bent neck CRT pins...

[eccerr0r]

Ugh... I think I may have broke a CRT in a 'scope.

I have one of those CRTs with pins on the neck for the deflection plates... they were bent really badly. Had to bend them back at risk of cracking the tube... and may well have...

What kind of characteristics do leaked CRTs have? how does a slow leaking tube behave as the tube fills?

Might have to buy a new CRT And hope it does not come with bent pins...

[stj]

Re: bent neck CRT pins...

internal arcing between the anode & internals.
and *maybe* the heater will be brighter or burn-up in the presence of oxygen.

[eccerr0r]

Re: bent neck CRT pins...

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?

[stj]

Re: bent neck CRT pins...

it will probably work until the heater or anode act up.
once you lose the vac the phosphor will start to fall off the face and leave a mottled effect btw - a good way to spot stuff with broken tubes at a car-boot sale.

[keeney123]

Re: bent neck CRT pins...

For future reference always use two long nose pliers. One to hold up near the tube and the other to bend the pin back.

[eccerr0r]

Re: bent neck CRT pins...

I was lazy and I guess I'm paying for it now. I would have to remove the tube to do that.

Now the other question, can a cracked tube instantaneously destroy the anode power supply? It looks like a fuse blew... not a good sign...

[stj]

Re: bent neck CRT pins...

no, never.

[eccerr0r]

Re: bent neck CRT pins...

Interesting... Maybe something different happened? I didn't hear any loud sucking sound when it seemed something cracked but perhaps it was just the outside layer... I hope.

But the strangest problem is that now my HV supply is instantly blowing fuses now. Was thinking the cracked tube could have caused this... hmm.

It's a 15 volt supply, and I tried sticking in 40 ohms of resistance instead of using the original fuse and almost 400 mA flowed - so the supply is virtually a dead short... The switching transistor (the circuit looks like a Joule Thief type oscillator) didn't short out. So something on one of the secondaries apparently shorted out.

Hmm. I wonder if I should fix the HV PSU or still look into buying a new CRT with the PSU...

BTW, before the phosphors flake off, is there any other way to tell without a working HV PSU if the vacuum leaked out ..err... air leaked in?

[stj]

Re: bent neck CRT pins...

the HT is very low current, and it is not reliant on a vac-tube.
i know of people how test high-voltage supply's by putting the anode-cap in a glass jar to insulate it for safety.

if your HT is blowing, it may be a shorted focus signal.
that is connected through the base.

[stj]

Re: bent neck CRT pins...

BTW, before the phosphors flake off, is there any other way to tell without a working HV PSU if the vacuum leaked out ..err... air leaked in?

i dont think so.
or maybe there is - but it's dangerous to your test gear.

a crt is a giant capacitor - but i wouldnt put my meter across one!!

[stj]

Re: bent neck CRT pins...

try isolating the HV unit from the tube, and then power it up.

[Sparkey55]

Re: bent neck CRT pins...

Was the scope working before you bent the crt pins? We need photos of the pins now. If the tube has a vacume leak then the cathode heater will be burnt and not light. You can test for High voltage coming from the power supply using a small neon bulb, should light up by just holding it near the flyback transformer.

[eccerr0r]

Re: bent neck CRT pins...

I did not know whether the scope was totally working before I bent the pins back to where I could reconnect the pins - I acquired it as "nonfunctional". After reconnecting them, it actually worked - I saw traces show up ... but then after a few minutes the focus got really bad and eventually the HV PSU apparently burned out. It's now apparently shorted (it's burning fuses now). What I'm wondering is if I need a new tube and PSU or perhaps I can get away with just getting or fixing the PSU - which might not be easy in itself...

It'll take quite a bit of effort to take the thing apart to take a picture (it's really embedded in there) so I'm going to try to avoid doing it until I can do it electrically or visually

(Hmm... I found a 66 megohm resistor... and a few 10M ohm resistors... Might be able to make a high voltage probe and electrocute myself )

[Sparkey55]

Re: bent neck CRT pins...

Ugh... I think I may have broke a CRT in a 'scope.

I have one of those CRTs with pins on the neck for the deflection plates... they were bent really badly. Had to bend them back at risk of cracking the tube... and may well have...

What kind of characteristics do leaked CRTs have? how does a slow leaking tube behave as the tube fills?

Might have to buy a new CRT And hope it does not come with bent pins...

INFO needed: Make and Model of Oscilloscope. Analog or digital. All tube or solid-state. Were the pins bent before or after you received it? Does the neck of the crt stick out past the rear of the chassis? Did it have a cover over the neck to protect it? In what manor did you try to striaghten the bent pins?

[Sparkey55]

Re: bent neck CRT pins...

It is difficult to say the least to be able to help you without the proper information on the scope. You can check to see if the heater is open or shorted to the cathode with an ohm meter. Pics are needed also.

[stj]

Re: bent neck CRT pins...

if it slowly lost focus, check every diode and resistor in the HT circuit.
disconnect the anode from the tube first!

[stj]

Re: bent neck CRT pins...

btw, what scope is it, do we have available schems?

[Sparkey55]

Re: bent neck CRT pins...

btw, what scope is it, do we have available schems?

It seems that we would have better luck pulling hens teeth than getting any info.

[eccerr0r]

Re: bent neck CRT pins...

It's a Tek 2465 analog. I found the service manual and schematics, but I'd thought a CRT is a CRT and all of them have similar behavior, hoping things would be as generic as possible and not specific to a certain device.

The pins on the tube were badly bent before I got it, almost 90 degress off. The issue was that the pins were not pointing into the aperture in the shield where the wires need to connect to them, there's no way that the wires could be reconnected without bending them back. As I was too lazy to take the whole thing apart I propped the pin with a plastic stick, pushing the base away from the direction I wanted to bend the pin, and used a hook to pull the pin the way it needed to unbend. I heard a glassy tick sound and that's when it sounded bad. But I did not hear a sucking sound, so hoping that it didn't leak.

The behavior of the HV circuit depends on the circuit of course, but I'd imagine most of them would work similarly to each other. The worry is that the HV circuit has an epoxy potted multiplier module which is one of those "replace whole thing" types of deal. Boo. However now that I know that the HV module is now bad or at least it's frying fuses. I suppose I've seen many people leaving the HT lead flying causing long sparks with no damage to the PSU. Just wondering if it's typical to all HV PSUs?

I don't see any burned areas other than discovering the burnt fuse. And it looks like the CRT heater is still a good 6 or so ohms after having it powered on for a few minutes after the unbending... The heater is apparently like most CRTs and is another winding on the secondary of the HV "flyback" (but it's not a true flyback like in TVs...)

[eccerr0r]

Re: bent neck CRT pins...

hmm...Waited a day of waiting, measured the resistance of the heater with my Fluke (instead of the inaccurate Radio Shack)... 14.5 ohms.
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.

Hmm... Should have measured it accurately yesterday ...

14.4 ohms seems a bit high does it? Not sure...

And guess what. The HV PSU is a different rev compared to the schematics I have. I guess I still have to trace some of those components not on the schematics.