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Online grumpydoc

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Odd CRT failure mode
« on: November 18, 2015, 11:44:12 pm »
I'm currently trying to fix a Phillips/Fluke PM3394B 'scope and I have come across what seems to be a very odd failure mode for the CRT.

Basically the deflection sensitivity seems to have gone down in both X and Y planes. The Phillips manual says X should be 8.5V/div although the data sheet says 7.7V/div. However even with the final X amp gain turned up full traces are squashed horizontally (ditto for Y). I have confirmed that the voltage on the plates is identical to a PM3394A which works fine and swapping another CRT into the 'scope solved the deflection problem (but brought even more issues for various reasons).

I haven't measured all of the voltages but the cathode supply which is supposed to be -2200V reads about -2230 which is within the tolerance of my HV probe. Focus and astigmatism controls work as expected so the voltages on those plates should be OK and if the -2200V is good then the final anode should be at the correct voltage as it is generated off the same winding on the EHT transformer (only the 2200V is regulated anyway).

Does anyone know if it is even possible for a CRT to "fail" like this and what the mechanism might be?
 

Online tautech

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Re: Odd CRT failure mode
« Reply #1 on: November 19, 2015, 12:02:32 am »
I won't even try to tell you how to fix a CRO grumpy, but are you sure you haven't missed something?

Back to basic checks? PSU?
DC value checks throughout the schematic?

Will the position pots allow full trace movement?
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Offline TerraHertz

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Re: Odd CRT failure mode
« Reply #2 on: November 19, 2015, 01:39:43 am »
Interesting. I had a CRT in a video waveform monitor where the deflection was screwed up. But that was clearly due to the instrument being dropped heavily in shipping, resulting in the inner structures in the tube bending out of position.

As I'm sure you're aware, possible causes of reduced electrostatic deflection are:
 * Below-spec voltages on the deflector plates,
 * Electron beam with higher than spec acceleration voltage,
 * Wrong shape of post-deflection plates electric field.

Can't you measure the anode EHT? If it's too high maybe that could be the problem?

Another idea: some tubes seem to have a kind of electrostatic lens in the wide portion of the tube, formed by a resistive spiral trace going round and round inside the glass. If that's to provide more deflection 'gain', than maybe if that trace went open circuit the trace would appear squashed. Or... there would have to be an electrical connection to the end of that spiral closest to the gun. Perhaps that went open? It would be a finger contact to the inside surface of the tube, probably. Maybe try giving the tube a good hard thump? (with a soft object, after all else fails!)

Ha ha. I can't believe I'm suggesting that.
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Offline aargee

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Re: Odd CRT failure mode
« Reply #3 on: November 19, 2015, 07:01:45 am »
The tube hasn't gone 'gassy' has it? i.e. partial loss of vacuum. This can happen with a failure around the glass envelope on CRT pins.
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Online grumpydoc

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Re: Odd CRT failure mode
« Reply #4 on: November 19, 2015, 01:05:03 pm »
Thanks guys - good thoughts.

Quote from: tautech
but are you sure you haven't missed something?

No and I have a nagging doubt that I am missing something, hence the post.

The problem affects both X and Y deflection so it is likely to be something "global".

Quote from: tautech
Back to basic checks? PSU?
Quote from: TerraHertz
Can't you measure the anode EHT? If it's too high maybe that could be the problem?

The PSU low voltage rails all check out - the 10V ref might be microscopically low at 9.99V (+/- 5mV spec - another 5mV would be within the DMM inaccuracy).

Yes, too high an EHT voltage was one of my thoughts. I haven't measured the final anode voltage directly - I do have a 40kV probe but it would be impossible to get it to the CRT connector with the tube in situ. There is a spec for a dummy load in the service manual but I don't have a 14kV rated 290Mohm resistor handy :(

As a surrogate for measuring the EHT I checked the cathode -2200V supply which is OK at -2230 or so (well within the 2% accuracy of the HV probe). Both supplies come off the same winding of the EHT transformer and the regulation is feedback from the -2200V rail anyway.

Intensity, focus and astigmatism controls adjust as expected so, although I haven't measured all the tube voltages I'm not currently suspicious that they are out of spec.

Quote from: tautech
DC value checks throughout the schematic?
Will the position pots allow full trace movement?
Quote from: TerraHertz
* Below-spec voltages on the deflector plates,
DC checks throughout the schematic whould be a tall order and I'm not sure how much light it would shed. Strictly there aren't any pots - it's all DAC controlled but there are good ranges of trace movement.

I have checked the deflection plate voltages - the final X amplifiers (two in antiphase) give a swing of between 80 and 96V p-p which is the same as a 3394A which works and in that 'scope about 88V of swing is needed.

I have checked that a signal is affected and that this is not just the vector generator in the 'scope and it is - I can't quite stretch a 10Vp-p 1kHz square wave to read correctly in either X or Y. I haven't checked the timebase frequency but I don't think that's it as the vector generator is entirely voltage driven and anyway that would not explain Y axis problems and the cursor system measures the signal perfectly.

Quote from: TerraHertz
* Wrong shape of post-deflection plates electric field.

Another idea: some tubes seem to have a kind of electrostatic lens
Mechanical damage isn't out of the question but to produce a perfectly symmetrical problem seems odd. The tube has some internal magnetic elements to help geometry.

Quote from: aargee
The tube hasn't gone 'gassy' has it? i.e. partial loss of vacuum. This can happen with a failure around the glass envelope on CRT pins.
I don't believe so. EDIT: At least I don't believe the 1st CRT (the one with funny deflection) is gassy, the dim one with low cathode voltage on the other hand is a different kettle of bananas.

The plan was to replace the CRT as a diagnostic test as this has become a two 'scopes into one kind of exercise. Unfortunately the EHT connector on the 2nd CRT was not the same as the first so I needed to bring the multiplier over with it which in turn meant bringing the whole PSU.

That did "fix" the deflection problem but at present that is not terribly useful. The 2nd PSU is making "I am not a happy PSU" noises and the cathode supply is definitely too low by 10-15% (about -1900 - 1980V) so the final anode voltage will be too low as well. This is likely to make this CRT too sensitive to deflection voltages.

I'm going to swap the EHT multiplier this evening and see where that gets me. If the voltages are OK but I still have the deflection problem with the replacement CRT I will know to look elsewhere, if the voltage is still low then I will know that the 2nd CRT or EHT multiplier have problems (though that won't help much with the original problem).
« Last Edit: November 19, 2015, 01:42:03 pm by grumpydoc »
 

Offline Ian.M

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Re: Odd CRT failure mode
« Reply #5 on: November 19, 2015, 01:33:33 pm »
There's always the old beerglass trick for getting at the EHT or patching in a temp. EHT lead with a more suitable connector.  Obviously its as dangerous as ****, so be careful.

Take one straight sided pint glass.  Wash out thoroughly, and rinse with IPA (97% alcohol, not export beer), hot air dry and stand it on a good high voltage insulator (e.g. a chunk of perspex) next to the chassis.  Attach the bare end of the temp EHT lead to the clip on the original lead, and put the joint inside the beerglass right at the bottom.   You may get a little corona when you power up, but the glass surface usually charges up enough to stop that.  DO NOT attempt to touch or move the glass while power is on, or after poweroff before discharging it through an EHT probe or meter.   

That was our go-to method for checking a TV with a suspected gassy tube that was overloading the EHT and triggering the protection circuits, or if we had to bench test a chassis without a tube but had a good yoke from a scrap set.
 

Online PA0PBZ

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Re: Odd CRT failure mode
« Reply #6 on: November 19, 2015, 01:55:40 pm »
Just my thoughts...

- Can you adjust the original PSU/CRT combo so that the cathode is at the same too low level? Does that change anything?
- Is the display still linear?

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Online grumpydoc

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Re: Odd CRT failure mode
« Reply #7 on: November 19, 2015, 01:58:09 pm »
Just my thoughts...

- Can you adjust the original PSU/CRT combo so that the cathode is at the same too low level? Does that change anything?
It would take some component changes.

Quote
- Is the display still linear?
As far as I can tell.
 

Online PA0PBZ

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Re: Odd CRT failure mode
« Reply #8 on: November 19, 2015, 02:17:38 pm »
It would take some component changes.

I know, but it still seems a very weird problem, and one you have one working with too low voltages and one not working with the correct voltages.
Hmm...
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Offline calexanian

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Re: Odd CRT failure mode
« Reply #9 on: November 19, 2015, 04:56:13 pm »
I have seen things like this before. The CRT mat have a conductivity leakage issue on the deflection plates. This happens in scopes that have been on forever every day. Over time remaining things break down inside the tube and adhere to the internal structures causing electrical leakages internally. Additionally barium can be evolving from the electron gun and causing problems. Its simply new tube, or new scope time if this is what has happened.
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Offline Artlav

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Re: Odd CRT failure mode
« Reply #10 on: November 19, 2015, 05:08:24 pm »
Try waving a magnet nearby (but not too close), and see if any field direction would make it improve a little.
I had a problem that sounds similar from the internals of a CRT being magnetized (someone was putting a magnet too close to it), and it got solved by degaussing the gun part of the tube.
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Offline T3sl4co1l

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Re: Odd CRT failure mode
« Reply #11 on: November 19, 2015, 07:42:06 pm »
Try waving a magnet nearby (but not too close), and see if any field direction would make it improve a little.
I had a problem that sounds similar from the internals of a CRT being magnetized (someone was putting a magnet too close to it), and it got solved by degaussing the gun part of the tube.

Heh, reminds me of an old problem: misalignment of the ion trap magnet.  That was a common sight on magnetic deflection (and occasionally magnetic focus also) CRTs in the 50s.  If it's not quite right, the picture gets cropped by internal geometry (probably hitting the 2nd anode inside the gun, before making it out to the phosphor screen).

Demagnetization (of a CRT that's not supposed to be in the first place!) would be helpful, if that's what's going on.

Nickel and iron were commonly used in CRTs, and are magnetizable.  I believe Tektronix used stainless steel, which isn't as magnetizable, and probably doesn't suffer from this problem.  YMMV.

If it has been magnetized, I'd also be concerned about the integrity and magnetization of the mu-metal shield (if present).

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Online grumpydoc

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Re: Odd CRT failure mode?
« Reply #12 on: November 19, 2015, 10:12:42 pm »
Well,  the CRT is exonerated!

Swapping the EHT multiplier to the "good" PSU and I get a bright trace on the previously dim CRT, and the fault is back - the trace is slightly squashed horizontally and I can't stretch it out enough with the available gain on the X amplifier. Y will be the same looking at the traces but I haven't formally checked.

One CRT with a bizarre failure mode is enough, two is implausible - it has to be something else.

The grid bias (or "dark level") won't adjust on this CRT which is different and might be relevant.

My initial instinct was that the EHT was too high, I'm still not sure about that but it is a good candidate. The next step is clearly to measure all of the CRT voltages whether they are tricky to get at or not.

Now, just to find that 290Mhom 20kV resistor.......
 

Offline Artlav

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Re: Odd CRT failure mode?
« Reply #13 on: November 19, 2015, 11:30:58 pm »
Now, just to find that 290Mhom 20kV resistor.......
Why the odd value?
I use a 50V panel meter (which have 50kOhm) with a 50MOhm resistor on the positive lead.
Hook it up, get away, turn things on, observe the voltage, turn things off.

There are plenty of HV 50Meg resistors, from cheap USSR NOS to tingly-priced new ones on Digikey.
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Online grumpydoc

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Re: Odd CRT failure mode?
« Reply #14 on: November 20, 2015, 08:38:06 am »
Now, just to find that 290Mhom 20kV resistor.......
Why the odd value?
I use a 50V panel meter (which have 50kOhm) with a 50MOhm resistor on the positive lead.
Hook it up, get away, turn things on, observe the voltage, turn things off.

There are plenty of HV 50Meg resistors, from cheap USSR NOS to tingly-priced new ones on Digikey.
That's what the service manual suggests as the dummy load - there is a table of currents and suitable resistor combinations which for the EHT is 50uA and 290Mohm (implying 14.5kV which is still low for the tube spec of 16kV)

50uA is probably just a bit more than the tube will draw under normal conditions.

50Mohm might load the supply down too much.

Anyway Farnell sell 3.5kV 47Mohm resistors for pennies - 6 of those in series would do the trick.
« Last Edit: November 20, 2015, 08:39:40 am by grumpydoc »
 

Online grumpydoc

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Re: Odd CRT failure mode
« Reply #15 on: November 20, 2015, 09:42:05 am »
There's always the old beerglass trick for getting at the EHT or patching in a temp. EHT lead with a more suitable connector.  Obviously its as dangerous as ****, so be careful.

Take one straight sided pint glass.....
I was thinking about this as I walked to work - I now have a mental image of a pint glass full of angry electrons fizzing around and wanting to bite something :)
 

Offline Ian.M

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Re: Odd CRT failure mode
« Reply #16 on: November 20, 2015, 10:15:55 am »
That's an excellent mental image - it will keep you safe if you ever have to put an EHT lead in a beer glass.
 

Online grumpydoc

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Re: Odd CRT failure mode
« Reply #17 on: November 24, 2015, 10:59:06 pm »
Just my thoughts...

- Can you adjust the original PSU/CRT combo so that the cathode is at the same too low level? Does that change anything?
- Is the display still linear?
I am still trying to figure out the best way to build the dummy load specified for the 14kV supply - 4x 100Mohm 7.5kV resistors in parallel with the 1Gohm of the HV probe should be OK at 285Mohm but they are tending to be expensive (about £3.50 each is the cheapest I found). Plus I want to combine the order with some other stuff and it looks like DigiKey, Farnell and Mouser each have part of what I want but not everything :(

So, I was thinking about PA0PBZ's suggestion of tweaking the voltage and looking at the EHT supply schematic and have realised that there are a few things I don't understand.

Here is the schematic

That's a bit small to see what's going on so click on the image for the larger, readable, version.

I understand broadly what's going on, there's a self resonant flyback oscillator, the duty cycle of which can be tweaked slightly by the output of the op amp N1101 however the exact operation is confusing me a bit.

Feedback is via the 31.6Mohm resistor R1103 which forms a voltage divider with R1123 and R1101 - the top of the chain is the 10V reference voltage. The non-inverting input is held at 0V via R1106 - I presume that the inverting input should be about 0V when the -2200V line is at the correct voltage but I get about -1.6V by calculation (it's pretty close to 0V when measured).

Also given that the -2200V supply is DC (well, nominally) I don't understand what the role of C1104, C1103, R1104 or C1102 is.

Could anyone explain in detail what is going on with this supply?

To further add to the confusion in the working 3394A the -2200V rail is about -2015V which is quite a bit off -2200V (but slightly more in line with the CRT datasheet which suggests -2kV is optional for the first accelerator), but I have the 3382 as well which is running at -2245V. The 3382 does have a different CRT (D14-374GH-123) which I can't find a datasheet for but the service manual does not show a different PSU in the 100MHz 'scope.

I'm starting to feel a bit  :-// and  :scared: trying to get my head around this problem, maybe knowing exactly what the EHT voltage is in the working/non working 'scopes will help but I'm not sure - it's going to take me a while to get the bits to do the measurement though.
 

Offline T3sl4co1l

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Re: Odd CRT failure mode
« Reply #18 on: November 24, 2015, 11:08:34 pm »
Looks like V1101 (who came up with these ridiculous designators) is simply controlling DC bias into the oscillator.  Something about V1103-4 (seriously, who?) pulling down on the base bias, when the signal swings sufficiently low, turning it off and allowing the cycle to proceed.  It should be roughly something like this,



where the "swing under and catch" behavior is permitted by V1108 (...gah!).

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Online grumpydoc

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Re: Odd CRT failure mode
« Reply #19 on: November 24, 2015, 11:18:01 pm »
Looks like V1101 (who came up with these ridiculous designators) is simply controlling DC bias into the oscillator.  Something about V1103-4 (seriously, who?) pulling down on the base bias, when the signal swings sufficiently low, turning it off and allowing the cycle to proceed.  It should be roughly something like this,

where the "swing under and catch" behavior is permitted by V1108 (...gah!).

Tim
Yes and your trace matches the one in the notes so that's fine.

I'm less clear on the details of the regulation and why the RC networks around the op amp, especially before trying to tweak the value of R1103 to lower the voltage a bit and see what happens..

 

Offline Ian.M

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Re: Odd CRT failure mode
« Reply #20 on: November 24, 2015, 11:40:23 pm »
Re Vnnnn - the drafting department was obviously still classifying all discrete semiconductors as 'valves'  :scared:

I wouldn't touch R1103 (other than cold testing).  Adding 15K in series with R1123 and R1101  would probably be a better bet to drop the 2200V supply a bit.  Use a 22K trimmer with the wiper wired to one end of the track so it *CANNOT* go open. and you should be able to 'dial an EHT'.
« Last Edit: November 24, 2015, 11:58:13 pm by Ian.M »
 

Offline T3sl4co1l

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Re: Odd CRT failure mode
« Reply #21 on: November 25, 2015, 02:21:19 am »
The RCs are for filtering and compensation.  Without (or with incorrect values), it will most likely oscillate rather than deliver stable regulated DC.  Or be overdamped and too slow, which is probably the side it's erring on.

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Offline TerraHertz

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Re: Odd CRT failure mode
« Reply #22 on: November 25, 2015, 02:23:30 am »
I'm not sure you really got what I meant by this:
Quote
* Wrong shape of post-deflection plates electric field.
Some tubes seem to have a kind of electrostatic lens in the wide portion of the tube, formed by a resistive spiral trace going round and round inside the glass. If that's to provide more deflection 'gain', than maybe if that trace went open circuit the trace would appear squashed. Or... there would have to be an electrical connection to the end of that spiral closest to the gun. Perhaps that went open?

Here's an example of what the 'spiral' I referred to looks like. I've seen this on various tubes, not just this rather old one (a GEC type 1074H, dual beam. No idea what it was from. Just happened to have it handy to take pics.)


One end of the spiral is connected to the EHT (anode) aquadag. The other end has it's own aquadag area down at the tube neck, with finger contacts and an independent connection pin. The final set of beam deflection plates are inside that neck aquadag area, so the beam emerges from the deflection plates directly into the graded E-field created by the spiral resistor.


Here's your schematic, edited with what I *think* is going on, *if* your CRT has a similar resistive field shaper electrode.


The CRT-internal resistor provides both the EHT voltage feedback to the regulator circuit, and also increases the overall CRT deflection gain. It's a bit tricky visualizing how that works. Simplest way to think about it, is the electrons spend more time being deflected radially towards the tube sides, while they haven't yet picked up much extra velocity due to the axial EHT acceleration field.

There is deflection gain due to the Anode EHT, but grading the field via that resistor increases the overall deflection gain.

My crude attempt to illustrate the 'gradient', doesn't have anything to do with E-field direction. It's just E-field intensity. Effectively the tube interior sees field strength proportional to voltage along the spiral resistor on the inside wall of the tube. I have a hard time trying to mentally model the field direction, and I don't have any modeling tools. But I do recall seeing some writeup (long ago) of how that spiral system increased defection gain.

Annnnyway... if that CRT spiral resistor went open circuit at some point, the whole system would be screwed in much the way described. Poor deflection, EHT too high (further decreasing deflection gain), -2200V rail wrong, etc.

Edit to add: try measuring the current at the CRT pin of the -2200V supply. It should not vary (much?) with beam intensity, but if there is an internal resistor in the CRT (many hundreds of Mohms) between Anode and -2200V, you'll be able to see that current. Hence whether it's open circuit on the bad tube.
« Last Edit: November 25, 2015, 02:46:53 am by TerraHertz »
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Offline T3sl4co1l

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Re: Odd CRT failure mode
« Reply #23 on: November 25, 2015, 02:44:25 am »
No, that curve is going the wrong way.  The beam is being accelerated towards the face, increasingly so as it drifts along the lens region.  It should be more of a parabolic curve, starting at a wider deflection angle and straightening out towards the face.

I think it's more a matter of, you get more deflection for a given spot intensity.  In the simple CRT case, the anode is at the same potential as the deflection plates (average), so whatever intensity you get, it's determined by cathode voltage only.

You can't simply amp up the anode in a simple CRT, because the field between the last deflection plates (usually the horizontal pair) and 2nd anode, has a warped shape because it's mostly defined by the edges of the plates, and whatever the aperture of the 2nd anode looks like, at the gun end (usually a proper cylindrical electrode, otherwise internal aquadag).  Indeed, anode voltage is usually variable, near 0V, to provide astigmatism control.

The only way you can smoothly go from the potential on the deflection plates, to a higher potential at the screen, is by adding a shield, or changing the field gradually.

Later Tektronix tubes used the shield: a fine mesh screen, following the last deflection plates, biased by the ASTIG control.  The beam's position, as it passes the screen, is a significant fraction of the total deflection: the path straightens out as it passes down the acceleration region.  The screen is visible by its effect on the "texture" of the display; it necessarily absorbs some beam current, and makes the beam a bit more diffuse.  On the upside, these often used quite high voltages (up to 13kV I think?).

Tubes as pictured, implement a similar gradient, at the expense of DC bias.  The lack of shielding between accelerating region and deflection plates (maybe I'm making some assumptions here, as I don't have an x-ray view handy!) means it won't be perfect (some residual distortion from the deflection plate tips should be present), but maybe the linearity and dot size consistency will be good enough for an o-scope.

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Offline TerraHertz

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Re: Odd CRT failure mode
« Reply #24 on: November 25, 2015, 03:30:44 am »
No, that curve is going the wrong way.  The beam is being accelerated towards the face, increasingly so as it drifts along the lens region.  It should be more of a parabolic curve, starting at a wider deflection angle and straightening out towards the face.

True in a simple case where the beam sees only a linear field between the anode and the deflector plates. My diagram would be more correct if the beam path approaching the anode was as you say, bent the other way. You try drawing subtle beam path curves in photoshop on a small diagram.
Mainly trying to make the point that with the graded field produced by the spiral resistor, the beam in the early part of the tube doesn't see the final anode EHT field yet. So, it spends longer with about it's original deflection (whether increasing or decreasing by the radial field, I'm not confident to argue about that) and so has an overall greater deflection in the graded field.

Quote
I think it's more a matter of, you get more deflection for a given spot intensity.  In the simple CRT case, the anode is at the same potential as the deflection plates (average), so whatever intensity you get, it's determined by cathode voltage only.
What? Deflection is not affected by spot (beam) intensity. Thankfully. Imagine how messed up trace intensify highlighting modes would be if that were the case.
Edit to add: Oh wait, by 'spot intensity' you meant EHT voltage (higher EHT allows maintaining spot intensity despite faster sweep speeds.) Not variations in beam intensity during operation.

Also "anode is at the same potential as the deflection plates"... What? Is that a typo, or did you mean it?

Quote
You can't simply amp up the anode in a simple CRT, because the field between the last deflection plates (usually the horizontal pair) and 2nd anode, has a warped shape because it's mostly defined by the edges of the plates, and whatever the aperture of the 2nd anode looks like, at the gun end (usually a proper cylindrical electrode, otherwise internal aquadag).  Indeed, anode voltage is usually variable, near 0V, to provide astigmatism control.
Wait, by 'anode' you're referring to the final focusing electrodes? Strictly speaking, the 'anode' is the EHT terminal and CRT face, since that's where the electrons end up. Other 'high positive voltage' elements are numbered anodes, but don't take any beam current.

Quote
The only way you can smoothly go from the potential on near the deflection plates, to a higher potential at the screen, is by adding a shield, or changing the field gradually.

Which is what I'm describing.

Quote
Tubes as pictured, implement a similar gradient, at the expense of DC bias.  The lack of shielding between accelerating region and deflection plates (maybe I'm making some assumptions here, as I don't have an x-ray view handy!) means it won't be perfect (some residual distortion from the deflection plate tips should be present), but maybe the linearity and dot size consistency will be good enough for an o-scope.

On that tube, the internals are visible and there's no shielding. The final deflection plates (definitely the horizontal, since the Y plates are twinned due to being dual beam) are in the middle of the spiral low-end aquadag area.
But any distortions are less than you'd get with exposure to the full EHT field, since they are inside that much lower voltage shell. There are some internal planar elements at the same potential as the aquadag, but they are well back from the exit aperture of the H-deflection plates.

But this is all kind of irrelevant. Point is, does grumpydoc's CRT have that kind of internal resistive EHT field shaping? (Which is primarily to achieve higher deflection gain, regardless of fine details of which way the beam curves.)
And if his CRT does have that, is the resistor open circuit?
« Last Edit: November 25, 2015, 03:40:06 am by TerraHertz »
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Offline T3sl4co1l

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Re: Odd CRT failure mode
« Reply #25 on: November 25, 2015, 04:58:29 am »
What? Deflection is not affected by spot (beam) intensity. Thankfully. Imagine how messed up trace intensify highlighting modes would be if that were the case.
Edit to add: Oh wait, by 'spot intensity' you meant EHT voltage (higher EHT allows maintaining spot intensity despite faster sweep speeds.) Not variations in beam intensity during operation.

Yes, not in an operating sense, but as a design tradeoff.  You can make wide deflection, dim CRTs, or narrow deflection, bright CRTs, but doing both is harder.  (Doing both, and with high efficiency in terms of light per beam power, even more so!)

Quote
Also "anode is at the same potential as the deflection plates"... What? Is that a typo, or did you mean it?

"Simple CRT case", like a 5AQP1 or whatever -- a CRT simpler than the ones we're talking about here.

Sorry for the lacking clarity :)

Quote
But this is all kind of irrelevant. Point is, does grumpydoc's CRT have that kind of internal resistive EHT field shaping? (Which is primarily to achieve higher deflection gain, regardless of fine details of which way the beam curves.)
And if his CRT does have that, is the resistor open circuit?

Yeah, definitely, there'll be something funny with the deflection sensitivity, and probably linearity too, if some of that trace burns off.

Got a megger handy?  Or.. maybe a picoammeter? ::)  Not sure offhand what resistance those things are, but I'm guessing it's more than most DMMs read.

Also, if there were a broken spot, would that break down, even in vacuum?  Would it have burned away enough that there's a gap or burn, visible by inspection perhaps?  (I don't know exactly how much clearance is needed in vacuum, but I would think it must be at least sparking if it's dropping a large fraction of the HT across a << 1mm gap in the stuff.  ...Would the sparking be detectable, say by a sensitive amplifier or radio?)

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Online tautech

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Re: Odd CRT failure mode
« Reply #26 on: November 25, 2015, 05:03:34 am »
Would the sparking be detectable, say by a sensitive amplifier or radio?
Hmm
SMPS HV supply and trebler noise is probably all you'd hear.  :-\
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Online grumpydoc

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Re: Odd CRT failure mode
« Reply #27 on: November 25, 2015, 06:46:21 pm »
Quote from: TerraHertz
But this is all kind of irrelevant. Point is, does grumpydoc's CRT have that kind of internal resistive EHT field shaping? (Which is primarily to achieve higher deflection gain, regardless of fine details of which way the beam curves.)

Not that I am aware. As luck has it the CRTs have available data sheets. The 200Mhz 'scope has a D14-383GH/123 and the 100MHz 'scopes have D14-373GH/123 although I found a D14-374GH/123 in the PM3382

Note 5 in the data sheet says that the CRT uses permanent magnets for fine tuning geometry and astigmatism

 D14-383GH: http://frank.pocnet.net/sheets/186/d/D14383-123.pdf
 D14-373GH: http://frank.pocnet.net/sheets/186/d/D14373-123.pdf

I assume that the D14-374 is very similar but I cannot find a data sheet - there are, however, photos including the neck assembly

http://koalakits.net/Tube_Images/D14-374GH-123_FP.jpg
http://koalakits.net/Tube_Images/D14-374GH-123_2_FP.jpg

Quote
And if his CRT does have that, is the resistor open circuit?

Don't forget I have tried two CRTs so a fault in both is unlikely.

Quote from: T3sl4co1l
The RCs are for filtering and compensation.  Without (or with incorrect values), it will most likely oscillate rather than deliver stable regulated DC.  Or be overdamped and too slow, which is probably the side it's erring on.
OK, good - that makes sense.

I still don't quite understand the divider there though - the total resistance is 31.7666Mohm and the voltage should be 2210V - 2210*(31.6/31.7666)  is 2198.41 so the top of R1103 should be at -1.59V. However it is connected to the non-inverting input so, given that the inverting input is at 0V the top of R1103 should be a virtual ground.

Quote from: Ian.M
I wouldn't touch R1103 (other than cold testing).  Adding 15K in series with R1123 and R1101  would probably be a better bet to drop the 2200V supply a bit.  Use a 22K trimmer with the wiper wired to one end of the track so it *CANNOT* go open. and you should be able to 'dial an EHT'.

I might try that. R1103 seemed an easier target as it is through hole and on the "large component" side of the board whereas R1123/R1101 are SMD on the other side of the board crammed in with all of the other stuff around the op amp. However there aren't too many values to choose from in VR37 (which is the Vishay high voltage range used) - 30, 31.6 and 33Mhom and only DigiKey stock all three.

I agree about making sure the wiper can't go open but there is also protection from V1100 and V1105 which clamp the non-inverting input to +/-11.6V or so.
 

Offline Ian.M

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Re: Odd CRT failure mode
« Reply #28 on: November 25, 2015, 06:58:31 pm »
In that case *IF* you decide you need to tweak the EHT for testing, its probably easiest to desolder the SMD R1123 and patch in the preset and an 18K resistor, soldered to a scrap of perfboard.  I'd keep the wiring short and twisted together to minimise the risk of instability, and due to the small pads you'd be soldering to, I'd probably choose Kynar wirewrap wire.
 

Online grumpydoc

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Re: Odd CRT failure mode
« Reply #29 on: November 25, 2015, 09:24:20 pm »
In that case *IF* you decide you need to tweak the EHT for testing, its probably easiest to desolder the SMD R1123 and patch in the preset and an 18K resistor, soldered to a scrap of perfboard.  I'd keep the wiring short and twisted together to minimise the risk of instability, and due to the small pads you'd be soldering to, I'd probably choose Kynar wirewrap wire.
Interesting - just about to look at getting a trimpot in there and I notice that they have been playing with the values of R1123 and 1101. The original 338x and 339x had 19.6K and 121k, the "A" had the same and then the "B" has 19.6k and 147k.

Of the two PSUs that I have out the original 3394 one has the values as per the schematic whereas the 3394B one has 11k and 147k

Curious.

HighVoltage is sending me another PSU to play with - it will be interesting to see what values it has.
 

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Re: Odd CRT failure mode
« Reply #30 on: November 25, 2015, 11:06:11 pm »
I replaced R1123 with a 50k multi-turn pot mounted on the top edge of the PSU board with a bit of hot snot.

Tweaking the cathode supply down to -2150V allowed the calibration marks to be aligned with the graticule and the autocal runs past the point that it gave up before (it still runs aground later on but HighVoltage thinks that is a problem on the digitiser board). I need most of the gain from the final X/Y amps - perhaps coming down another 50V would help.

The CRT isn't especially bright and I can't quite adjust the grid bias/dark level but that might just be the CRT that is in the 'scope which is the one from the 3394B. The one originally in this chassis was better.

I suppose that I could have a look where the trimpot is now set and replace the 19.6K resistor with that value. I don't much like that as a solution - too much like a hack but it is a solution.

I suppose the fact the Phillips themselves put so many different values here makes it more acceptable.

 

Online grumpydoc

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Re: Odd CRT failure mode
« Reply #31 on: November 29, 2015, 01:24:15 pm »
The PSU from HighVoltage turned up yesterday

Its cathode supply is around -1950V which is interesting. The CRT is bright with this PSU so there is probably another problem with the other "B" series PSU that I have. R1101 and R1123 are 147k and 11k which is different to the B series service manual that I have but the same as the other B series PSU.

I'm now pretty certain that Philips lowered the cathode supply in the "B" series 'scopes (and maybe, in practice, some A series instruments) but did not fully/correctly update the service manual to reflect the change. We go from -2200V/140k (121k+19.6k) in the original series to -1950V/158k (147k + 11k) in the "B" - the PSU I modified is sitting somewhat in the middle with a total resistance of 150k (121k + 30k on the trimpot) and cathode supply of -2100V.

I'm not sure why they might have done this - perhaps they had problems with deflection sensitivity. I am also not at all clear why the combination of the CRT in the original 3394 and the PSU from the same chassis would not work.

However it does make me comfortable in increasing the resistance at the top of the R1123/R1101/R1103 divider to something in the region of 150-160k

Just need to fix the A8 boards now (hopefully)
 

Offline justanothercanuck

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Re: Odd CRT failure mode
« Reply #32 on: November 30, 2015, 01:19:21 am »
Over time remaining things break down inside the tube and adhere to the internal structures causing electrical leakages internally. Additionally barium can be evolving from the electron gun and causing problems

can't you tilt the tube vertically and blast the guns with a crt tester?  should fry that stuff off without damaging the tube phosphor.  :-//
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