Oscilloscope fault diagnosis

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Hello everyone,

I just purchased a 60MHz dual trace scope (a Sampo SSI-2360 to be
specific) which I thought was working. Unfortunately, it appears that
it is not quite right. I was hoping somebody could point me in the
right direction with fault finding it. With the inputs grounded, I can
get fairly focused traces, but not particularly sharp. The scope
triggers fine and the vertical amplifiers seem to be working perfectly.
However, if I connect the input of either channel to the calibration
square wave then I can get a semi-square displayed but the trace seems
to be "smeared" so that the high and low parts of the waveform overlap.
It's hard to explain so I took some pictures:

http://thompson.homedns.org/scope%20fault/1.jpg
http://thompson.homedns.org/scope%20fault/2.jpg

Playing with the focus control will give me a waveform like this:
http://thompson.homedns.org/scope%20fault/3.jpg
but if I ground the input with this focus setting the flat line trace
is very much out of focus.

The waveform looks exactly the same on CH1 and CH2, and any timebase
setting, so I suspect a problem with the HV section? My experience is
mainly with digital electronics so I'm not sure where to start. Can
anyone please offer some advice?

Thanks very much for any help,
Dean


Re: Oscilloscope fault diagnosis





**   Groper Alert   !!!!!!!!!!!


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** I take it YOU do not have the user manual.

Try to find a trim control for "astigmatism" or maybe just "astig".

Could be on the back panel or even internal.

When YOU find it,  set it for best overall trace quality with a SINE wave.

If that looks sharp and of consistent trace width  -  the rest is all OK.






.......  Phil







Re: Oscilloscope fault diagnosis


Hello,
He does have the operators manual - bought the unit off ebay - a lesson
learned no doubt

could be astigmatism or perhaps eht. if the trace is clearly visible with
intensity turned down then check the eht adjustment but i didnt notice
blooming on the pics so it's probably not. if with intensity turned down the
trace just about or just disappears then eht is ok and its likely to be
astigmatism

if the control is not on the front panel or back then it's inside. Check the
main board for potentiometer naming. If it's not there then perhaps on a pcb
on the back of the crt. astigmatism adjusts the voltage on the anode and/or
screen relative to the deflection plates. depending on the complexity of the
crt there may be other adjustments (ie pincushion or geometry) so be careful
to note position of the pot before adjusting if its not labelled. a
permanent marker is a good way to do this, mark the pot and the pcb so the
marks line up - adjust away.

Cheers
Greg



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Re: Oscilloscope fault diagnosis


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Thanks for the reply Greg, did you have your eye on ebay for this one
too? I guess I can't really complain since I got it for $150.

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The EHT seems fine, that was my first thought too, but the intensity
adjustment works perfectly.

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Thanks again Greg, this is exactly the kind of info I'm looking for!
There's no labelling but I'll take a closer look at the CRT PCB again
tomorrow. And before someone says something, yes, I do know about the
high voltages...

Dean


Re: Oscilloscope fault diagnosis



"gcd"

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**  LOL

Only TV sets have " picture tubes " .







......   Phil




Re: Oscilloscope fault diagnosis


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I do have the user manual, but is doesn't have any details on fault
finding or the location of any adjustments. There are not trim controls
on the back panel, there is a label that lists some of the calibration
adjustments but no astigmatism.

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I have had it apart, but none of the trim controls are labelled, just
to make things difficult. I think it has been stored for quite a long
time. On the plus side it does seem to be rather well made, using
mostly Japanese parts. Not that it does any good when it doesn't work
properly...

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I'm not familiar with these kinds of circuits, any tips on what I'd
find it near?

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I did also apply a sine wave when I was testing it:
http://thompson.homedns.org/scope%20fault/4.jpg
I guess I should have though of astigmatism... I'm used to DSOs

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Thanks for the advice Phil, hopefully that is all the problem is.

Dean


Re: Oscilloscope fault diagnosis



"Dean"

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** My gawd -  what on earth do YOU  call a "sine wave".

Some shit awful step-wise approximation is not one.






........  Phil




Re: Oscilloscope fault diagnosis


I finally had some time to play with the scope, if anyone is wondering
what happened with it. I managed to find what seems to be the
astigmatism pot and I can get a fairly decent trace if I have both the
focus and astig pots at their extremes. This part of the circuit is a
bit hard to follow as it is spread over a number of boards. I've
checked the high value resistors, pots, and various other components
and they seem to be in spec. So I'm not sure what's causing the lack of
sufficient range on the controls, any ideas? In case anyone wants to
know, the astig pot is on the main board at the top of the scope, near
the back. It's labelled VR80x. I can't remember the last number and
I've put everything back together now. It's the only pot numbered
in the 800's on that board though.

Thanks,
Dean


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