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Author Topic: What Is Going On Here??  (Read 1126 times)

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Online AF6LJ

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What Is Going On Here??
« on: June 20, 2017, 03:31:13 AM »
Okay...
This might become a project in the new future...
A friend picked up this Tek 2240 for a too good to be true price.
It doesn't seem to be working correctly even though it passes all the power on tests.
Anybody have any ideas before I dig in...
Or is this a case of cockpit error???
Sue AF6LJ
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Offline The Soulman

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Re: What Is Going On Here??
« Reply #1 on: June 20, 2017, 03:41:46 AM »
What type/shape signal with what amplitude/frequency are you feeding it?
Tried increasing the Volts per division?
No point in making a point just to make a point, that would be pointless.
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #2 on: June 20, 2017, 06:57:46 AM »
What type/shape signal with what amplitude/frequency are you feeding it?
Tried increasing the Volts per division?
No signal, What you are seeing is the scope with no input signal.
It won't see an input signal from an outside source....
Sue AF6LJ
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Offline Andy Watson

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Re: What Is Going On Here??
« Reply #3 on: June 20, 2017, 07:10:58 AM »
It won't see an input signal from an outside source....
It might, if the scope is not grounded. The signal looks too "good" to be a fault!
Has it been got-at? Has some of the internal shielding been left off?
 

Online edpalmer42

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Re: What Is Going On Here??
« Reply #4 on: June 20, 2017, 07:18:44 AM »
Both channels?  Does the volts/division control affect the signal?  Are you really sure that no input signal appears between the bursts?  Does the timebase affect anything?  C'mon Sue, more details!

I agree with Andy.  The signal looks too good.  But I wouldn't think that missing shielding would give something like that.  It looks more like something is shorted into the signal line.  Could be anything from a wire clipping to a misplaced ground finger.  Maybe a broken internal ground lead.

Ed
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #5 on: June 20, 2017, 07:45:01 AM »
Both channels?  Does the volts/division control affect the signal?  Are you really sure that no input signal appears between the bursts?  Does the timebase affect anything?  C'mon Sue, more details!

I agree with Andy.  The signal looks too good.  But I wouldn't think that missing shielding would give something like that.  It looks more like something is shorted into the signal line.  Could be anything from a wire clipping to a misplaced ground finger.  Maybe a broken internal ground lead.

Ed

None of the controls have any effect
My first guess is there is a power supply issue of some kind.
Sue AF6LJ
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Online AF6LJ

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Re: What Is Going On Here??
« Reply #6 on: June 20, 2017, 07:48:31 AM »
I'll check to see if there are any missing shields next time I see it.
It is not here, it is over at my friends place.
Sue AF6LJ
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Offline Andy Watson

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Re: What Is Going On Here??
« Reply #7 on: June 20, 2017, 08:28:56 AM »
Er, I have to ask the obvious question. Since this is a storage scope with non-volatile memory, is it simply stuck on displaying the waveform that the last user was looking at?
 
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Online AF6LJ

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Re: What Is Going On Here??
« Reply #8 on: June 20, 2017, 09:57:01 PM »
Er, I have to ask the obvious question. Since this is a storage scope with non-volatile memory, is it simply stuck on displaying the waveform that the last user was looking at?
That is certainly a possibility.
I will know more in a couple of weeks when we get together again.
Sue AF6LJ
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Offline David Hess

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Re: What Is Going On Here??
« Reply #9 on: June 21, 2017, 11:45:35 AM »
Does it do that on both channels?  What happens at different time/div settings?  What happens at 200ns/div and faster?

There is a detailed manual diagnostic test which should be run.
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #10 on: June 22, 2017, 02:03:16 AM »
Does it do that on both channels?  What happens at different time/div settings?  What happens at 200ns/div and faster?

There is a detailed manual diagnostic test which should be run.
It does that regardless what you do.
I haven't had a chance to open it, and the individual it belongs to doesn't own torx bits.
Sue AF6LJ
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Offline Jwalling

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Re: What Is Going On Here??
« Reply #11 on: June 22, 2017, 02:05:42 AM »
I've seen 2430/2440 scopes with that problem. IME it's the CCD chip(s) although they usually fail self test as well.
Jay

System error. Strike any user to continue.
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #12 on: June 22, 2017, 02:43:55 AM »
I've seen 2430/2440 scopes with that problem. IME it's the CCD chip(s) although they usually fail self test as well.
Thanks for your input, I will keep everyone posted as we pursue this problem.
Sue AF6LJ
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Online metrologist

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Re: What Is Going On Here??
« Reply #13 on: June 22, 2017, 04:08:12 AM »
What are the CCD chips?

Mine does the exact same thing. I took out the power supply and started measuring caps, took a few off to compare with in-circuit measurements. They all seemed to measure fine, except the paper metal film (I forgot what they're called) around the transformer - the ones that usually go bad, mine were bad.

I replaced those caps and it fired up great, passed all self tests and then it went back to locking up and failing again. I was thinking of wholesale PS cap replacement, but bought a Rigol and forgot about it. I'd still like to fix mine if possible. I'm just lost as to what to do.  :-//

 

Offline David Hess

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Re: What Is Going On Here??
« Reply #14 on: June 22, 2017, 07:52:20 AM »
What are the CCD chips?

These DSOs sample the input signal and store it in charge coupled devices configured as bucket brigades.  When the trigger is recognized, a further number of samples are taken and stored providing variable pretrigger capability and then the entire record is slowly read out into a digitizer.  The advantage of a CCD based DSO was that up until relatively recently, it could support a higher sample rate than a fast ADC and memory.

As far as what to do with this particular instrument, do a detailed inspection with the case off, check the low voltage power supply outputs for level, noise, and ripple, run the full diagnostics, reseat various cables between the boards, and maybe pull the hybrids on the bottom board and clean their contacts.

Do not operate this oscilloscope with the cover removed without a fan blowing on the hybrids along the bottom because otherwise they will overheat.
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #15 on: June 23, 2017, 05:34:33 AM »
That scope is like my 475 it must have air blowing across the power hungry hybrids.
The bit about how the sampling works makes a lot of sense.
Sue AF6LJ
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Offline Bud

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Re: What Is Going On Here??
« Reply #16 on: June 23, 2017, 02:44:04 PM »
This may be a corrupted image from the scope channel memory, when you have these scopes powered off for a prolonged period of time the memory becomes corrupted. You should press the Acquire button to put the scope in acquisition mode and it will clear the screen if working. Your bigger concern is going to be if the scope does not start rolling when you press Acquire and remains locked. This is where fun will begin.

EDIT: just checked my 2430, Acquire does not clear the memory screen, you should press Display Ref and uncheck all memory buffers using buttons under the screen.
Anyway, attached is somewhat similar image to yours.
« Last Edit: June 23, 2017, 03:03:43 PM by Bud »
 

Online AF6LJ

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Re: What Is Going On Here??
« Reply #17 on: Yesterday at 12:35:52 AM »
Thanks I'll have a chance to look at it this weekend.
Sue AF6LJ
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Offline voltz

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Re: What Is Going On Here??
« Reply #18 on: Yesterday at 04:14:18 AM »
That scope is like my 475 it must have air blowing across the power hungry hybrids.
The bit about how the sampling works makes a lot of sense.

Also have a 475 here Sue if you ever need a comparison scope for future fault finding. I understand the tants are quite prone to give up. And yes the air flow is vital on these.
 
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