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Old 14th Mar 2014, 9:35 pm   #121
davidgem1406
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Default Re: Tektronix 465B Scope - Some problems.

I fitted the cover and it does make a difference. I tried to take comparison pictures but daylight had by then gone. I will do it again tomorrow and post them here.

As for the reduction in bandwidth to the 20MHz limit it does seem to make some difference but it is so marginal it is difficult to be sure.

Dave.
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Old 15th Mar 2014, 10:44 am   #122
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Default Re: Tektronix 465B Scope - Some problems.

Having the cover on should make a big difference. If you ground the input, and turn the brightness down to a sensible level, you should now have a perfect circular spot/ thin trace with no ripple. In my experience, the display is not as bright and sharp as some simpler, basic scopes, and I think this is something to do with the design of the crt itself. It also depends on correct crt cathode volts as per the manual, -2450V, and correct accelerating potential. You will need a high impedance eht probe to set the cathode volts correctly. Also as the crt ages, they can go a little 'soft' depending on the amount of use they have had. If you are trying to photograph the trace, it can be difficult to get a good picture without it looking over exposed.
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Old 15th Mar 2014, 10:54 am   #123
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Default Re: Tektronix 465B Scope - Some problems.

Way back in the '70s when these scopes were de rigeur, I recall it was generally accepted that the 465/475 display was slightly softer than the competition (eg HP1740 series)
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Old 15th Mar 2014, 2:44 pm   #124
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Default Re: Tektronix 465B Scope - Some problems.

I have added the 2 pictures, 1 is with the cover on and 2 with cover off.
There is a difference between them, not as much as I had expected it to be. Maybe I'm expecting to much from it. I looked at them on the PC monitor at a large size.

I have not made a specific check on the spot size since when I replaced the CRT but if I remember rightly it was not all that special. Not what I would call small and sharp anyway. I will do another check on it to be sure.

A high impedance eht probe is something I do not have.

However for the cathode voltage the manual states a DC voltmeter with range 0 to 3KV at 20K per volt, the AVO 8 will do that.
I think the cathode voltage was one of the things checked quite early on in this thread.

Dave.
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Old 15th Mar 2014, 3:43 pm   #125
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Default Re: Tektronix 465B Scope - Some problems.

The invisible verticals and the broader horizontals look to me as if the focus and astig controls are still not set for the smallest roundest spot. Trying to set them up while the timebase is still running is much more difficult.

Al
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Old 15th Mar 2014, 10:00 pm   #126
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Default Re: Tektronix 465B Scope - Some problems.

Personally I wouldn't use an AVO, this is a high impedance circuit and it will load it, thus giving false results. You can easily fashion a hv probe using a string of 10 meg resistors in a plastic tube, ie a biro tube, and measure across the last resistor, grounding the end. Calibrate it from the measured ht line and using the multiplication factor derived therefrom. You don't need to spend a fortune.
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Old 15th Mar 2014, 10:57 pm   #127
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Default Re: Tektronix 465B Scope - Some problems.

For the record, The TQ D83 (and the D75) use CRTs manufactured by TEK. In fact I think the same CRT is in D75 as 465B.
I feel the HP CRTs look nicer than the TEK ones. Recently looking at a fine pulse sitting on a TQ D75 triggering wave trace, the pulse was barely visible on another D75, but "clear as a bell" on an HP1703.
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Old 20th Mar 2014, 1:02 pm   #128
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Default Re: Tektronix 465B Scope - Some problems.

IIRC, the TEK's poor sharpness is down to the use of an expansion mesh. This effectively allows faster sweep speeds without having to use too high a deflection voltage, which is a problem for semiconductors. I've seen razor sharp traces on some TEK scopes, sans expansion mesh, but they needed some fancy tube circuits to swing enough voltage.

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Old 20th Mar 2014, 2:21 pm   #129
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Default Re: Tektronix 465B Scope - Some problems.

Or a small tube faceplate requiring less deflection energy for a given electron beam velocity.

The first Tek portable of this generation, the 453 has a pin-sharp trace and uses about the same acceleration potential as the 465. There's no expansion mesh on this small tube though.

You can't buck the Laws of Physics.

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Old 20th Mar 2014, 4:37 pm   #130
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Default Re: Tektronix 465B Scope - Some problems.

Agree Leon. The 453 has a very sharp fine trace. The acceleration potential of the 465 is 14.5kv, but I dont think its as high on the 453. The screen is a lot smaller on the 453. Both 'scopes are predominantly semiconductor, particularly in the deflection circuits. You are correct Dave, re the tube mesh, and any sharpness problem is magnified by incorrect voltages at the cathode and final anode.
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Old 20th Mar 2014, 5:29 pm   #131
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Default Re: Tektronix 465B Scope - Some problems.

I've just checked the (excellent) 453 documentation and the acceleration potential is indeed a bit lower than that of the 465, as you point out. The total accelerating potential for the 453 is 12 kV (2 kV gun) - not bad for a smallish tube.

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Old 20th Mar 2014, 6:02 pm   #132
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Default Re: Tektronix 465B Scope - Some problems.

This made me look up the anode voltage for my 7603, a mere 12kV and that is with a 10*12cm face. These have a 'some what' poor spot size, no doubt due to a lot of magnification by the expansion mesh, but the large screen makes up for that !

I understand the difficulty of identifying the cause of a 'worse than normal' focus as I have a scope sat under my bench that has resisted numerous diagnostic attempts to make it focus as tightly as a good un

dc
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Old 20th Mar 2014, 7:56 pm   #133
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Default Re: Tektronix 465B Scope - Some problems.

I recently bought a 465 for spares, it had a blown multiplier. If it had been only that at fault, I would have repaired it. So having lashed up a multiplier from spares and connected it up, there was a trace, but it was in soft focus, and although cathode voltage was spot on at -2450, and the multiplier output at + 14.5kv, nothing would get that tube to sharpen up. It had either been subjected to a knock or had just become 'soft' with age. This thing had plenty of black muck inside and had obviously had a hard life, probably been left on for days on end. So that would tie in with the ageing theory.
Bill
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Old 22nd Mar 2014, 11:53 pm   #134
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Default Re: Tektronix 465B Scope - Some problems.

Im now looking at checking the cathode voltage as the spot is not to good but I don't have a high voltage probe to use and it seems an AVO 8 will load the circuit to much.

So I have just purchased 20 x 10Meg resistors. I know what the circuit of the probe should be to use it with a Fluke 77 DMM, that has an input impedance of 10Meg so needs a 10Meg as the final resistor.
The problem is that I don't know how to calculate the series resistor required to have the 1000V range read 5000V.
All the information I have found so far seems to point at using thousands of Meg in the series resistor?

However whilst searching earlier this evening it suddenly occurred to me that I have a x100 Tek scope probe, theoretically I should be able to use that.
Any comments please.

Thanks Dave

Last edited by davidgem1406; 22nd Mar 2014 at 11:55 pm. Reason: Correction
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Old 23rd Mar 2014, 1:54 am   #135
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Default Re: Tektronix 465B Scope - Some problems.

I would do a string of 9 times 10M + 9M (99M) in series with the meter AND the meter shunted by 1.11M to make the meter look like 1M so you get a X100 probe. Without having a meter shunt resistor, if the meter goes open circuit changing ranges, its gets thanked by getting the full applied voltage (via 99M). That assumes a 100M load is high enough ...

dc
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Old 23rd Mar 2014, 12:35 pm   #136
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Default Re: Tektronix 465B Scope - Some problems.

Hi dc,

Thanks for the reply.

Maybe I should have not used the term 1,000V range as the Fluke is auto ranging with a maximum DC input of 1,000V, so no switching involved.

Sorry for the misdemeanor.

I want to increase the maximum input by a factor of x5 to 5,000V, or maybe x20 to 20,000 I could then check the Anode voltage as well by using a probe. Although I don't know how I would gain access to that. Anyway the Anode is just another thought the important one is the Cathode.

I just measured the DC resistance of the Tek x100 probe and that comes out at 9.87Meg as the series resistance and 112K as the shunt resistor. Also from the probe tip there is another shunt resistance of 9.98Meg.

Dave.
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Old 23rd Mar 2014, 1:25 pm   #137
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Default Re: Tektronix 465B Scope - Some problems.

Switching is involved, it just isn't manually controlled which makes series resistor range multipliers riskier.

Good voltage range multipliers are potentiometer networks, with ground leads integrated, some protection for the meter, and values calculated to account for the impedance of the meter.

David
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Old 23rd Mar 2014, 1:29 pm   #138
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Default Re: Tektronix 465B Scope - Some problems.

Dave, the number or exact value of the resistors doesn't really matter. For the sake of an example, take say 3 x 10 meg resistors, connect in series, then at the end of the chain add 1 meg, this is the resistor that your Fluke will be attached across, the end of the resistor is earthed, as is your black fluke lead. The red lead is attached to the other end of the 1 meg. Put a flylead on so you can attach the Fluke. Put the chain in some shrink wrap tube and insert in a plastic tube, bringing the +ve eht test connection out of the far end, probe-style. Seal it up with some hot melt glue, and there's your probe. Now you need to calibrate it from a known hv source of say 100v, connect the probe, and read off the value on the Fluke. If this is say 0.1v(100mv), the conversion factor will be 100/0.1, so once connected to the cathode, if you get 2.45v, then 2.45x100/0.1=2450v
Hope this helps
PS. Make sure the earth is connected, or the whole thing including the meter will float at -2450v, and keep your hands away when its under test.
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Old 23rd Mar 2014, 2:25 pm   #139
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Default Re: Tektronix 465B Scope - Some problems.

Hi maninashed,

Thanks for the information, I do have information on making the probe so that is all OK. It is the resistor value for the series chain that is my problem.

As you say I could just use a number of resistors in series and then calibrate it, there in lies my problem. I don't have any means of a calibration supply sufficiently high and of a sensible value.
The only chance may be the power supply of an Advance SG66, I will have to open it up and find out exactly what the HT value is although I previously marked the rail on the circuit at 200V.

An update note on the Tek x100 probe. This is definitely not suitable as it has a maximum input voltage of 1,500V. DC/RMS.

Will be back in due course.

Dave.
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Old 23rd Mar 2014, 3:05 pm   #140
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Default Re: Tektronix 465B Scope - Some problems.

Just use the 115v (?) ht supply in the scope itself. Why make life hard.
Bill
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