Help troubleshooting function generator issue

[giosif]

Hello,

I bought a used (and cheap) function generator off ebay and, when I turn it on, I see the signal being periodically disturbed, like in the first picture below.
This picture shows the problem affecting a square wave, but the same happens for sine and triangle waveforms.
Other bits of information that might be relevant:
- I set the frequency all the way down to almost 0 Hz, and I got the output in the 2nd picture attached.
- Another thing I noticed is that, although the setting on the function generator was for a 4Vp-p signal, my oscilloscope was showing something like a 2.4Vp-p signal (you can see that in the first picture).
- The function generator is a Digimess/Caltek/others FG200.

Anyone has any ideas where I should start looking in my troubleshooting process?

Thank you!

Regards,
George

[vaualbus]

Are you have your oscilloscope / function generator impdence matched?

[fcb]

Start with getting the manuals 

Expand the time-base on the 'scope and see if that saggy bit occurs at 50/100Hz - check the power supplies in the FG.

[mij59]

Hi,

Start by checking the power supply, could be  a case of bad caps.
Also look for mechanical things like bad connectors, switches,  solder joints, pcb traces.

[edavid]

Are you have your oscilloscope / function generator impdence matched?

Ignore this, not relevant.

Start with getting the manuals 

Chinese product, no manuals available

Expand the time-base on the 'scope and see if that saggy bit occurs at 50/100Hz - check the power supplies in the FG.

Look at the second photo... 100Hz disturbance.

[giosif]

Thank you everyone for your replies and my apologies for not replying any sooner!

I've been looking into this a bit more and I believe at least one of the problems is with one secondary on the transformer.
Because of that and also because of the sheer build "quality" of this unit (I think the soldering on this FG could be used as a prime example of how to do bad soldering), I have decided to put it in the spares/source of components box.

Regards,
George

[alex.forencich]

Those pulses are at 100 Hz.  If you line freq is 50 Hz, then the rectified AC will oscillate with a period of 100 Hz.  My guess is the power supply filter caps are failed open. 

[devanno]

The reason for the impedance question is that sig gens that I'm familiar with, outputting to 50 ohm into a 50 ohm 'scope should show accurate signal amplitude... if the 'scope is high impedance, then you will not get an accurate voltage reading at the 'scope.

[RJFreeman]

as others have said, 10mS apart, therefore 100Hz, you are in the UK, so 50Hz mains, start at the power supply....

[diegosfb]

I second that, checking the caps on the PSU is probably worth it.

[giosif]

Thanks again for the good tips, everyone!
I will give it another go next time I get to it.

Regards,
George