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Old 4th Jul 2013, 3:14 pm   #61
davidgem1406
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Default Re: Tektronix 465B Scope - Some problems.

Hi Bill,

If you mean the A/B sweep generator circuitry then no I have not done that, I am waiting for a proper copy of the 465B manual to arrive.

This is the progress so far:

With X and X plates disconnected and +30V applied to them from an external PSU.
Astig set to full C/W, focus set to the best point (Image_x1)
Any movement of either the astig or focus just increases the distortion.
The spot really is not a spot it is just distortion that can be made bigger or smaller.

Removing the +30V leaves things much the same but the astig control needs to be full A/C.

I then reconnected the X plates.

With the time base running at 0.5mS:
The best focus we can get (Image_x2)
Focus at full A/C and astig about 3/4 A/C (Image_x3)
Astig now moved to full C/W (Image_x4)
The effect of moving the focus towards C/W (Image_x5)

Other than checking any components around the focus and astig circuits I don't know where to move to next!

Dave
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Old 4th Jul 2013, 9:34 pm   #62
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Default Re: Tektronix 465B Scope - Some problems.

With the deflection plates shorted and on an external supply, then there is no deflection happening as a result of any sort of noise on the amplifiers.

Maybe some electrode along the tube is at a wrong potential, but otherwise it begins to look like the tube. I have some spare HP tubes, but nothing for Tek.

You need the manual in order to know what all the electrode voltages should be. You'll need an eht probe too.

This tube might have a "Helical PDA" where there is a spiral resistor deposit around the inside of the tube between two different potentials of electrodes, and the thing gives an artfully graded electric field intensity. Thee things work as electron lenses after the deflection stages. Sometimes the helix can fail open part way along.

David




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Old 4th Jul 2013, 10:08 pm   #63
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Default Re: Tektronix 465B Scope - Some problems.

I'm not a scope repair guru but could you try using a camcorder/digital camera to film it and then adjust the manual settings in the camcorder to try and see if you can capture an alias effect to see if the spot is actually a smaller spot that is being deflected over time. I would guess you would have to do this at low intensity to get the best visibility of any deflection over time.

Last edited by G0HZU_JMR; 4th Jul 2013 at 10:22 pm.
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Old 4th Jul 2013, 11:03 pm   #64
davidgem1406
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Default Re: Tektronix 465B Scope - Some problems.

Hi David,

Yes I have to check on the CRT voltages but am waiting the arrival of the manuals on CD. I will be able to check up to 3KV beyond that I have no means to do.

I wont be giving up on the tube as yet.


Hello G0HZU_JMR,

Thanks for your input but unfortunately I'm not understanding what you are saying. Can you enlarge on it please.
Also the only video camera I have is in my phone or as part of my Fuji DSLR


Thanks
Dave.
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Old 4th Jul 2013, 11:17 pm   #65
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Default Re: Tektronix 465B Scope - Some problems.

Hi Dave, I'm not really sure if my suggestion is a useful or valid one but the camera should sample the trace in the same way a camera samples the way a car's wheels move and if you catch a low frequency alias term in the camera sampling then you get the impression the car's wheels are moving slowly or even backwards. We have all seen this effect on TV for example. Sometimes you can see it on a propeller on a plane if it is filmed. You can see they are slowly 'rotating' rather than just seeing a blur.

Maybe you could see the same effect on the trace spot if you filmed it and played with the camera to find an alias effect. i.e. you might see a small trace spot rotating slowly to create the bigger spot.

But despite my limited experience in tinkering inside scopes I agree with others in that the tube itself may well have a problem.
Regards
Jeremy

Last edited by G0HZU_JMR; 4th Jul 2013 at 11:25 pm.
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Old 4th Jul 2013, 11:18 pm   #66
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Default Re: Tektronix 465B Scope - Some problems.

An AVO with a 3kV range?

If so, I'm afraid the current it takes could be a problem, these voltages are usually measured with a long string of big resistors working into a 10 megohm input electroic meter. It's usual to use an EHT probe capable of 30kV just to measure a couple of kV or less.

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Old 4th Jul 2013, 11:29 pm   #67
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Default Re: Tektronix 465B Scope - Some problems.

I still suspect the focus and/or the astig supplies may have excess ripple on them. It is possible these voltages are generated using a high frequency inverter so the focus voltage could have ripple of 20kHz upwards.

Al
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Old 4th Jul 2013, 11:45 pm   #68
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Default Re: Tektronix 465B Scope - Some problems.

Not sure if the camera could detect an alias effect if it runs that fast but it might also be possible to fish for an alias/beat effect by injecting a function generator into the regular scope Y input and playing with the frequency and amplitude to see if it ever beats with the deflection rate caused by the ballpark 20kHz ripple (assuming there is ripple).
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Old 5th Jul 2013, 12:07 am   #69
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Default Re: Tektronix 465B Scope - Some problems.

All the anodes, grid, cathode, mesh, pda potentials come from the same inverter via a multi-tapped potential divider, and a multiplier for the PDA. THere is usually a pair of windings to allow control of the DC between grid and cathode for DC coupled Z-mod. If it goes ripply you most usually see moving 'strings of beads' which can easily be resolved by timebase sweeps.

This hasn't shown up, also joining plates has removed the possibility of ripple on power supplies deflecting the spot....

Now, if the fuzzy spot is caused by power rail ripple, the movement will be repetitive, and there will be a correlation between the effects on X and Y. We would see photos of a shape in the best-focused spot, not a regular fuzz. What's more, steady adjustment of the timebase speed would resolve the movement in a free-running sort of way. No camera needed, this thing is an oscilloscope and can do an awful lot to analyse its own symptoms.


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Old 5th Jul 2013, 12:20 am   #70
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Default Re: Tektronix 465B Scope - Some problems.

Dave, you wont get a proper reading testing the cathode voltage with an Avo, you need a high voltage probe as Radio Wrangler suggests. The high impedance of these supplies means that the loading will drag the true voltage down and show a false reading. The cathode voltage should be -2450v. You posted that you measured it at about -2500v with the Avo, which means that it may be much higher than it should be. Maybe there is some member of this forum who lives a bit closer than I, who may be able to assist you with a decent probe. Just while you set it up properly.
In the meantime, can you tell me , when the intensity control is set to minimum, can you still see a trace or not?
Try the set eht voltage pre-set, and reduce the cathode voltage and see if the reduction improves the spot image and focus control, note the original setting.
Personally, I don't think its the tube, but we have to keep an open mind!
Bill
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Old 5th Jul 2013, 1:17 am   #71
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Default Re: Tektronix 465B Scope - Some problems.

For a bit of fun I dug out my camcorder and charged it up a bit and used it on my 465 having set it to rotate the spot in a circle at an audio rate to draw a 'bigger dot'.

I then turned on the camera and played with the manual settings and soon got it to show a single spot rotating very slowly even though the human eye saw a solid circle.


It took me just a few seconds to get this info because I simply whizzed through the manual settings and could see the alias effect quickly appear in the LCD display so I knew instantly that the fat dot was drawn in a circle over time
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Old 5th Jul 2013, 8:10 am   #72
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Default Re: Tektronix 465B Scope - Some problems.

I've not played with the z input on a 465 but it might also be possible to look for an alias effect by connecting a pulse generator into the z-axis input in the rear panel. This might prove a useful diagnostic method on a single spot that is moving in x and y because the sampling effect of Z could appear to slow down the XY movement making it visible to the human eye. I'll have a play tonight but I've no experience of the Z input so this might not work...
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Old 5th Jul 2013, 1:19 pm   #73
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Default Re: Tektronix 465B Scope - Some problems.

Hi Jeremy,

Thanks for the explanation, I see what you are saying now, however it is rather precluded as I don't have a camera capable of doing that.

I do now have to look further into the focus and astig supply rails but need the manuals to arrive before I can attempt that.

As for the tube supply voltages from what has been said I have no way to check any of those. I don't have electronic meter or HV probe.

Hi Bill

Quote:
Originally Posted by maninashed View Post
In the meantime, can you tell me , when the intensity control is set to minimum, can you still see a trace or not?
I will check that out and get back.

Dave.
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Old 5th Jul 2013, 1:44 pm   #74
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Default Re: Tektronix 465B Scope - Some problems.

Dave;

There are a few crt electrode voltages you can check to eliminate them from the enquiry!

1. Pin 7 (= Pin 4 on connector P4035): c. +33v dc: also check no ac present.
2. Pin 5 ( ditto Pin 1? =wiper of astig): runs from 0 to 110v dc: check no ac.
3. Pin 8 (ditto Pin 3): +55v dc, and again check no ac present.
4. Pin 12 -150v dc (high impedance, check with 10M probe).
Finally, there appears to be a control R4059B labelled "Focus Tracking", which is a second gang on the intensity pot? This 2.5Mohm variable is in series with the focus pot. Check it's OK (i.e. measures 0 to 2.5Mohm as the brightness is rotated to full scale ('scope off of course!) By implication, the value of R4127B needs to be correct (25.6 M ).

John

Last edited by John_BS; 5th Jul 2013 at 1:53 pm. Reason: Bullet 4 added
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Old 5th Jul 2013, 3:32 pm   #75
davidgem1406
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Default Re: Tektronix 465B Scope - Some problems.

Hello Bill,

The answer to the earlier questions:
With the intensity control at minimum no trace to be seen.
Moving the "CRT Bias" (I cant see a "Set EHT") this has no effect other than to increase or decrease the intensity.


Hi John,

Thanks, I will check the voltages that I can do.

I know of the "Focus Tracking" pot but can only see 1 connection to it, the one between that and the focus pot.
Again I think this will have to be done when the manuals arrive and I am able to trace out the PCB.

As for R4127B I think this is the thick film resistor at the other end of the focus pot. I did check that and if I remember it was 26+ Meg, did not have a figure for the value at the time so will do it again.
The focus pot was as it quotes on the back.

Will return with the results.

Dave
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Old 5th Jul 2013, 4:12 pm   #76
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Default Re: Tektronix 465B Scope - Some problems.

Quote:
I know of the "Focus Tracking" pot but can only see 1 connection to it, the one between that and the focus pot.
Yes, the Tracking pot is wired wiper-to-one-end, and that end is 0v chassis. So if you connect an ohmmeter to the wire between the focus & tracking pot, it should measure 0 to c. 2M5 to chassis.

John
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Old 5th Jul 2013, 10:32 pm   #77
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Default Re: Tektronix 465B Scope - Some problems.

Hi John,

Thanks for the CRT drawing.

The results of the tests.

Focus control 5.07Meg
Focus tracking 2.33Meg (Within 10%)

Resistor R4127:

From the terminal @ J4325 end to the other end @ TP4127 measures 22.20Meg.
Then from TP4129 end to the focus pot connection 6.57Meg
From the next terminal to the end of the resistor @ J4325 body and the focus tracking pot 23.49Meg
The focus and focus tracking pots were disconnected from R4127 for these checks.

Voltages @ P4035

Pin 1 (CRT pin 5) 0V - 110V
Pin 3 (CRT pin 8) +55V
Pin 4 (CRT pin 7) +32.8V

Pin1: With zero intensity there is some AC present at about 10mV and a lot of HF stuff.
When the intensity is increased we have a train of pulses that continue to increase with the intensity, they vary in frequency with the timebase speed. I assume these are blanking pulses.
On the leading and trailing edges of the pulses there are beads like water running up and down the edges.

Pin 3: This has up to 15mV of various HF frequencies. This is again effected by the timebase speed setting.
With the timebase in off there is up to 10mV of HF present

Pin 4: About 2mV of AC with HF on top

Pin 12 on the CRT @ -150V can this be checked with a Fluke DMM? or maybe a 40K per volt analogue meter?

Thanks
Dave.
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Old 5th Jul 2013, 10:47 pm   #78
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Default Re: Tektronix 465B Scope - Some problems.

I don't see the problem of using an AVO for EHT - checks. At 20,000 ohms/volt, and a range of 3KV, that is 60Meg ohm, drawing a max of 40 micro amps. I use my Selectest Super 50 with its 2,500v range with no apparent problems, and it reads the same (with read errors borne in mind) if I use its Xs 30 probe, which I use if I need to check PDA voltages etc..
Les.
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Old 5th Jul 2013, 11:03 pm   #79
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Default Re: Tektronix 465B Scope - Some problems.

Quote:
Originally Posted by davidgem1406 View Post
Pin 12 on the CRT @ -150V can this be checked with a Fluke DMM?
yes, that should be OK

John
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Old 6th Jul 2013, 7:55 am   #80
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Default Re: Tektronix 465B Scope - Some problems.

While you're probing around with the scope, also probe a known ground on the area you're working in. It's a quick check to see if you're getting HF pickup in your measuring gear.

It's easy for this to happen, and very easy to then assume the HF is on the signals you probe.

Probe grounding in an area with an inverter running can be.. um.. fun.

David
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