Author Topic: (classic) Tektronix 502 scope repair / restoration  (Read 15344 times)

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Offline tautech

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #75 on: November 16, 2024, 10:14:21 pm »
FYI, a great PDF on phosphor types first posted here by rf-loop is attached.
Enjoy.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #76 on: November 17, 2024, 12:53:20 pm »
Wow that's a looooot of CRT's in there !  :o
I guess when it's the CRT era, you do everything you can think of with CRT's.....

I started work on the intensity balance issue.

Tek was nice, it's all very tidy and organised in there, and the intensity control circuitry has its own dedicated area, its own set of ceramic strips.
Only Problem is that it's partially obfuscated by the big handle bar...



So I removed it. Excellent access now !



It was not that hard... I should have done it when I washed the scope, would have made my life a tad easier and I would have been able to clean that area easily. Never too late though, so I proceeded to do just that... clean the tiny bit of grime that escaped the wash.



Of course I was as frugal as possible... I simply sprayed one squirt of cleaner directly into the bristles of a soft brush, then lightly wiped the grime off the ceramic strips and components. Then wiped the excess with a kitchen towel.



Then I started investigating the issue.

I noticed there was a bare wire that was running underneath the trimmer uh.... I lifted the trimmer to have a better look, and confirmed the "issue"... much more of an issue when you see that the trimmer has a huge, long screw sticking out of it, very much likely to be touching that wire !
So I pushed the wire as far down as I could, so that it clears that screw.





Then I measured the value of the trimmer, because there are two manuals for the scope, and early ones are supposed to have a 1M trimmer and the later ones like mine, a 3.5M one.

Mine measured at.... 400K !  :o
I don't know it's that's intended or not, could not find markings on the trimmer that looked like a resistance value.
I can't be 100% sure whether it's normal or not, because I noticed this scope does NOT stick to the manual !
They modified the circuitry like this I think.. I tried to make a drawing of it :




As a reminder, here is again the official schematic :




So basically instead of having each beam be controlled by each end of the trimmer, what they did is wire the trimmer with only 2 pins, as a simple variable resistor, and it controls only one of the two beams. For the other beam, they replaced the trimmer with a fixed resistor of 1.8M, which is about half of the 3.5M value that this trimmer is supposed to have... hint hint... so maybe that trimmer measuring at 400K is not normal. I don't know.


Then I powered up the scope to see if lifting that trimmer away from that wire underneath it, would make any difference at all.
It certainly did : I got sizzling noise and a few pops and tiny flashes... then the CRT losing life. Pulled the plug... maybe the kitchen towel did not dry the couple drops of cleaner well enough then...  :palm:

So I tried to be more thorough this time. I shoved a micro fibre towel through the components, to reach the surface of the deck, helped with a screw driver.
Then blew some hot air using the hot air rework station.
Then powered up the scope again. Zero life on the CRT.... I killed that scope again !  :palm:

Told you, I should not be allowed to work on these scopes.... still, I want to believe that overall he is still better off in my company.. the alternative would have been the junk yard. At least with me he got a good clean, lots of diagnosis and repairs, and a nice warm dry place to live in.
So I might not be perfect, but I am still much better than the other option...  No, not trying to justify my mistakes, just trying to feel less bad about them  :-//

OK so I hope I can fix it and make it go again....  :-//
« Last Edit: November 17, 2024, 01:13:39 pm by Vince »
 
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #77 on: November 17, 2024, 03:33:58 pm »
Looking at my schematic, the trimmer needs to be at least 1.8M in order to match the fixed 1.8 Tek put in the other branch of the circuit, so as to get a balanced intensity. Plus, of course it needs to be able to go above 1.8M to be able to lean the other way. So.... that means that trimmer really is supposed to be 3.5M like the manual states... so the measured 400k just is not right.

What could possibly make it measure so much lower ? Boggles the mind.
Idea : since we established that the scope was most likely the victim of a heavy smoker... and since HV attracts particles... maybe the track of the trimmer is covered with a layer or cigarette smoke, contaminating it, lowering its resistance ?

I will unsolder that trimmer, try to take it apart non-destructively and see if cleaning the track restores the nominal resistance...

« Last Edit: November 17, 2024, 03:35:38 pm by Vince »
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #78 on: November 17, 2024, 04:48:22 pm »
Then powered up the scope again. Zero life on the CRT.... I killed that scope again !  :palm:


Maybe something just reached it's "best before" date.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #79 on: November 17, 2024, 05:49:14 pm »
I don't know...

I checked all 4 power supplies : -6.2V, -150V, +100V and +350V = all OK
That's good.

Then dug out my HV probe. I do get HV so that's good, the HV transformer, oscillator and rectifiers and HV disc caps, are also good, that is.

Manual states cathode(s) voltage should be "about" -2900V and I measure -3,000V, on both of them, close enough I guess.

Then measured the grid(s) voltages / intensity voltage for both beams. manual states "about" -3100V (depending on the setting of the intensity control) , i.e 200V lower than the cathodes.
I measured a lot less than that : almost -3,600V  Say I got -3,550 initially, then after a bit (warming up, I don't know), it got down to -3,575V

Unlike what the manual states, that voltage does not vary depending on the (main) intensity control knob on the front panel. At all.
I checked that pot, looks alright to me : measures 25% high at 2.5M instead of 2M, not the end of the world. Resistance varies smoothly and linearly from lock to lock, as it should.

I played with the intensity control on the front panel, while measuring the grid voltages. Doing that, I can see voltages vary between about -3,200V (intensity control set to max) and -3,450V (set to min). So that means the grids are at least 200V lower than the cathode. Would that be enough to reduce intensity to the point of not seeing the beams on the display ?

I will try to adjust the HV from 3,000 down to the indicated 2,900, see if that changes anything... I guess not, since grid voltage would also move in unison, but well...

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 
Of course the bad news is that since voltages look about fine, that means maybe I killed the CRT...  :palm:
I hope not...
I guess the worst that could have happened during that brief fireworks in the intensity control part, is the grid voltage going way less negative than they should, like... zero volts potentially... but if CRT's are like other tubes, a grid at zero should be perfectly fine and sustainable, no ?

Oh my....  :-BROKE



 

Offline tautech

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #80 on: November 17, 2024, 06:18:40 pm »

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 

Yet it may have died on its own.

Post the CRT circuitry for our study please as your reported voltages don't seem right.  :-//
« Last Edit: November 17, 2024, 06:30:46 pm by tautech »
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Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #81 on: November 17, 2024, 06:37:13 pm »
So what have you possibly done, pulled those disk capacitor sides closer together?
How long that sizzling noise lasted?
You have those bent CRT pins.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #82 on: November 17, 2024, 06:50:59 pm »
Post the CRT circuitry for our study please as your reported voltages don't seem right.  :-//


Attached.

I desoldered the trimmer. My in situ measurement was wrong it seems... it's not 400k anymore, but 3.6M as it should, and working smoothly. So I put it back.
Checked every resistor in the circuit, all fine. Many read are a bit high, but like 5% high only, so within spec (10%).
The two 15M resistors in each branch of the circuit, read 17M, so again within spec, and since they read the same they aren't introducing any imbalance of their own.

Just as a sanity check, I disconnected all wires from the deflection plates of the CRT, and shorted the terminals, just to make sure the spot was not just lost off screen. It's not, still no spot to be seen.

« Last Edit: November 17, 2024, 06:56:53 pm by Vince »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #83 on: November 17, 2024, 07:01:58 pm »
So what have you possibly done, pulled those disk capacitor sides closer together?

Not sure what you mean ?
The pairs of disc caps ? Tehy are already wired in  // so making them closer will not change anything I would guess...

How long that sizzling noise lasted?


A few seconds, 5 at worst I would guess...

You have those bent CRT pins.

Only one bent pin, and only affecting one beam... unless you mean that as envisaged, when I straightened it, I created a microcrack that compromised the vacuum...  yeah.
I don't see the relation to the sizzling in the intensity control circuit, but maybe the two events, though unrelated, happened at the same time...
I guess I should try swapping the CRT and see for myself....
 

Offline tautech

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #84 on: November 17, 2024, 07:39:04 pm »
I don't know...

I checked all 4 power supplies : -6.2V, -150V, +100V and +350V = all OK
That's good.

Then dug out my HV probe. I do get HV so that's good, the HV transformer, oscillator and rectifiers and HV disc caps, are also good, that is.

Manual states cathode(s) voltage should be "about" -2900V and I measure -3,000V, on both of them, close enough I guess.

Then measured the grid(s) voltages / intensity voltage for both beams. manual states "about" -3100V (depending on the setting of the intensity control) , i.e 200V lower than the cathodes.
I measured a lot less than that : almost -3,600V  Say I got -3,550 initially, then after a bit (warming up, I don't know), it got down to -3,575V

Unlike what the manual states, that voltage does not vary depending on the (main) intensity control knob on the front panel. At all.
I checked that pot, looks alright to me : measures 25% high at 2.5M instead of 2M, not the end of the world. Resistance varies smoothly and linearly from lock to lock, as it should.

I played with the intensity control on the front panel, while measuring the grid voltages. Doing that, I can see voltages vary between about -3,200V (intensity control set to max) and -3,450V (set to min). So that means the grids are at least 200V lower than the cathode. Would that be enough to reduce intensity to the point of not seeing the beams on the display ?

I will try to adjust the HV from 3,000 down to the indicated 2,900, see if that changes anything... I guess not, since grid voltage would also move in unison, but well...

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 
Of course the bad news is that since voltages look about fine, that means maybe I killed the CRT...  :palm:
I hope not...
I guess the worst that could have happened during that brief fireworks in the intensity control part, is the grid voltage going way less negative than they should, like... zero volts potentially... but if CRT's are like other tubes, a grid at zero should be perfectly fine and sustainable, no ?

Oh my....  :-BROKE
Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....
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Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #85 on: November 17, 2024, 07:41:15 pm »
A negative grid-cathode voltage controls the electron beam current, and avoids grid current.
A positive grid-cathode voltage would "incite" even more beam current, but substantial current would flow in the grid circuit and affect the grid-driving network.
If you must measure the grid-cathode voltage, it is quite dangerous since you want to connect a voltmeter between the grid and cathode, both of which are at a negative high voltage with respect to ground.
Measuring the two voltages from ground separately, with a reasonable voltmeter resistance, will not give an accurate value for the difference.
« Last Edit: November 17, 2024, 07:43:22 pm by TimFox »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #86 on: November 17, 2024, 08:10:24 pm »
Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....

Didn't know that. I thought CRT's were just tubes, and tubes give their full electron flow just fine with the grid tied to the cathode / same potential.
Never saw them needing the grid to be higher than the cathode. Strange. You lost me...
Every tube datasheet I have looked at give current flow being maximum when the grid is at the same potential than the cathode, not higher.
Then if you want to restrict the electron flow you make the grid negative relative to the cathode. The more negative it is, the lesser the current.

I don't understand what you mean  :-//



 

Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #87 on: November 17, 2024, 08:23:06 pm »
Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....

Didn't know that. I thought CRT's were just tubes, and tubes give their full electron flow just fine with the grid tied to the cathode / same potential.
Never saw them needing the grid to be higher than the cathode. Strange. You lost me...
Every tube datasheet I have looked at give current flow being maximum when the grid is at the same potential than the cathode, not higher.
Then if you want to restrict the electron flow you make the grid negative relative to the cathode. The more negative it is, the lesser the current.

I don't understand what you mean  :-//

In a CRO with CRT, varying the grid-cathode voltage (always negative) changes the beam current and therefore the brightness of the trace.  It is controlled by the "brightness" panel potentiometer.
In vacuum tubes in general, going from zero grid-cathode voltage to positive grid-cathode voltage will increase the cathode current further, but some of that current flows through the grid.
Some vacuum tubes' ratings exclude positive grid voltage, due to excessive current through the relatively fragile grid.
However, high-power tubes used for RF power amplifiers often run with substantial positive grid voltage.
Here is the datasheet for a 6C4 general-purpose triode:  https://frank.pocnet.net/sheets/093/6/6C4.pdf
Note the two graphs of characteristic curves on p. 3 of the datasheet, for positive and negative values of "Ec", the grid-cathode voltage.
The max specs give a limit on negative grid voltage, and another limit for grid current.
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #88 on: November 17, 2024, 08:28:00 pm »
A negative grid-cathode voltage controls the electron beam current, and avoids grid current.
A positive grid-cathode voltage would "incite" even more beam current, but substantial current would flow in the grid circuit and affect the grid-driving network.

Do you mean to say that as I suggested earlier, maybe during the "accident", the grid potentially having gone to zero volts or something, might have induced a high current grid, hence might have destroyed it/them ?

If you must measure the grid-cathode voltage, it is quite dangerous since you want to connect a voltmeter between the grid and cathode, both of which are at a negative high voltage with respect to ground.
I don't see the need to do that, but call me stupid (I am) but if I measure that voltage using a handheld/ battery powered/ floating voltmeter, how would that ground reference matter to the meter ?

Measuring the two voltages from ground separately, with a reasonable voltmeter resistance, will not give an accurate value for the difference.

I don't understand ?  The HV probe is rated for 40kV, 1:1000 ratio, 10G resistance.. it draws bugger all current so it should not load the circuit enough to give a false reading I would think ?!  :-//

Still, the voltages indicated in the manual aren't even taken with a 10G probe, they only talk about a "voltmeter", with no particular indication.

Anyway, from what you say it's likely I damaged the grids, so will try swapping the CRT...


 

Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #89 on: November 17, 2024, 09:57:19 pm »
Actually, the "grid" in a CRT is usually not a physical grid, but an electrode with a hole in the center.
If you accidentally shorted it to ground, that might damage it--I've no experience with that.
It might damage the components in the brightness control circuit.

Even though your HV probe has a high resistance, the effective resistance of the voltage divider for grid bias ("brightness") is high and will shift with the probe loading more than the cathode voltage.
In general, deriving a small quantity by taking the arithmetic difference of two very large quantities is bad for accuracy.

A "handheld" voltmeter would measure the grid-cathode voltage accurately, but the entire multimeter package would be at a dangerously high potential--do not hand-hold it!
 

Online George Edmonds

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #90 on: November 17, 2024, 10:00:59 pm »
Hi Vince

I cannot recall the last time I read a topic containing so much incorrect information.

As was standard at the time the scope was manufactured ALL voltage measurements were made using a 20K/V meter, for example a Simpson 260.

Your chances of having damaged the CRT are very low to none existent unless you have caused mechanical damage to it.

 Your biggest problem is likely to be the Allen Bradley resistors as they are hydroscopic. They also ALL drift high in time.

G Edmonds
 

Online George Edmonds

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #91 on: November 18, 2024, 04:44:20 am »
Hi Vince

The CRT used in the 502 is what is known as a dual gun one, ie there are two independent electron gun assemblies in one CRT envelope.  This was quite common for a time in the 1960’s with most scope manufacturers using such CRT’s, for example the Telequipment D43.

Now I have no intention of giving a full description of how CRT’s work other than to provide the information that will help you.  The 502 series used a considerable range of Tektronix CRT’s some of which I can find data for and some I cannot, it may help if you can provide the Tektronix part number of the one you have, in this posting I will be using the data for a CRT used in the 502A as I have it.

With one exception the actual CRT voltages are not critical as most are adjustable, the 502 uses a Cathode voltage of some 3KV, this exact voltage in not critical, the voltage that is critical is the Cathode to G1 voltage as this controls the intensity of the electron beam, the G1 voltage MUST always be negative with respect to the Cathode, most CRT manufacturers call the voltage that causes complete extinction of the electrum beam the pinch off voltage.  With the 502A CRT the pinch off voltage looks to be about -95V with respect to the Cathode so that any G1 voltage between say -90v and say -40V should show a CRT trace or spot.  Be aware that a very bright spot or trace can and will burn the CRT phosphor if displayed for more than a very short time.

The simplest way of measuring the G1 voltage is to do it directly with a BATTERY powered hand held DMM, however NEVER hold the DMM or touch the probes whilst the scope is turned on.

I am going to try to attach some Allen Bradley information to this posting, it is very relevant information.

G Edmonds


 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #92 on: November 18, 2024, 01:35:44 pm »
So wet resistor values are more than dry values.
But can they be so wet this time that drying is noisy.

CRT beam I understand so that more negative grid is more exporting electrons.
But what high negative potential means, is it more negative than low negative potential?
No positive is present here.


When cathode is heated enough it start emitting electrons, letting them loose from heated material.
(cold cathode is not heated, sort of)
But nothing there is regulated in any other way than how much those loose electrons happen.

Between cathode and grid those freed electrons are bouncing around.
That continues until something catches then or they pass the grid.


When grid material is negatively charged it pushes electrons away and more when charge is more.
If negative charge is enough it also start emitting electrons, what obviously is not an intention.
So grid pushing electrons away is an electron lens.


If basic level of freed electrons in vacuum tube is so high that operation is overshooting it's possible that grid must be positive.
But if electron lens is also needed it must be another grid.
« Last Edit: November 19, 2024, 02:47:13 pm by m k »
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Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #93 on: November 18, 2024, 02:45:10 pm »
No:  the relevant control voltage is Vgk, the voltage on the grid with respect to the cathode.
Just as in any vacuum tube, a negative Vgk reduces the cathode current.
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #94 on: November 18, 2024, 04:01:55 pm »
So grid voltage over chamber to cathode is a closed loop and is sucking freed electrons back to cathode, more than anode.
Then I don't understand the lens.

Maybe I don't understand what negative means.
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Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #95 on: November 18, 2024, 04:16:28 pm »
No.
The negative grid voltage affects the electrostatic field at the cathode, reducing the number of electrons that can exit the hot cathode.
This was all worked about by Child and Langmuir over one hundred years ago, and is in all the textbooks.
Lacking a textbook, try googling "triode".
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #96 on: November 18, 2024, 06:16:30 pm »
So grid level is under cathode level and is partially guiding electrons to anode and partially to another "anode" somewhere around a circuit of grid and cathode.
So stream of electrons is turned to two streams.
So mechanical structure decides what part is for what function.
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Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #97 on: November 18, 2024, 06:39:23 pm »
If you are interested in this, you should look for information on the "electron optics" of CRT electron guns.
Note that the control grid in electron guns is often called a "Wehnelt", and is not the same shape as the grids used in vacuum tubes for amplifiers.
Above all, you need to understand the physics behind thermionic emission and space-charge limited current in vacuum tubes.
 

Offline Wallace Gasiewicz

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #98 on: November 18, 2024, 11:34:55 pm »
The cathode in a CRT emits electrons based on the voltage potential.  This would result in a valve "Diode" if it was not controlled with all power available transferred thru the tube to the Anode.  The grid changes the electrostatic environment in the tube and limits the cathode emission.  The grid voltage is about 60 volts Negative compared to cathode.     
The idea is to get the grid voltage to a level so that at minimum brightness, there is just a barely visible trace or spot on the screen. You then increase the brightness control to make the electron beam stronger. The voltage on the grid is maximum Neg with the beam almost turned off.     
This actually regulates the electron emission of the Cathode or "Electron Gun" rather than absorbing or otherwise steering electrons elsewhere.     
The other plates in the CRT "steer" the electron beam to make a trace.   

A Triode tube does the same thing and the grid is Neg with respect to the Cathode usually.  A "Self Biasing" triode uses a resistor from the Negative supply to the Cathode to make the Cathode more Positive with respect to the Neg supply, which is usually chassis ground. That way the Neg supply  is more negative than the cathode and can give you a neg supply (with respect to Cathode) for the grid.   

Without a Negative Bias on the Grid the Triode tube will act as a Diode and pass all the current it can in ONE direction.  There is a reason people call these things "Valves" which I think is more helpful than calling them "Tubes"  (but I speak "United States" rather than English)

I realize this is a simple minded explanation and apologize in advance. I am leaving out the inter electrode capacitances and probably other things that I have either forgotten or am ignorant of.
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #99 on: November 19, 2024, 02:45:31 pm »
I was around CRT monitors for decades, but unfortunately never really "inside" a tube.
And no wonder my understanding is a bit off when many times everything is anodes without voltages.

One example has positive voltages after G1, maybe it's an error.
It means that all electrons away from center of G2 are attracted by G2 and go more away from the center.
Much more rational it would be if G2 voltage is negative, then it would collect electrons towards the center.
This example has G2 as pre-accelerating anode, maybe its mechanical shape can change the situation.

I remember that long long ago I tried to understand how accelerating anode is actually accelerating, but it was left at that.
Seems that my understanding also includes more parts between cathode and G2 than there really are.
No wonder either when some use general name grid and other anode, and then some has much more Gs and none has voltages.

But it's Vince who's CRT must be made clear and simple.
The control grid hole is not a lens, the hole is a pinhole like a pinhole camera, but energy goes to other direction.
And yes, control grid level is under cathode, is it a must, can't say, maybe it is not, but pinhole start acting differently.
It's also possible that control grid is emitting some rouge electrons, but that's not important and mainly it's a static repelling plate.
When G1 is repelling it's also repelling the cathode, so some electrons are not emitted when repelling force is low.

If G1 has no voltage at all the surface of it is not repelling nor attracting, so it's merely a mirror.
Can't say how that actually behaves, but for sure it's different to situation where voltage is much above what cathode has.
Maybe different is a wrong word, maybe it's just weaker, there's still a route for those electrons, so maybe some electrons are pushing G1 voltage down.

When electron bundle exit from control grid its shape is like over bent umbrella.
So before the spread is too much something must start collecting it towards the center.
But maybe that is for later.
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