the gasses inside the tube, over a fairly wide range of pressures. At about 3 torr, the pressure inside of the typical CFL, the Tesla coil was capable of exciting the gasses. Pressures too high will not allow the excitation of the gasses to be visible. On the opposite end of things, a pressure too low won't allow one to observe excitation of the gas from a Tesla coil either.

Once we get below about 10^-3 torr, the excitation of the gasses from the Tesla coil should no longer be visible. A CRT with a pressure above about 10^-4 torr is useless and will start arcing. A tube at 10^-6 to 10^-9 is ideal.

Given what I've observed thus far, I can safely conclude that the tube must be at a pressure no greater than 10^-3 torr. Even a Tesla coil isn't as fool proof as I had thought... Nick is right, the only good test is a working chassis.

Last edited by benman94; 10-12-2017 at 12:16 PM.
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  #35  
Old 10-12-2017, 03:15 PM
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Quote:
Originally Posted by miniman82 View Post
The actual terms are next to meaningless, but I recall someone telling me it’s actually more correct in most cases to call narrowband circuits ‘equiband’ since most difference circuits have about the same frequency response in both axis. Maybe it was wayne?
Yes, the major difference is non-equiband (I/Q) vs. equiband; a secondary difference is how wide the equiband demods are. Generally, if the demods are "high level" (have high gain and are coupled to the CRT without further amplification) they will have more roll-off due to stray capacitance than a "low level" demod that is followed by an additional stage of amplification (like the RCA X/Z system). The X/Z system was truly a well-optimized trade-off of cost and performance, which was not improved on for many years.
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  #36  
Old 10-12-2017, 04:18 PM
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Question for the 15 inch gurus: does pin 13 of the CRT itself typically show signs of having had 10 kV or so on it? Or would you only expect to see marks on hole 13 of the socket?

The CRT has absolutely no signs of having had convergence voltage on pin 13. I mean none. But the corresponding hole in the socket is pretty chewed up...

Maybe the tube was replaced at some point.
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  #37  
Old 10-12-2017, 06:05 PM
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The CRT is good:
  • The heaters lit normally, and drew normal current
  • The neck didn't get any warmer than a typical color CRT neck
  • Neither Dave nor I saw ANY glow whatsoever
  • There is no discoloration of the cathodes or grids
  • Multiple passes with three different tesla coils revealed no gas
  • Cutoff on all three guns responded normally, though the pot needed to be in a different position for each gun
  • And finally, the Sencore showed emissions results very similar to the Beltron



Dave was fairly confident when he left that the tube would produce a good picture; so am I.

Now to get the chassis to Nick...
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  #38  
Old 10-12-2017, 11:08 PM
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Now that the CRT is know to be good are you going to pony up some more cash, or was it an all sales final kind of deal
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  #39  
Old 10-12-2017, 11:15 PM
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Congrats Ben! I would bet your assumption that the tube had been replaced at some point, is correct. The tube seems to test like NOS. And the fact that you found some carbonizing only on the socket at pin13, I think your assumption that this is a replacement crt is probably correct.

In addition, it is my belief that late production crt's from RCA were more likely to be free from leak defects as RCA got better at producing good leak free tubes.

When you get a chance to pull the tube, it will be interesting to see if it has been re-necked which would indicate a tube that was either rebuilt at RCA or failed QC and reworked during original production. I have seen a couple 15Gs that were obviously renecked/rebuilt/reworked by RCA. Although this is just conjecture, I think that after the original production run was over, RCA rebuilt duds to have a supply of 15Gs for RCA's replacement crt division. It is well known that replacement crt price lists showed the 15G to be available for quite a number of years.
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  #40  
Old 10-12-2017, 11:18 PM
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Quote:
Originally Posted by Kevin Kuehn View Post
Now that the CRT is know to be good are you going to pony up some more cash, or was it an all sales final kind of deal
Dwight and I have been in contact, and he's happy the tube is operational. The trade was pretty damn fair considering the missing parts on the Westy. If he wants to reveal the details, that's his prerogative. The deal is done.
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  #41  
Old 10-12-2017, 11:28 PM
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Quote:
Originally Posted by ohohyodafarted View Post
Congrats Ben! I would bet your assumption that the tube had been replaced at some point, is correct. The tube seems to test like NOS. And the fact that you found some carbonizing only on the socket at pin13, I think your assumption that this is a replacement crt is probably correct.

In addition, it is my belief that late production crt's from RCA were more likely to be free from leak defects as RCA got better at producing good leak free tubes.

When you get a chance to pull the tube, it will be interesting to see if it has been re-necked which would indicate a tube that was either rebuilt at RCA or failed QC and reworked during original production. I have seen a couple 15Gs that were obviously renecked/rebuilt/reworked by RCA. Although this is just conjecture, I think that after the original production run was over, RCA rebuilt duds to have a supply of 15Gs for RCA's replacement crt division. It is well known that replacement crt price lists showed the 15G to be available for quite a number of years.
Thanks Bob, it feels good to know I don't need to hunt for a good CRT. Eventually I'll want to pick up a spare for the Westy, but I have time yet.

It's funny you mentioned re-necking: the tube has a very obvious, some might say ugly looking, neck weld. Dave noticed it immediately. Either a QC failure, or the grand father of all Coloramas.

I'm guessing a later rebuild; there are four very small (a triad or two) regions of messed up phosphors. There's also a long black human hair caught between the dot plate and the front of the tube, but that would have to be from the initial stages of manufacture, no? She ain't pretty, but a like-NOS tube is like hitting the lottery. I can live with the occasional dead dot.
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  #42  
Old 10-13-2017, 01:03 AM
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Quote:
Originally Posted by benman94 View Post
Dwight and I have been in contact, and he's happy the tube is operational. The trade was pretty damn fair considering the missing parts on the Westy. If he wants to reveal the details, that's his prerogative. The deal is done.
Of course I wasn't really too concerned, just being a smarty pants tonight.
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  #43  
Old 10-13-2017, 09:26 AM
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Originally Posted by Kevin Kuehn View Post
Of course I wasn't really too concerned, just being a smarty pants tonight.
No problemo; I have a difficult time recognizing sarcasm/jokes.

On another note, I feel incredibly stupid about the swirling pattern on tube. It is not a phosphor defect; it is a defect in the front glass of the tube. At my parents home, there are two ceiling sockets in the main room of the basement, each with a pair of 300 watt lightbulbs. The table that the chassis is on is sitting maybe a foot "behind" the ceiling fixture. The light from the lamp was coming in at such an angle so as to strike an aberration in the glass near top right of the screen and then reflect that onto the bottom left of the screen. When I held the log paper up to it earlier, I wasn't seeing the distortion of the pattern itself but rather the shadow and mistaking it for the distortion (my eyesight isn't so great).

TL;DR The phosphor is fine, the front of the tube has some bubbling that created the weird pattern. In a normally lit room there should be no problem.
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  #44  
Old 10-13-2017, 10:49 AM
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Great news about the CRT tests. Hopeing for the best.
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