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Old 13th Oct 2018, 1:29 pm   #1
Anthony
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Default Tektronix 475: No Horiz Sweep

May I have some guidance on a fault(s) which hit my long-service 475 today. Before it all gets too serious I wonder whether anyone might recognise the symptoms? It just lost Horiz. Sweep and curiously the Power Neon does not come on. It is just possible the neon fault has been there for some time and I have simply not noticed it (?). Both Y-axis channels seem fine.

My initial attack on the formidable Tek manual have been worrying in that I cannot even locate the Power On neon on the circuit! But this is the first time I have looked at the service manual. It does not really make sense to me that the Sweep and Neon problems are related but I suppose the neon maybe driven from somewhere odd.

An additional fault, I suspect, is that the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen.

Clearly I am going to have to do my homework but I just thought to ask for some initial hand-holding before I actually do anything.

Many thanks. Anthony M0WWV
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Old 13th Oct 2018, 2:34 pm   #2
ColinTheAmpMan1
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Default Re: Tektronix 475: No Horiz Sweep

Quote:
Originally Posted by Anthony View Post
May I have some guidance on a fault(s) which hit my long-service 475 today. Before it all gets too serious I wonder whether anyone might recognise the symptoms? It just lost Horiz. Sweep and curiously the Power Neon does not come on. It is just possible the neon fault has been there for some time and I have simply not noticed it (?). Both Y-axis channels seem fine.
First off, is this a 475 or a 475A? I ask because there are differences. I have a 475A and the Service Manual.

Quote:
My initial attack on the formidable Tek manual have been worrying in that I cannot even locate the Power On neon on the circuit! But this is the first time I have looked at the service manual. It does not really make sense to me that the Sweep and Neon problems are related but I suppose the neon maybe driven from somewhere odd.
I have looked in my manual and on the scope - there isn't a "Mains On" light; it is a "Low Line" indicator which indicates that the mains voltage is too low. It should only illuminate when that condition is true, not simply when the mains is switched on.

Quote:
An additional fault, I suspect, is that the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen.
That is _the_ fault! This sounds very much like there is something wrong either in the sweep and trigger circuits, the horizontal amplifier, or any combination of the three. I would first try looking at the low-voltage power supplies and see if they are all within spec.

You could also tell us what your "Beam Finder" does (also when twiddling the "Horiz. Position control), whether the A- and B-Gate signals available on the rear panel are present, whether the time-base will trigger or not (check the "Trig" lamp) and what effect, if any, the "X10 Magnify" button has.

Good Luck, I doubt this is insurmountable.
Colin.
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Old 13th Oct 2018, 2:44 pm   #3
Anthony
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Default Re: Tektronix 475: No Horiz Sweep

Colin
Very grateful. It is a 475. `Ouch re: the `Low Line`- the neon is unlabelled on my front panel. Will get back to you.
Anthony
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Old 13th Oct 2018, 4:04 pm   #4
ajgriff
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Default Re: Tektronix 475: No Horiz Sweep

You may already have a copy but the Tektronix Troubleshooting Your Oscilloscope guide is a very useful document. It's widely available for free download including here:

http://www.sphere.bc.ca/test/tek-par...ing-scopes.pdf

Definitely start by checking power supply rails.

Good luck. I'm sure you'll manage to track down the fault.

Alan
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Old 13th Oct 2018, 4:27 pm   #5
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Default Re: Tektronix 475: No Horiz Sweep

Alan.Thank you for the encouragement Yes I have that very helpful slim volume. Best wishes. Anthony
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Old 13th Oct 2018, 6:49 pm   #6
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Default Re: Tektronix 475: No Horiz Sweep

First thing on these is check the power supply voltages. 99% of the problems are power supply related.

And the problems are almost universally shorted tantalum capacitors! The blue blobs or striped ones.
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Old 13th Oct 2018, 7:28 pm   #7
Anthony
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Default Re: Tektronix 475: No Horiz Sweep

Mr. Bungle & confreres

That is my project for tomorrow. i just odo not want to rush it. Having taken off the `scope cover i am amazed at the component density!
Best
Anthony M0WWV
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Old 13th Oct 2018, 7:39 pm   #8
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Default Re: Tektronix 475: No Horiz Sweep

Power supplies first

Then try to get the unit going in X-Y mode

Then try to get it triggering in 'norm' 'ext' trigger mode.

Then try to get the timebase going in 'auto' 'ext' trigger mode

Then try to get it 'int' triggering from Y amp signals.

Each of these steps brings in a bit more circuitry. Having the spot(s) far to the right with a timebase fault doesn't automatically implicate the X amp or the power supplies, nor does it exonerate them.

Simple stress factors make the power rails an order of magnitude less reliable than the things they power.

Dsvid
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Old 13th Oct 2018, 8:17 pm   #9
Anthony
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Default Re: Tektronix 475: No Horiz Sweep

David
Many thanks. I appreciate the graded approach. I think I'll pause until the alcohol levels have fallen.
Sincerely
Anthony
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Old 13th Oct 2018, 9:58 pm   #10
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Default Re: Tektronix 475: No Horiz Sweep

One key thought to hold onto is that this timebase is designed not to keep running, but to run once and then stop, waiting for another trigger. Repetitive operation only happens in 'norm' if there are repetitive triggers from whatever source the switches select, OR from an added timer in 'auto' mode.... there's an extra timebase speed switch pole to select different capacitors for the auto-run self-trigger timer so the rate is appropriate to the timebase speed.

David
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Old 14th Oct 2018, 1:57 pm   #11
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Default Re: Tektronix 475: No Horiz Sweep

Have a lot more in the way of clues but cannot integrate them into a single fault area:
All 6 PSU DC levels and ripple are depressingly well within spec. (surprised by the very low ripple checked with both a DVM and `scope ?odd).

X-Y mode was interesting. Firstly I noticed that in both X-Y mode and 0.1/0.2 and 0.5 secs/div the spot goes off screen and needs Beam Finder to bring it back On all other sweep settings the spot is visible without BF.

Putting the Calibration signal into Ch.1 in X-Y mode (using BM to see trace) and selecting Ch. 1 there is a trace at 45deg with of about the right for the 300 mV Cal signal. If Ch.2 is selected the trace is horizontal.

It is as if Ch. 1 is feeding X and Y while Ch. 2 is feeding only X. if Ch. 1 is selected .? Red Herring.

Going to a sweep setting, will not trigger in Norm/Ext or Auto/Ext.

I looked at the A+GATE signal using a sort of working 465. On the 465 itself there are pulses but nothing comparable on the 745 under `repair`.

Am floundering at the moment and don`t feel I have localised to Trigger, Sweep or Horiz. Amp. Though the lack of the A+GATE signal suggests sweep (?)

Am floundering a bit (sorry for the fish metaphors!)

Thoughts

Anthony

P.S. David - understood.
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Old 14th Oct 2018, 2:49 pm   #12
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Default Re: Tektronix 475: No Horiz Sweep

Should have said 475 not `745` also the lack of A+GATE signal could be either Triggering or Sweep -yes?
Anthony
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Old 14th Oct 2018, 5:10 pm   #13
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Default Re: Tektronix 475: No Horiz Sweep

Proper look art manual indeed states CH2 to be selected for XY mode - ?why - so herring is indeed red on that point. This is a more complex instrument than I thought!
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Old 15th Oct 2018, 6:14 am   #14
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Default Re: Tektronix 475: No Horiz Sweep

" the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen." might be another red herring down a rabbit hole, but this to me indicates one half of the X amp is not working.

Another watery metaphor, sounds like your drowning here, flopping about all over the shop. After a PSU rail check, break the scope into sections (helped by the manual block diagram) and proceed in an orderly logical way, else you'll get lost; you need to be methodical.

For a start have you given it a good visual with a magnifying glass and checked all the tant's? Never owned one of these but the tantalum and dry joint issue is a well known issue with these scopes. Also check daft easy things first like connectors, iffy pots and switches.

Lastly post some schematics of the section in question, helps folks help you.

Good luck, Andy.
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Old 15th Oct 2018, 8:03 am   #15
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Default Re: Tektronix 475: No Horiz Sweep

I'm working on memories, I don't have a manual.

In X-Y mode it parks the timebase and only the X amp is in use, taking signal from one Y channel. This is a lot simpler than what's in play in the usual timebase mode! You appear to have an X amp fault, or maybe the voltage from the parked timebase is wrong. Whatever it may be, you need ti fix this first. 1) because it's easiest, 2) because testing/fixing all the rest needs it to be working.

This looks like a complex instrument, but it's really just an awful lot of simple enough circuits flying in very close formation. Slow, methodical, orderly work will get you there, anything else will get anyone lost and confused.

But before you start. Look at the schematics of the X amplifier. Spot all the places each power rail appears, notice any local filtering components and check the voltage is correct on all of them, and that it makes it through the filtering OK. Tedious? yes. but it can save more anguish later.

I used to design stuff of this complexity (and worse!) for the 'other brand' and this meant prototyping my designs and debugging them. When you do this, you don't even have the comfort of knowing that it once worked and you can't even expect there to be only one fault. This is the troubleshooting equivalent to extreme ironing, and it teaches you to be methodical, and just how precious certainty is.

David
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Old 15th Oct 2018, 8:10 am   #16
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Default Re: Tektronix 475: No Horiz Sweep

Good advice. They’re not particularly intimidating really. They just look it!

The good bit with the X amp failing is it’s on the big interface board so it’s easy to trace out there, unlike the vertical which is all over the place and buried.

Last X amp failures I had were a shorted tantalum and the deflection plate wires were loose.
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Old 15th Oct 2018, 8:14 am   #17
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Default Re: Tektronix 475: No Horiz Sweep

As bungle says - check the power supplies before you do anything else. Apart from shorted tantalums, the key supply failure mode is Sprague twist-lock capacitors on the supply board. These have a failure mode that the internal connection foil to the can (so negative) rots through by electrolytic corrosion. So the capacitor just disconnects itself internally. These are a nightmare to desolder when one has failed, and someone at Tekscopes has gone as far as to layout an entire replacement board.

So check power supply voltages and ripple.

Another supply failure is the bridge rectifier that gives confusing symptoms. Tek's power supply parts list usually lists three or four bridge rectifier types while they arrived at a solution!.

Good luck. And join TekScopes@groups.io which is the community for all things Tektronix.

Craig
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Old 15th Oct 2018, 10:36 am   #18
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Default Re: Tektronix 475: No Horiz Sweep

One simple fault that hasn't been mentioned is that the push button switches become unreliable with age. It is worth cycling the horizontal mode buttons (A, mix, A intensified, B delayed ) many times. It might also add a clue if you describe what you see selecting 'B delayed' (A trig auto, B trig starts after delay).

Roger
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Old 15th Oct 2018, 1:48 pm   #19
Anthony
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Default Re: Tektronix 475: No Horiz Sweep

I am very grateful for these suggestions. Yes, I am intimidated by the 475. It has numerous features which I never used. The only equipment at all `similar` to this which I have worked on is HP of this era - is that David`s `other brand` - which I have almost always, after some effort, been able to debug. The 475 will take some getting used to and I must be ever so humble and RTFM + Tek troubleshooting guide again and again. Will update;Anthony
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