Oscilloscope noise - is this normal? [With video]

[Augmental]

Hello,
I am an MCU enthusiast and have just invested in an Oscilloscope to help me through the analogue side of things.

I bought a basic LS OS-5020G from eBay. I think it may be slightly malfunctioning so I am looking for some reassurance or advice.

Basically, the trace seems to be noisy. I have linked to a video below so you can see for yourself. This is more apparent on lower time-bases such 10ms and 20ms/division. The amount of jitter on the beam appears to be consistent over all 'voltage division' settings which says to me that the noise is being introduced on the output side rather than the input.

From the video; is this level of tracing noise normal, or is there a problem with my scope?

Video of tracing a square wave from the function generator, as well as GND:
**broken link removed**

Photograph of front panel:
**broken link removed**

 

[Sceadwian]

Would help if we knew the frequency you were testing at, it looks like some kind of AC line noise, it could be from the square wave generator or from electrical noise on the scope line. Is the scope properly grounded and wired and what else is running on the same branch circuit? You should ask the same question about the square wave generator. I would definitely start with the local power situations first.

 

[crutschow]

The noise is not normal and appears to be at the line frequency. If so, then it's likely a failed power filter capacitor somewhere in the oscilloscope.

 

[audioguru]

LG (Lucky Goldstar or Life's Good) is Korean and was called Goldstar and Lucky. I saw some of their electronics 30 years ago and the circuit design, parts and pcb material were horrible.
The soldering was the worst that I have ever seen. Maybe they have improved?

Are you using a shielded probe cable? Without a shield then a wire is an antenna that picks up mains hum from the air.

 

[Augmental]

I suspect crutschow is correct - that one of the power smoothing caps has failed.

As I said, the noise is remarkably consistent in character, even on a flat line, when the probe is disconnected and the input is set to GND - i.e. the function generator is NOT being used. In fact it doesn't seem to be affected by any control on the input side.

I even tried attaching the ground wire to some exposed pipework to try and ensure the device has a proper ground, but there was no change.

Luckily I have found a copy of the service manual online and ordered a capacitance meter. So, I will be able to disassemble the unit and go through all the caps to check they are still working to spec.

Does anyone have any other ideas for diagnosis of this kind of scope fault, or tips on finding & replacing broken caps?

Thanks for you everyones suggestions so far! - Chris

 

[crutschow]

Are you using a shielded probe cable? Without a shield then a wire is an antenna that picks up mains hum from the air.

My impression from his discussion was that the noise level did not change with the input gain settings, thus the effect has an internal source.

 

[ghostman11]

i have just sorted out my DSO wich had a simmilar problem, i found a straight forward cap meter didnt help as the caps tested out ok, in the end i borrowed a ESR meter and found the bad cap that way. infact in mine i had to replace 4 caps.
nice looking scope by the way, should serve you well i like the function generator feature

 

[Augmental]

Wow am I glad you posted that ghostman, I hadn't realized that a cap could show correct capacitance but still have bad charging characteristics, revealed by an ESR meter. I feel like it could be a wasted exercise to test the unit without using an ESR meter now, but unfortunately the only ones available to buy are rather pricy.

There seem to be various projects online to build your own which I will have a look at.

 

[crutschow]

Sometimes when electrolytics fail, they will get warm, so check for that.

 

[ghostman11]

i dont want to cloud the issue any, but i should add that i also found a dud one with just a multimeter with a cap setting on it.... i guess in your shoes my own approach would be to first look for any bulging electrolytics and replace theese as a matter of course, then look at all the caps in the power supply area. personaly being tight and if i didnt have access to a ESR meter i would desolder and test them one by one and see if any were showing iffy readings on a cap meter. if i found any i would replace. if this didnt work then and only then would i probally consider a ESR meter.
like you say they cost a few ££. you may well get lucky just using your eyes and a cap meter