bent neck CRT pins...

[momaka]

Re: bent neck CRT pins...

If the tube has a vacume leak then the cathode heater will be burnt and not light.

+1

How about when it's a slow leak... I'm sure at first it will still work somewhat? What characteristics would it have before it degrades to death?

Loss of focus and/or darker screen image.

The more air you have inside the tube, the more the electrons traveling in the tube will collide with air molecules. When they collide with the air molecules, they will tend to scatter and not hit the intended spot on the screen that they were fired at. Also, some electronics will scatter to such an extent that they never reach the tube phosphor screen in the front - hence the darker screen image.

internal arcing between the anode & internals.

Yes, that is one way to spot a tube that has lost vacuum. One of my Sony GDM-FW900 is like that. Luckily, Sony designed it properly with all kinds of safety circuits, so nothing will blow even if the HV supply (anode, and G1 focus 1 and 2) is shorted out.

and *maybe* the heater will be brighter or burn-up in the presence of oxygen.

Actually, if anything, the heater will not light up at all if the tube has lost vacuum. It's the same with incandescent light bulbs (and why you see them blow sometimes if the bulb has lost too much vacuum). The vacuum allows the heating element to become very hot and glow, since there are no molecules inside the tube to carry away that energy. When the tube looses vacuum, now you have air molecules inside that basically cool down the heater, so it won't be able to run as hot (and thus not glow).

An easier way to think of this is a heating element running in free air and the same heating element running in a tub of water, both with same power applied. Obviously, the element running in free air will reach much higher temperatures than the one in the water.

So a good way to test a tube is to apply proper voltage to the heater pins and see if you get a nice yellow-orange glow. If you don't, tube likely has gone to air or heater has went O/C.

Connected a somewhat discharged 6V NiMH battery to the heater for 5 seconds (more because it's annoying to hold the wires to the heater pins)... Saw voltage go down...
Measured resistance of the heater again... 14.4 ohms.

Is 6V what the heater is supposed to run on for your set? Check with the service manual (if you have one). Also, while you are applying a voltage on the heater pins, check the current draw as well. If the heater is drawing normal or higher current with the rated voltage but NOT glowing, the tube likely has lost vacuum.

[eccerr0r]

Re: bent neck CRT pins...

The normal voltage, at least as it says on the manual, is 6.3VAC @ high frequency.

Hmm.... I would have thought a heater when exposed to air would increase its resistance as it burns out, and thus current draw would go go down...

I just wonder if I can correlate a cracked tube with a burned HV PSU and really should be looking for both for a repair... I really didn't see much of a brightness loss until the PSU fried somehow, but did have a severe loss of focus. I'll need to pull the psu and hope it's not a fried inductor or the potted multiplier.

Question is if this is worth fixing any more. I usually mark broken crt = give up Hence seeing if I can figure out exactly what caused what...

[stj]

Re: bent neck CRT pins...

most likely the focus pin was the bent one and didnt contact the socket properly.
or even shorted the spark-gap in the socket grounding out the focus voltage!

[eccerr0r]

Re: bent neck CRT pins...

The pin that was bent dealt with the electrostatic deflection (they're in the neck of the tube!) instead of the focus/cathode/heater which are in the round base of the tube! The neck pins I'd imagine are significantly more fragile unfortunately.

So the focus circuitry is more delicate and can damage the HV supply if shorted or even open?

How many days does it take to consume all of the getter assuming it's a fast leak and it's full of air now? I still haven't removed the shields so I don't know where the getter spot is however...

[eccerr0r]

Re: bent neck CRT pins...

HV circuit pr0n




It looks OK still...

Need to look for schematics that match this board... the ones I found were close but not identical. The resistors in the lower left look a bit toasty but can't tell what they were from the schematics I found.

[eccerr0r]

Re: bent neck CRT pins...

The HV PSU supposedly should work at 15V 750mA (at least the original fuse was 750mA). Initially I tried a 500mA slow blow fuse (did not have a 750mA 5x20mm fuse, so I used clips and extension to a 1.25" fuse), the slow blow fuse got red hot and burned. The 40 ohm resistor instead of the fuse pretty much had all 15V across it.,.

A quick ohming out the tube base pins didn't imply any shorts other than the currently intact heater.

I tried powering the PSU from a bench psu and it does not seem shorted... The mystery continues. Then again I did not power the regulation circuitry which may be shutting the HV off and thus not appear to short; may have to fake that by plugging in feedback voltage or by using multiple supplies.

Is it typical for TVs to actually regulate HV output? Looking at the schematic, there is indeed negative feedback voltage regulation. Based on the number of precision resistors on this board, this has to be very delicate stuff... A power supply (unlike PC power supplies, etc...)

[momaka]

Re: bent neck CRT pins...

The normal voltage, at least as it says on the manual, is 6.3VAC @ high frequency.

Hmmm, I wonder why they say that. It's a light bulb element, basically. I never thought it would matter.

Hmm.... I would have thought a heater when exposed to air would increase its resistance as it burns out, and thus current draw would go go down...

Perhaps when it oxidizes enough, I suppose.

[eccerr0r]

Re: bent neck CRT pins...

Actually I added that, whoops. They did write "AC" but it's sort of implied by the fact it's connected to a winding the high voltage transformer which operates at high frequency...

---

I suspect if you have the heater heated, oxidation will occur fairly rapidly in presence of air?

[momaka]

Re: bent neck CRT pins...

I suspect if you have the heater heated, oxidation will occur fairly rapidly in presence of air?

At the stock rated voltage? I would guess probably not, but I have no idea, TBH. Here's an interesting video I found:
https://www.youtube.com/watch?v=35NiLqmLGtU
I wonder what voltage this tungsten element was rated for.

[eccerr0r]

Re: bent neck CRT pins...

As each bit of oxygen attacks and generates an insulative or flake-off tungsten oxide, there will be less material left to conduct electricity, so that's why it would make sense that as long as it's oxidizing, resistance would go upwards.

Not sure where you cold get fairly large piece of tungsten, probably a 60-100W bulb perhaps? I suspect that most smaller bulbs would be really thin...

[stj]

Re: bent neck CRT pins...

obviously none of you have switched on an incan light with a broken seal.
you get a bright flash as the element vaporises like a fuse and often a bang and a load of broken glass.

[goontron]

Re: bent neck CRT pins...

obviously none of you have switched on an incan light with a broken seal.
you get a bright flash as the element vaporises like a fuse and often a bang and a load of broken glass.

Guess you've never dealt with old vacuum tubes that went to air suddenly (of your own fault).

[eccerr0r]

Re: bent neck CRT pins...

The video seems to have implied it got the tungsten from something like http://cpc.farnell.com/1/1/91062-ge-...00w-gy9-5.html - so the movie is actual undervoltage. Then again this is also a halogen, so it should burn even hotter than a regular vacuum/inert lamp.

So in any case they should burn up in air if it glows. I don't know how much damage tungsten takes when it's not heated as hot as in a light bulb, and yes, the CRT is a huge vacuum tube.

[momaka]

Re: bent neck CRT pins...

I don't know how much damage tungsten takes when it's not heated as hot as in a light bulb, and yes, the CRT is a huge vacuum tube.

As long as there is no air (presence of Oxygen), it hardly matters. That said, the only reason why low-power incandescent light bulbs usually last longer compared to high-power ones is because the high-power ones have to dissipate more heat (and thus get hotter). Because they get hotter, the ON-OFF cycles kill them faster (due to seals and glass being stressed more).

obviously none of you have switched on an incan light with a broken seal.
you get a bright flash as the element vaporises like a fuse and often a bang and a load of broken glass.

Like I said, the resistance of the tungsten element is low when the element is cold. What you see happens because the air inside the bulb doesn't permit the tungsten element to reach a high temperature and thus increase the element's resistance. Since the light bulb is usually directly connected to the line voltage, it will draw a huge current (and hence a lot of power). When that happens, the tungsten element simply can't keep up with the current and blows up. And even if the current is limited, the tungsten element oxidizes as it heats up due to presence of Oxygen.

Now back to the thread's topic...

They did write "AC" but it's sort of implied by the fact it's connected to a winding the high voltage transformer which operates at high frequency...

This and the above info got me thinking: perhaps when the tube went to air, the heater drew a much larger current than it did before. Since it is connected to a winding on the high voltage transformer, perhaps this is what threw-off the HV supply and made it blow up? Anyone else also find this to be a possible scenario? Or am I having brain farts here?

[stj]

Re: bent neck CRT pins...

maybe test the psu NOT connected to the tube - that would be logical.

[eccerr0r]

Re: bent neck CRT pins...

Hmm... The tube does not seem shorted which is weird, plus the PSU does not appear to be shorted either. I think I'll have to swap the 750mA fuse with a light bulb so I can stop burning fuses while debugging this. Now the question is, what kind of light bulb can I use. Thinking about a 12V 12W lamp for this 15V supply, 750mA fuse as that's all I have that's close by, else I should hack up a LM317 to pass 750mA...

[eccerr0r]

Re: bent neck CRT pins...

Ahh... I found schematics and placement information that's closer than found in the service guide; it looks like a later version of the board was in the device. Still no idea why it seems shorted when plugged in (light bulb in place of the fuse burns brightly) but without the PSU the light is out.

When disconnecting the anode and cathode/filament (by the base socket), the light bulb still burns brightly.

One oddity was that I hooked up my sencore to the "flyback" output (before the multiplier). At first I was getting a Q factor of around 9... Then after a bit of futzing with it, the meter got a Q factor of 29. I hope I don't have a fried flyback that has an intermittent short. I was testing in-circuit...

Woah
http://www.ebay.com/itm/TEKTRONIX-GB...-/272182368706 - What was Tektronix thinking? Look at the bodge job at the lower left of the board...

Sigh... hope that I can fix mine somehow...

[stj]

Re: bent neck CRT pins...

i wonder, is the psu oscillating, or just sticking dc into the transformer?

[eccerr0r]

Re: bent neck CRT pins...

The PSU is not oscillating at least with the 12W bulb as the DC "dim bulb tester." I used another scope to determine this However I wonder if I should up the wattage of that bulb...

It uses a single TO220 PNP transistor as the oscillator, and uses same circuit topology as a Joule Thief, except power is taken from the secondary. This transistor is just to the lower/right of the 'flyback,' plastic body facing left. Negative feedback is done by changing the voltage on the feedback winding (on a Joule Thief, the feedback winding is connected to V+; on this circuit, an op amp output through a resistor.)

The transistor does not appear shorted but apparently having trouble oscillating. I should check the feedback voltage, and assuming it works like a Joule Thief, ... hmm...

[stj]

Re: bent neck CRT pins...

so a bad cap or shifted-value resistor maybe.