[Moved] Strange oscilloscope screen problem...

[Tricka90]

I've bought an used Hameg HM304 oscilloscope. I'm a beginner so that's my first one.
I found that the oscilloscope screen has a strange problem when I use "X-MAG X10" function to zoom the waveform a lot.
In the video below you can see how the wave deforms in different ways.
The more I increment TIME/DIV value the more the wave deforms. But if X-MAG X10 is not active the wave show perfectly (but it's too little to be measured accurately, of course).
In this video I'm measuring a 12MHz wave from a PIC external crystal oscillator and moving it around horizontally. The probe is set to 10X.
Can I do something to fix this problem?
Do you think is an hardware issue or it can be solved just with some calibration?

Here's the video: SAM 0060

 

[PA4TIM]

I do not know what wafeform a pic crystal oscillator is but your scope is 35MHz. If the oscillator is a squarewave you need 5 to 10 times 14 MHz to show it correct. So you need a 100-150 MHz scope for that. Your scope will transform a square into something that looks like a sinewave.

But the strange trace with the increased risetime for the first part of the trace in your video I have never seen before. I would look for the 10X part in the timebase (but I think this scope has a microprocessor and is some kind of hybrid)

 

[Tricka90]

I'm sorry, I didn't mean a Pic oscillator. I just wanted to say I was using the crystal for the clock of a Pic. That's a normal 12MHz cristal oscillator.

So do you think I can fix the problem looking for the 10X part? What should I look for?

 

[E-design]

It looks like it could be a magnetization problem. If other waveforms are correct on x1 then it is not. If you can get hold of a demagnetization coil, or even ask a TV repair shop that still service old type CRT type TVs to do over the whole scope for you. There are cheap ones on eBay **broken link removed**

 

[Tricka90]

Yes, other waveform (4MHz, 8MHz, and also a 1KHz square) are correct on X1. They deforms just when X-MAG X10 is active.

 

[E-design]

The x10 sweep normally activates a x10 current multiplier in the sweep circuit, so it could be a problem there causing non-linearity. Do you have a schematic for this model?

 

[PA4TIM]

E-design, a scope has an electrostatic deflection CRT that uses plates, not electromagnetic deflection like in TV's

 

[FvM]

X10 horizontal magnification can be found with many classical analog oscilloscopes. It's switching x10 gain to the X amplifier. We can expect a defective part somewhere in the X amplifier circuit.

HM304 is an upgraded HM303 with additional digital control, providing autoset and other features, if I understand right. The basic XY amplifier and CRT circuit should be almost identical between both models. At least the HM303 service manual is freely available from internet.

 

[Tricka90]

Thank you for the help, thanks a lot.
I'm sorry, I can't find the HM304 schematics on internet, I've just found the manual: https://www.akh.se/hameg/hmosc/hm304.pdf

I found something that could be interesting, I hope!
When I turn clockwise the DEL. POS. HOLD OFF knob, which controls hold off times between sweeps, the problem becomes way less significant! Before the knob was almost entirely turned counterclockwise, near the X1 sign!
Can this help you to understand what's the problem?

 

[E-design]

E-design, a scope has an electrostatic deflection CRT that uses plates, not electromagnetic deflection like in TV's

Yes, I know but in my many years having a service and calibration facility (before retirement) for test equipment, I have seen the effects of magnetization of the chassis and metal parts around the tube on the beam deflection. We always had a degaussing ring around when we encountered some weird deflection problem, and all the waveforms looked normal. It can basically affect all sorts of CRT based test equipment. In some instances, the magnetization was so bad that you hardly could get the trace on the screen.

 

[PA4TIM]

Thanks, good to know. I used to have a Tek 2445 in my scope collection and that showed also some strange trace behauviour. The cause was a high power 500V powersupplu that I placed some days befor that next to it.

i got my own private cal lab:
https://www.pa4tim.nl/?cat=29

 

[Tricka90]

Thank you.
First of all I'm going to try measuring the ripple on the power supply rails with a VAC multimeter.
Cold you please tell me how can I find the power rails when I open the oscilloscope? I have no circuit diagram...

 

[FvM]

As said, HM303 seems to be similar in many regards.

The screenshot didn't particularly look like excessive supply ripple, by the way.