Help for hard repair attempt

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Hi, I'm Paolo from Italy.
I'm attempting to repair a Radio Test Set (Advantest R3361) that exhibit  
a strange fault that (I suppose) is originated in some digital stage.

https://s26.postimg.cc/9hwc9nr15/IMG_2473_1.jpg

Do you know where I can try to search fo fix it?
I have the electric diagrams.
Many thanks.

Re: Help for hard repair attempt
On Tuesday, June 12, 2018 at 2:05:02 PM UTC-4, Eremita Analogico wrote:


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Paolo:

A picture is truly worth 1,000 words, but a more complete description of the fault would be useful. And, in addition to the schematic, do you have the user-manual? This may be a set-up issue.  

http://www.g7lwt.com/documents/advantest/ocr_r3361a.pdf

I know that 455 pages may be a struggle - but when in doubt, read the directions.  

Good luck!

Peter Wieck
Melrose Park, PA  

Re: Help for hard repair attempt
Il 12/06/2018 20:26, snipped-for-privacy@aol.com ha scritto:
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Hi Peter, thanks for your reply and for the manual!
The fault consist in a "toothed" displayed waveform.
The Tracking Generator seem to be also malfunctioning, but I think that  
this is another issue; the toothed waveform is displayed with an  
external input signal.
I have both service and user manual, and I (and my colleague) have tryed  
a lot of set-up with no results.
Waiting for new ideas, I will try to read more deeply the manual, thanks.


Re: Help for hard repair attempt
On Tuesday, June 12, 2018 at 2:38:34 PM UTC-4, Eremita Analogico wrote:

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Do you have another scope to verify that the input signal is truly smooth, and that the toothed display is the actually the fault of the device?  

Just making sure that the diagnostics have covered all the necessary ground.

Peter Wieck
Melrose Park, PA  

Re: Help for hard repair attempt
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I'd check for the obvious, first:  scope out all of the power-supply
busses and see if you see any kind of noise on them.

The toothed waveform, and the fairly consistent height of the tooths,
makes me suspect that the output of the log amplifier is being
corrupted... possibly by a regular variation in voltage.  If the
Advantest uses a switched-mode power supply to create its analog
supplies, there might be a fault in the supply which is causing it to
dither between two different voltages... and this could throw the log
amp all out of whack.

The same fault might be affecting the tracking generator.





Re: Help for hard repair attempt
Quoted text here. Click to load it
corrupted... possibly by a regular variation in voltage.  If the
Advantest uses a switched-mode power supply to create its analog
supplies, there might be a fault in the supply which is causing it to
dither between two different voltages."

If you are right it may be as simple as a filters. I have a tendency to scope the supplies first unless indications are clear that that is not the problem  

What we do not see with the still is whether those square type waves are stationary or they move, roll across the display. If they do move and they change direction or velocity with line voltage the answer is right there.  

That's the simple answer, if that ain't it then that could be a problem.  

Re: Help for hard repair attempt
Il 13/06/2018 01:08, snipped-for-privacy@gmail.com ha scritto:
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Hi, preliminary tests was done some times ago, and if I remember right,  
the various supply lines do not have ripples. When I got time, I will  
try again to verify supplies.
The tooths are stationary over the waveform.

Re: Help for hard repair attempt
Eremita Analogico wrote:
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These tooths look pretty clean and perfect to come from a failing capacitor.
If this scope has a probe calibration signal or output maybe some signal is leaking into the input. Or some other source is leaking in the signal path. Some hints can be obtained observing how it changes with amplitude adjustments and scan timings.
I would start scoping the input with nothing connected to it to check if that square signal is there and if possible trace thst input through its stages.

Re: Help for hard repair attempt
Check the scope input coupling, also the probe. Looks like input coupling might be set to AC.

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