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Old 3rd Jun 2015, 3:07 pm   #1
phimargin
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Red face "Self-Diagonosing" Tek475

The attached image shows that this intermittent fault seems to be a trace flyback (blanking?) issue or the like.

Any ideas? This is my only scope.

I have before now fixed a few PSU issues.

I'm rather hoping to avoid a major dismantle (the PSU is fairly easy to get at).

Thanks in advance...
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Old 3rd Jun 2015, 4:05 pm   #2
dave cox
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Default Re: "Self-Diagonosing" Tek475

Is there an input signal ?
What are the test conditions ?

dc
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Old 3rd Jun 2015, 8:24 pm   #3
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Default Re: "Self-Diagonosing" Tek475

I very recently fixed a Telequipment scope that looked for all the world like a blanking problem (but not intermittent so easier to trace). It had the same slightly fuzzy main trace and a distorted and displaced fainter trace that I thought was poor blanking. Quite by chance I found that on very slow sweep speeds (the ones I never use) the ghost spots moved across in the 'right' direction during the scan but not at a constant rate. It turned out to be a failed reservoir capacitor on the 175V line that supplies both X and Y amplifiers. The Telequipment scope has an inverter power supply so the ripple was at 30kHz or so and the waveform was moderately flat most of the time (the fuzzy main trace) and then dropped rapidly giving the offset and distorted ghost trace.

I am not sure if the 475 has an inverter or not, I don't think a 50Hz supply could produce a similar visual effect . You should be able to use the 475 to look at its own supply voltages, to see if there is a major problem.

I have had a couple of failures in Tektronix Z-axis / blanking circuitry, both were transistor failures and can usually be spotted just with DC measurements. If you have the manual you can swap many of the Tek plugin originals with 2N3904 / 2N3906 which are quite adequate for all except the high voltage applications, at least to show if there is a failed transistor somewhwre.

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Old 3rd Jun 2015, 9:10 pm   #4
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Default Re: "Self-Diagonosing" Tek475

The reduced size picture and suppression of blanking seen in that image look like the beam finder button is pushed (or stuck in, or contacts dirty)

This control reduces the bias in the X and Y amplifiers so you can see which direction a missing trace could be feflected in,and it de-blanks the beam so you can see whats going on even if the timebase isn't being triggered.

David
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Old 3rd Jun 2015, 9:30 pm   #5
phimargin
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Default Re: "Self-Diagonosing" Tek475

Hi,

I should have put more detail in. The 1 KHz self-cal was the input for this.

However, even with the AC-GND_DC switch at GND, I see a '>' shape.

My first question is
Where does the vertical "sweep" originate? Could it be bad switch contacts making the scope stay in 'something like' XY mode?

From a hard-to read (smudgy) diagram I got from Boat Anchors, I see the XY switch pulls up pin 18 of U600, labelled "Lockout In". I don't exactly know what is 'locked out' here though.

An hour ago, I removed the cover, jiggled U600 and sprayed some cleaner around... The scope suddenly worked - I put the cover on...

Now it's broken again - about the same fault: A bit easier to get something showing perhaps, but still the funny trace shape.
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Old 3rd Jun 2015, 9:43 pm   #6
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Default Re: "Self-Diagonosing" Tek475

The last time I saw a display like this (also on a 475) the fault was down to failed electrolytic caps on the power supply. I replaced all of the caps and normal operation was resumed.
The main caps are the 4 tag type and not easy to remove from the multi layer PCB without damaging it.

Definitely worth checking the power supply rails before getting involved in more detailed fault finding.

Phil
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Old 4th Jun 2015, 12:54 am   #7
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Arrow Re: "Self-Diagonosing" Tek475

Quote:
Originally Posted by Oldtestgear View Post
Definitely worth checking the power supply rails before getting involved in more detailed fault finding.
Which has been exactly my experience, too - applicable to the 455, 465 and the 475.

Good as they are - when fully working - it must be said that Tek. gave no consideration whatsoever to maintenance when they designed the 475 - which is a great shame; ship, tar, etc.

Al.
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Old 4th Jun 2015, 9:20 am   #8
dave cox
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Default Re: "Self-Diagonosing" Tek475

If all the PSU supplies are OK for voltage / ripple then I would switch to XY mode - CH2 at ground and a slow signal ( pot if necessary ) into CH1 and see if you can extend to full vertical deflection then reverse CH1 / CH2 and try for full horizontal. If both are compressed then check the bias (as David said) or tube voltages. If only 1 then suspect H or V output. I would only start looking at the time-base / blanking after this all checked out.

dc
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Old 4th Jun 2015, 11:23 am   #9
phimargin
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Default Re: "Self-Diagonosing" Tek475

Just reading and thinking about above advice - thanks.

"Beam finder button" suggestion and XY tests now pending...
All PSU voltages look good: (110,50,+/-15,+/-8). But this is admittedly in "its working again now I've taken the cover off mode".
[Unsure about checking ripple, maybe enough life in the scope to look...]

In the meantime, a couple of pictures:

nearly_shorting cap leads (closeup) Shows a possible source of intermittent shorting on cap leads: (I can't actually find this capacitor on the circuit diagram though...)
possible_trace_crack A real puzzle to me - are the two solder lands means to be connected? I think by comparison with circuit diagram "yes".

If someone is able to post a photo of the same board for comparison, that would be much appreciated.
Also-has anyone got nice, crisp scans of the circuit diagrams? the ones I've found seem to be scans of photocopies - really hard to read.
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Old 4th Jun 2015, 12:39 pm   #10
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Default Re: "Self-Diagonosing" Tek475

Quote:
Originally Posted by Skywave View Post
Good as they are - when fully working - it must be said that Tek. gave no consideration whatsoever to maintenance when they designed the 475 - which is a great shame; ship, tar, etc.

Al.
I think that is slightly unfair. They were, like most high quality test equipment, designed with a finite life, 10 years is normal.

They were easy to repair if you knew what you were doing and were in a suitably equipped workshop.

They were very easy to calibrate to.

I doubt if the designers considered the possibility that they would be in the hands of enthusiastic amateurs several decades after they became obsolete.

Yes, I was one of those who looked after them when they (and I) were in the first flush of youth. They were a revelation in terms of performance for their size.

The 475 was my favourite and I still have one in regular use today.

I wonder how many of the modern menu driven LCD based instruments will still be around and working in 2045?
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Old 4th Jun 2015, 12:42 pm   #11
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Default Re: "Self-Diagonosing" Tek475

Quote:
Originally Posted by phimargin View Post
Also-has anyone got nice, crisp scans of the circuit diagrams? the ones I've found seem to be scans of photocopies - really hard to read.
You have a PM.
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Old 4th Jun 2015, 4:23 pm   #12
phimargin
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Default Re: "Self-Diagonosing" Tek475

Thank you Brian,

This is the sort of thing I have, but would like in sharper form...

Regards
Stephen
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Old 4th Jun 2015, 5:04 pm   #13
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Thumbs down Re: "Self-Diagonosing" Tek475

Quote:
Originally Posted by Skywave View Post
Good as they are - when fully working - it must be said that Tek. gave no consideration whatsoever to maintenance when they designed the 475 - which is a great shame; ship, tar, etc.
Quote:
Originally Posted by Brian R Pateman View Post
They were easy to repair if you knew what you were doing and were in a suitably equipped workshop. They were very easy to calibrate too.
From my experience in the very environment you describe, I found exactly the very opposite of what you have said. I think we will have to agree to disagree on those points. It's perfectly clear that Tek. chose to make a high performance 'scope that was readily portable. And that meant getting a quart into a pint pot. And - as a consequence of that - accessibility for maintenance became way down the list of design features.

Al.
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Old 4th Jun 2015, 5:53 pm   #14
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Default Re: "Self-Diagonosing" Tek475

Quote:
Originally Posted by phimargin View Post
From a hard-to read (smudgy) diagram I got from Boat Anchors, I see the XY switch pulls up pin 18 of U600, labelled "Lockout In". I don't exactly know what is 'locked out' here though.
I'm no expert on this particular model, but "Lockout" is almost certainly to disable the timebase when in X-Y mode.

(I've seen that term used on another Tek IC in a different 'scope that I am very familiar with.)
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