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cmjohnson
Joined: 03 Apr 2006
Posts: 5180
Location: Buried under G90s
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| Posted: Sat Apr 22, 2017 12:21 am Post subject: Anode short on CRT? How is this even possible? |
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I've never seen this before.
I've gone through those Sony 07MX tubes (applicable to AmPro 4200 and Sony 1292, maybe others as well) and I've found one that just doesn't allow the HV to start up. (Using a Marquee chassis for testing)
The HVPS just clicks about once a second if this tube's anode lead is connected.
Simply disconnecting the anode lead allows the HVPS to power up just fine, but of course, while the other tubes will light up,
this tube will of course remain dark without any HV.
I'm trying to figure out how a tube can even BE faulty in this manner!
The only thing I can think of is a conductive path from the anode button to the outside grounding dag. There is no evidence of that issue on this tube as far as I can determine.
Got any ideas or experience with this?
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Tim in Phoenix
Joined: 21 Oct 2006
Posts: 4409
Location: Phoenix
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| Posted: Sat Apr 22, 2017 1:10 am Post subject: |
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Say CM
Any weirdness in the gun? Funny glow? Maybe that tube went to air.
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cmjohnson
Joined: 03 Apr 2006
Posts: 5180
Location: Buried under G90s
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| Posted: Sat Apr 22, 2017 1:22 am Post subject: |
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No evidence of that. I know how the tube changes appearance when its vacuum leaks out. No trace of that.
I'm going to ohm it out if I find my meter. I suspect it has an ohmic ground problem which would result in it drawing more current than the HVPS will allow.
What puzzles me is what that ground path is.
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Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
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| Posted: Sat Apr 22, 2017 2:51 pm Post subject: |
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If the tube has had air leak in, it's a dead short to the HV. Check the filament, if it doesn't glow, the tube is cracked somewhere.
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cmjohnson
Joined: 03 Apr 2006
Posts: 5180
Location: Buried under G90s
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| Posted: Sat Apr 22, 2017 6:50 pm Post subject: |
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Filament glow is normal.
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Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
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| Posted: Sun Apr 23, 2017 12:32 am Post subject: |
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Tim gave me a tube once to play with, it would cut out after 70 contrast. LCP Marquee. Blue. Ran completely normally below about 70 contrast. Never did figure it out, and I chucked it after trying to zap it on my Sencore.
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cmjohnson
Joined: 03 Apr 2006
Posts: 5180
Location: Buried under G90s
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| Posted: Sun Apr 23, 2017 4:50 pm Post subject: |
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Got it figured out. It had a bad anode cap coupled with a carbon track to ground, probably
due to a glycol leak from its previous installation in a flight simulator projector that uses an attached
faceplate/coolant tank.
Replaced the anode cap and cleaned the tube and it's fine.
However, I had another dead soldier that in fact did have no filament glow. I checked it,
open filament. So I cracked the button off the end of the tube and sure enough, no hiss
of escaping vacuum. Aired tube.
I have never fully trusted Sony's method of gluing the tube face together. Now I know why.
OK, granted, a precision flat faceplate for the phosphor is probably a superior way to go with
regard to the quality of the complete optical system, but if the seal fails that's just a waste of
effort to even build the tube.
A funny thing, though: Tektronix has used the separate glued faceplate system on their oscilloscope
tubes for many decades, and I've never yet heard of one of THEIRS losing vacuum due to a seal failure.
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