Hameg 303-6 problems

[Daddy-B]

Hello everybody, i got a Hameg 303-6 with a view problems.

I think that it's all related to each Other, but i am not sure.

The main problem is that the lines on the display are a bit jumpy, after a while (say about 15 min) the lines become steady.
The second problem appears after about 20 minutes. The calibration square wave is not square any more.
Now i have no experians (yet) with repairing measuring equipment. I bought myself a new oscilloscope, so the time for me to learn repairing measuring equipment is here.
I don't have a service manual of this scoop and i hope someone can give me some directions how to solve this problem.


Thnx in advance


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[WaveyDipole]

Manuals for the oscilloscope can be found here:
https://www.rohde-schwarz.com/uk/search/hameg-archive_229476.html?term=HM303

There is a service manual on Elektrotanya:
https://elektrotanya.com/hameg_hm303-4_oscilloscope_sm.pdf/download.html

It seems like your scope needs a warm up for a few minutes before things settle down. This could be a symptom of a PSU issue. The usual advice would be to check line voltages and it might be worth doing that just after cold start and then after the problems stop/start manifesting themselves to see if there is any change. Looking at the circuit diagram on page 18 of the service manual, this would be test points MPT5 (+185v), MPT6 (+141v), MPT7(+13v), MPT8(-13.5v), MPT9 (+12v) and MPT10 (-12v).  However a word of caution: The PSU board has some very high and potentially lethal voltages present on the output side! For example, test points MPT14 (-2025v), MPT15 (-1950v) should not under any circumstances be probed without a suitable DMM equipped with an appropriate high voltage probe. I would like to think that there is adequate shielding to protect these areas but there seems to be no indication of this by just looking at the service manual. Also please take note of  the safety information at the top left of the circuit diagram. This is a switched mode (SMPSU) rather than linear PSU and I will leave it to a SMPSU expert to advise on that aspect, but there will be high voltages present here too.

If you have no experience then I would not advise probing around the board without some assistance from an experienced person.

[Armadillo]

Clean all the contacts;

Don't follow this video but it gives you a general idea of the problems with the 303;



Use this method to clean the contacts with a eraser and contact cleaner.



Hope it helps;

[WaveyDipole]

Certainly cleaning the contacts is worth a try. If this settles the jumpy lines problem, then if you have not already done so, follow this up with compensating the probes after your oscilloscope has had time to warm up.

https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/using-an-oscilloscope

See the section entitled 'Compensating an Attenuated Probe'.
Lets hope it is a simple as that!

[StillTrying]

Going just by the 2 photos, I can see 3 possible clues that the calibration square wave is set on 1MHz, with the scope probe on X1, which would make the square's edges a bit rounded.
Make sure it's on 1kHz for now.

[Daddy-B]

Manuals for the oscilloscope can be found here:
https://www.rohde-schwarz.com/uk/search/hameg-archive_229476.html?term=HM303

There is a service manual on Elektrotanya:
https://elektrotanya.com/hameg_hm303-4_oscilloscope_sm.pdf/download.html

It seems like your scope needs a warm up for a few minutes before things settle down. This could be a symptom of a PSU issue. The usual advice would be to check line voltages and it might be worth doing that just after cold start and then after the problems stop/start manifesting themselves to see if there is any change. Looking at the circuit diagram on page 18 of the service manual, this would be test points MPT5 (+185v), MPT6 (+141v), MPT7(+13v), MPT8(-13.5v), MPT9 (+12v) and MPT10 (-12v).  However a word of caution: The PSU board has some very high and potentially lethal voltages present on the output side! For example, test points MPT14 (-2025v), MPT15 (-1950v) should not under any circumstances be probed without a suitable DMM equipped with an appropriate high voltage probe. I would like to think that there is adequate shielding to protect these areas but there seems to be no indication of this by just looking at the service manual. Also please take note of  the safety information at the top left of the circuit diagram. This is a switched mode (SMPSU) rather than linear PSU and I will leave it to a SMPSU expert to advise on that aspect, but there will be high voltages present here too.

If you have no experience then I would not advise probing around the board without some assistance from an experienced person.

Thank you for your reply. That is a lot of info you gave me. I have enough experience with electronics and smps, but no experiance with repairing measuring eauipment. Here in the Netherlands it's now almost 3am, so i will open up the scoop tomorrow and your info gave me enough info to spend some hours measuring thnx


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[Daddy-B]

Clean all the contacts;

Don't follow this video but it gives you a general idea of the problems with the 303;



Use this method to clean the contacts with a eraser and contact cleaner.



Hope it helps;

Thank you. It is now 3am sobi will look at it tomorrow.

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[Daddy-B]

Certainly cleaning the contacts is worth a try. If this settles the jumpy lines problem, then if you have not already done so, follow this up with compensating the probes after your oscilloscope has had time to warm up.

https://learn.sparkfun.com/tutorials/how-to-use-an-oscilloscope/using-an-oscilloscope

See the section entitled 'Compensating an Attenuated Probe'.
Lets hope it is a simple as that!

Thnx o will look in to that.

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[Daddy-B]

Going just by the 2 photos, I can see 3 possible clues that the calibration square wave is set on 1MHz, with the scope probe on X1, which would make the square's edges a bit rounded.
Make sure it's on 1kHz for now.

The calibration switch is set to 1 Khz, and they where square at first. A saw it happen that the corners started to round of. And they don't get rounder that it is displayed on the photo.

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[Daddy-B]

Yes i successfully repaired my Hameg 303-6 scoop.
The video with the cleaning of the contacts was very useful, and the problem with the square wave corners is also solved. There where 3 leaky capasitors, and i found 4 lose connections. 


I am so happy, that i took my right hand, placed it on my left shoulder, tapped 3x and told myself weldone

Thnx for the very useful replies you all.

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[WaveyDipole]

Leaky ageing capacitors in the PSU and sometimes dry joints are common problems in older equipment. Glad you got it sorted! 

[Armadillo]

Yes i successfully repaired my Hameg 303-6 scoop.
The video with the cleaning of the contacts was very useful, and the problem with the square wave corners is also solved.

I am glad that you actually clean the contacts and solved the problems.  You see, some would pinned it down as trivial and use the wedge hammer to solve the simple problem instead... LOL.