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Old 4th October 2007, 03:13 PM   #1
GeeVee is offline GeeVee  Australia
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Default oscilloscope help

Hi all

I know that this is a long shot, as the fault could be from a long list of possibilities, but if someone could have a look in their "crystal ball" for me, it will be greatly appreciated.

The matter concerns a TEK 2246 cro, which has substantial jitter on the displayed waveform, but dies down somewhat after a whlie, but never really stabilises.

The on screen menus however are perfect, just the waveform is jittery.

Could it be a problem with the input amplifier, or perhaps something to do with the vertical deflection?

Any thoughts will be greatly appreciated.
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Old 4th October 2007, 06:05 PM   #2
SY is offline SY  United States
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Could be either, someone more specialist in that scope line than me could shed light perhaps, but the first thing I'd do is open it up and clean all the switches and connectors.
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Old 5th October 2007, 02:24 PM   #3
GeeVee is offline GeeVee  Australia
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Hi SY.

Thanks for your response.

For what its worth, and wether or not it sheds any more light on the problem, is that the jitter of the waveform is present of all of the inputs.

With any luck, some else who has seen this problem before may respond.

Cheers
GV
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Old 8th October 2007, 02:48 AM   #4
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I have a question to add as well...

Just bought a nice Leader scope off ~bay, and am having lots of fun with it. HOWEVER, the first five to ten minutes after turning it on scares me. Makes a horrid squealing noise while it warms up. Is this natural? Gets even louder if I turn up the backlight or trace intensity. Within 5~10 gradually becomes quieter until becoming dead silent.

I know CRT scopes need a good warm-up period. Just the (power supply init?) squeal is unnerving.

As for GV's question... I have the opposite, however in a very minimal way. My digital readout can shake a touch depending on the input signal (lock) vs. trace frequency. Takes a short warm-up time to get pretty much unnoticeable. This I think is pretty natural, having to make/sync a second scan for the text.

Your case sounds different, but can you turn off the readout? Might help.
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Old 8th October 2007, 06:37 AM   #5
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Default Re: oscilloscope help

Quote:
Originally posted by GeeVee
The matter concerns a TEK 2246 cro, which has substantial jitter on the displayed waveform, but dies down somewhat after a whlie, but never really stabilises.

The on screen menus however are perfect, just the waveform is jittery.

It sounds like you need to adjust the triggering. Scopes usually have trigger threshold adjustment knob as well as several switches that deal with selecting polarity, etc. It frequently takes some fooling around with them to achieve a stable waveform, or even to get a trace to appear at all.

I_F
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Old 9th October 2007, 01:17 PM   #6
GeeVee is offline GeeVee  Australia
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Default Re: Re: oscilloscope help

Quote:
Originally posted by I_Forgot


It sounds like you need to adjust the triggering. Scopes usually have trigger threshold adjustment knob as well as several switches that deal with selecting polarity, etc. It frequently takes some fooling around with them to achieve a stable waveform, or even to get a trace to appear at all.

I_F

Hello and thanks for the response.
Unfortunately it is definately not a triggering or polarity problem.
After some investigation, and assistance by a friend (a self appointed expert!!) the fault is believed to be in the power supply

To make matters worse, the above mentioned problem has now been compounded by another problem.
The CRO now does not display any waveform whatsoever, including the cursors and the on screen text.
This now makes me even more convinced that the problem is in the power supply, or the CRT, or perhaps a combination of both.

Might be easier to replace the entire CRO. They are resonably cheap on ebay now.

Regards
George.
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Old 9th October 2007, 09:14 PM   #7
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Check the PSU electrolytic filter caps. It is the most common common failure in power supplies, especially switched ones. And it could explain the jittery waveform. The squealing scope is almost certainly due to bad filter caps on the output of the SMPS.
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Old 9th October 2007, 10:09 PM   #8
dnsey is offline dnsey  United Kingdom
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Quote:
The squealing scope is almost certainly due to bad filter caps on the output of the SMPS.
It could also be EHT leakage, especially if it's an irregular squeal (a bit like the noise made by a deflating balloon. Try cleaning the final anode lead and associated structures ( be careful to discharge everything first!)
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Old 14th October 2007, 01:38 PM   #9
GeeVee is offline GeeVee  Australia
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Default update

My oscilloscope is now in pieces on my friends bench, who is far more in the know than I am.

It appears that there are two completely separate problems, which is just coincidence and bad timing.

The first is the power supply. After replacement of most of the electrolytics, the CRT has come back to life, but the subststantial jitter in the waveform is still present.

The second, which is the jitter in the waveform appears to be a triggering problem, as mentioned by I_F.
Therefore I must apologise to I_F for previously dismissing the triggering suggestion so quickly.

I thought that just beceause the trigger light came on, it had satisfactorily captured the signal. No so.

Whilst i am still no wiser as to exactly what the problem / failure is with the jitter, at least the problem has been isolated (fingers crossed)

Regards
GV
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Old 14th October 2007, 04:35 PM   #10
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It's not likely to be the CRT. CRTs last a long time. Power supplies croak all the time. That's your starting point.

Jittery waveforms can be the result of noisy pots used to set the trigger threshold (I'm talking about the pot on the front panel with the trigger threshold knob attached to it). Dirty switch contacts in the trigger circuit can cause the same problem. Spray them all with contact cleaner and work them a few times.

I_F
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