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Old 6th Jul 2013, 11:30 am   #81
Leon Crampin
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Default Re: Tektronix 465B Scope - Some problems.

Very good advice.

Remember there's a meaty inverter inside your 465 as well. If the supply decoupling electrolytic dries up, the noise goes everywhere.

Leon.
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Old 6th Jul 2013, 11:53 am   #82
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Default Re: Tektronix 465B Scope - Some problems.

I tried the Z axis sample method last night and it works extremely well. I won't dwell on it too much because you may not have access to a suitable pulse generator but to give you an idea of how powerful this technique is, I was able to detect cyclical spot 'excursions' in XY mode of only a tiny fraction of the spot width even if the 'ripple' rate was at many kHz.

i.e. it's as easy as seeing a small circle wobbling a tiny amount. It's easy for the human eye to detect tiny movements like this once the slow moving alias is displayed on the CRT. The alias is created by the sampling set by the pulse generator so you don't need a camera with this method. However I don't believe you could easily see this degree of detail by simply playing with the timebase controls because the movement is in both X and Y and would make it very hard to 'defuzz' the trace. It takes literally a few seconds to find a slow moving alias with the Z axis method.

It's easy to make the spot appear to freeze in one place or make it move forward or backwards and slowly draw the spot distortion.

Obviously this technique assumes the ripple is cyclical in nature rather than random noise. The ideal device to feed to the 0 to 5V Z axis port would be a PIC or AVR chip rather than a basic pulse generator because the duty cycle needs to be fairly fixed but with the ability to easily vary the pulse repetition frequency (or sample rate)

Whilst this method won't tell you what component is causing the problem it might be able to prove if the distortion is caused by a tiny amount of cyclical ripple in XY somewhere as opposed to something like noise or a basic focus problem.

Last edited by G0HZU_JMR; 6th Jul 2013 at 12:00 pm.
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Old 6th Jul 2013, 12:23 pm   #83
Alistair D
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Default Re: Tektronix 465B Scope - Some problems.

If the problem was ripple on the focus supply, would you be able to see the spot get smaller and larger as the ripple voltage rises and falls?

Al
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Old 6th Jul 2013, 3:31 pm   #84
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Default Re: Tektronix 465B Scope - Some problems.

Hi David,

The -150V to the CRT checks out at 146.4V.

I did play with the earthing using the scope probe.
What I did was to check some of the earthing pints in the Tek to other earthing points. I found all ground points are not equal.
I put the probe onto a ground point on the main board and the earth clip to the rear metal work. That showed a reading of some 8mV of HF, I then moved the clip to the front metal work and it showed some 2mV of HF.
If i move the probe to another ground point the readings are again different.
I don't really know if this means a lot.

I now have the full service manual available, and very good it is as well, easy to read and all present. I have both versions for the high and low serial numbers and the user manuals.

I found under the information about the test setup for DC voltages that with DC voltages above 1KV a meter with 20Kohm/V is used and a 0 to 3KV range. The AVO is just the job.

Hi G0HZU_JMR,

Thanks for that insight and the explanation but as you suggest I do not have a pulse generator available, it is also somewhat over the top of my head, although it may be easier to actually do that it sounds by reading it. Maybe one day, you never know.

Dave.
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Old 6th Jul 2013, 9:22 pm   #85
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Default Re: Tektronix 465B Scope - Some problems.

OK, sorry I wasn't able to help much.

I guess we all have different ways of troubleshooting but with something like a scope that is 'nearly' working I would always spend a lot of time trying to diagnose the fault using external stimulus signals and also the various modes within the scope which is why I was suggesting the sampling method(s)

However, I can show you what spot size a tyical 465 can deliver. I've had this scope nearly 20 yrs and I took it through the whole service manual procedure over a long weekend when I first bought it and it works well. The earlier plots on the thread of a healthy 465 spot looked a bit big and blobby to me so I posted up an image of mine after setting astig and focus. Note that when I was doing the Z axis tests the 'enlarged' spot size with my deliberately added XY movement was smaller than the 'healthy' spot size shown earlier and I could still see the healthy (small) spot moving within the larger spot size when showing a slow moving alias term.

The trace sharpness isn't as good as my old Tek 585 but it is probably a typical example of a healthy 465. If I make it dimmer I can make it look even smaller but hopefully the image is useful.
See below:
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Name:	465_spot.jpg
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ID:	81666  

Last edited by G0HZU_JMR; 6th Jul 2013 at 9:30 pm.
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Old 6th Jul 2013, 9:32 pm   #86
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Default Re: Tektronix 465B Scope - Some problems.

Hello G0HZU_JMR,

Thanks for that picture, that is what I would expect to see as a spot. Unfortunately mine is far from that but we hope to get it to a small spot as your image shows.

Hello Leon,

Quote:
Originally Posted by Leon Crampin View Post
Remember there's a meaty inverter inside your 465 as well
Can you tell me what part of the circuit is the inverter, the only mention of an inverter is with an option enabling the scope to run from 12V or 24V DC.

Reading the part of the manual dealing with the CRT, it's supply and controls makes no mention of an inverter.

Thanks
Dave
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Old 6th Jul 2013, 10:16 pm   #87
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Default Re: Tektronix 465B Scope - Some problems.

Dave, I think Leon is referring to the hv power supply. It steps up from 110/55v levels to 2450v using an oscillator driven eht transformer and then via a Cockroft Walton multiplier to 14.5kv. The oscillator runs at about 50khz and creates a lot of radiated noise. That's why its important that the decoupling caps in the hv psu are in good order. These are usually tantalum types in Tek scopes.

Bill
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Old 6th Jul 2013, 11:11 pm   #88
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Default Re: Tektronix 465B Scope - Some problems.

Hi Bill,

Thanks for the info, I did think it was a reference to the oscillator.

There is a couple of tantalum's and electrolytics in the vicinity of the oscillator.
I will try to check those but i don't have replacements for tantalum's.



I have just been going over a couple of things and 1 was the -150 supply to the CRT pin 12. This is at -146.5V but that is OK I would think.
However I found that there is some AC content and a lot of HV stuff as well at 0.45V PtP as per the attached picture. The majority of it would seem to be 50Khz or higher.
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Old 6th Jul 2013, 11:52 pm   #89
Leon Crampin
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Default Re: Tektronix 465B Scope - Some problems.

Dave, I'm sorry if I did not make this clear. The whole EHT system + the tube heater is powered from an inverter in the 465 as are most other Tek 'scopes of the period.

I don't have the 465 manual immediately to hand but typically, the inverter will be powered directly from one of the reservoir capacitors for a low voltage supply. My 453 runs the EHT inverter at about 19V (unstabilised); this supply feeds the +12V regulator for the rest of the 'scope.

Very importantly, on the 453 there is a series 80 uH choke and a parallel 47 uF capacitor to decouple the EHT inverter from the power supply. I remember the 465 is very similar - I've been in there in the past. If the 47 uF capacitor dries up, the inverter noise goes everywhere.

Although the inverter is fed from an unstabilised supply, it is itself stabilised by being in a feedback loop which compares the tube negative cathode potential with a reference from one of the stabilised lines, via the "high voltage" preset. Thus, the whole EHT system + tube heaters are fully stabilised - as you would expect from Tek.

Good hunting...

Leon.

Last edited by Leon Crampin; 6th Jul 2013 at 11:58 pm.
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Old 7th Jul 2013, 9:10 am   #90
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Default Re: Tektronix 465B Scope - Some problems.

Dave, the eht oscillator runs at approx 50khz, and that noise probably originates from there.The failure of tantalum caps is well known on Tek scopes and you need to get some available or a source of good used spares. Tek used Tantalum caps because they have a very low ESR and are therefore good decouplers. Modern low esr electrolytics can be used to effect a repair, but I prefer to stick to the original design spec wherever possible.
Check all the tants in the area of the eht oscillator with a good esr meter, bad ones will be obvious.
Bill
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Old 7th Jul 2013, 9:30 am   #91
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Default Re: Tektronix 465B Scope - Some problems.

I think I had the brightness up a bit more than G0HZU, I'd just left the big Nikon settings pretty much unchanged with low ISO and small aperture. Dropping the intensity down, twiddling focus and astig give pretty much a spot size like his.

David
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Old 7th Jul 2013, 2:59 pm   #92
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Default Re: Tektronix 465B Scope - Some problems.

Thanks everybody for the input.

I will check the decoupling in the oscillator circuit. This will have to be by replacement as I don't have an ESR meter.

Circuit for CRT and supply is attached (pdf 4 x A4 pages)
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Old 7th Jul 2013, 11:38 pm   #93
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Default Re: Tektronix 465B Scope - Some problems.

Does anybody know if C4006 in the HV oscillator circuit is an axial tantalum or a standard electrolytic? It just looks to be rather different in case style to a standard electrolytic.
The parts list does not differentiate between Tantalum and electrolytic, it calls them all as electrolytic.


Thanks
Dave.
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Old 7th Jul 2013, 11:53 pm   #94
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Default Re: Tektronix 465B Scope - Some problems.

They are all different to what you are used to seeing, its probably an axial tantalum but substitute it for a good quality low esr electrolytic to rule it good or bad. You can always obtain the correct part later. Don't compromise on quality, these are quality instruments and were designed with quality components, you wont get premier league performance from conference league components.
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Old 8th Jul 2013, 8:36 am   #95
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Default Re: Tektronix 465B Scope - Some problems.

I recently saw the current prices of some axial tantalum capacitors I need for a spectrum analyser restoration... £70 each! but they used to be pennies.

The Tek scope (and my spectrum analyser) were built before certain characteristics of tants became known. An edict was issued to designers that tants must not be used without series resistance limiting ripple current... Um, enough resistance to ruin their use as decouplers. Next, the recommended derating of tants was cut to 50% of specified voltage. It seemed the makers had found that everyone was derating them and used this as an opportunity to reduce their actual voltage capability and save on increasingly pricy tantalum dust.

What we found was that modern aluminium capacitors from a few Japanese suppliers had become much more reliable, and their ESR at high frequencies had fallen, so we largely moved away from tantalum. Certainly tants in a 50kHz inverter shouldn't be needed. Tants in a power circuit do show reduced reliability, but in an application needing lowest ESR for lowest ripple on a rail, they may be necessary.

If a low ESR aluminium part works, don't bother with the original type, the aluminium one is probably more reliable and you won't ned to re-mortgage.

David
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Old 8th Jul 2013, 1:35 pm   #96
Leon Crampin
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Default Re: Tektronix 465B Scope - Some problems.

I have replaced the EHT inverter supply decoupling capacitor in several Tek scopes with a good quality Al electrolytic on several occasions with good results. The last one was fitted to a 7000 series mainframe and was originally a Sprague Al device which had dried out completely.

As David points out above, modern Al electrolytics are far more reliable than their predcessors (I would estimate 10x) and if kept cool and well within their ripple rating will have a very long life. I suspect the ripple current is a bit high in the Tek application - but I've never measured it.

Leon.
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Old 8th Jul 2013, 2:31 pm   #97
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Default Re: Tektronix 465B Scope - Some problems.

Hi All,

Thanks for all the advice on the Axial Tantalum/Electrolytic question.
I have actually found metal cased axial Tantalum 47uf @ 63V for £0.44 each.
Under the circumstances that sounds like a real bargain.

Another question, I have found a CRT for sale for £35.00 of the correct part number. Came from a working unit apparently and said to be in excellent working order.
Would it be worth buying, on the off chance, just in case the original CRT does turn out to have a problem?

Thanks
Dave
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Old 8th Jul 2013, 2:50 pm   #98
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Default Re: Tektronix 465B Scope - Some problems.

Before you go splashing out for a new tube Dave, replace that decoupler with a good quality electrolytic and see if there's any improvement. Ill be surprised if it is the tube.
Bill
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Old 8th Jul 2013, 3:35 pm   #99
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Default Re: Tektronix 465B Scope - Some problems.

Hi Bill,

I will be replacing the decouplers but I have had to order them as I only had 47uf electrolytic available.

The tube was just a thought really as it is available now, may not be latter.
I still hope it is not the tube

Dave
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Old 8th Jul 2013, 4:35 pm   #100
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Default Re: Tektronix 465B Scope - Some problems.

Just put that 47uf in to compare results. It should work fine.
Bill
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