HAMEG 203 - asymmetrical (standard) square-wave

[nimda-eev]

I have just bought an old (but nice) oscilloscope, a Hameg HM203. Next to me, all the functionality that should be there, is there. Besides that the scope needs some calibration, I wonder whether the next phenomenon could also be adressed during this adjustment (calibration):

When connecting a probe to the 200 mVpp square wave output eyelet, a square wave is shown. But, the positive horizontal line segments of the square-wave are significantly longer than the negative ones. This happens on both channels. I've included a photo of the result following the standard instructions for viewing the standard square-wave signal in the manual, and also a version with a time-base that shows the difference more pronounced.

With difficulty I found a hard to read copy of the adjusting plan. (It is a pity that I did not find a step-by-step calibration procedure, by the way..) Most text labels next to the adjustment pots are kind of self-explicable, but neither of them indicates or suggests (to me)that it will address the asymmetry in the signal projection.

Can one of you help me with this problem (if there is one..? :-)

[oldway]

No need to worry about this, it is not a true square wave generator, it is only a NAND oscillator.
The only ajust is for amplitude = 200 mV +/- 1%

[singapol]

Here is Hameg203-4 service manual in English (Good copy).

http://www.eserviceinfo.com/downloadsm/65162/Hameg_HM-203-4.html

I would check calibrator circuit first. The resistor,capacitor like C2001-C2003,C2004 (10uf 35V) and VR2001. Not to forget
IC 2001 (HEF4011BP). If calibrator circuit is good then check horizontal X deflection output circuit. Assuming
power supply voltages are good.

PS it does output a square wave.Your actual waveform is distorted at the bottom. Do you have another scope to check?

[oldway]

The only thing you can eventualy tweak, that's the horizontal amplifier gain.
The horizontal trace seems a little short.
If you are not an expert, let this oscilloscope as it is....don't look for trouble ! 

[tronde]

I agree with oldway. Don't make trouble for yourself.

The first thing you should do, is to make really sure whether the problem is within the calibrator or the scope.

I have an old HM203-1 just like yours, and the square wave should be symmetrical.

If you have a low voltage mains transformer you can try to measure the output of that. You should see a symmetrical sine wave.

You should NEVER mesure anything connected directly to the mains supply. Make sure to use a low voltage transformer.
If in doubt, ask here before you measure.

Even better is if you have access to a signal generator with sine out.

If the waveform is symmetrical, the problem is within the calibrator. If not, tell us exctly what i t looks like before you try to adjust anything. It's easy to introduce new problems if you don't know exactly what to adjust.

Here is the user manual for your version of the scope:
http://www.sm5cbw.se/hameg/hmosc/hm203-1-man.pdf

Here is the service manual with schematics and PCBs. It's in German, but schematics is a universal language.
http://home.vikenfiber.no/vv79/hameg_hm203_oscilloscope_sm_deutsch.pdf

[nimda-eev]

Thanks for all your responses! And the documents!

And yes.. it's unwise just to jump into the guts of the scope and star tinkering without knowing what I am doing. I do not have another scope at hand.. I am looking around for one though. But, there's another way I can check the scope first without opening the cover: A couple of days ago I ordered a simple a simple wave generator, an FG085 do it yourself kit. For me, putting that thing together will be an interesting project in itself. And it could help me to further analyse the situation with the scope.

I will keep you posted 

[cnqhdszq]

I have a  old  Digital Storage  Scope  HM208  , it works good .But one day ,I found the same thing. After nearly ten days hard think , I recognized that  there are no problem .