Hameg 203-6 Oscilloscope flickering problem.

[Calambres]

Hi, there!

I've recently got an old Hameg 203-6 oscilloscope. It seems to work OK but there's a very annoying issue with it: the wave flickers in certain timebase adjustments: 2ms, 5ms and 10ms. This happens in both channels and no triggering selection can solve it.  Just in those three settings. If I change to the previous setting, 1ms and below, the trace becomes rock solid. It even happens when I put the input coupling to GND: the flat line flickers. 

I suspect a problem in the time base circuit but I'm not able to find it. All the other scope's functions seem to work fine.

If you want to look at the schematic, the Time Base board and schem are in the pages 7 & 8 of the Service Manual:

Service Manual:
https://onedrive.live.com/?authkey=%21AC%2DprPxPkJ96%5Fp0&cid=D3F55CD5F0B2818B&id=D3F55CD5F0B2818B%213582&parId=D3F55CD5F0B2818B%21186&o=OneUp


User Manual:
https://www.manualslib.com/download/1246263/Hameg-Hm203-6.html

Any ideas?

Thanks a lot in advance.

[w2aew]

Just a silly question - you don't have it in CHOP mode, do you?  You can sometimes get flickering when the sweep rate is harmonically related to the internal CHOP frequency...

[Calambres]

That is not a silly question. In fact that was the first thing I thought when I observed the problem. Unfortunately that is not the problem.

[Calambres]

Attached the TimeBase schematic. I presume the problem is somewhere there...

[Calambres]

Hope this video can show what I mean:

https://youtu.be/LtLPAZceflU

- First 7 seconds: 100Hz 1Vpp into channel 2, 0.2V per div. and 1ms in TimeBase. The flicker you see is a camera artifact as in reality it is rock solid.

- From 8" to 10", TB in 2ms. The flicker is very noticeable.

- From 10" to 14", TB in 5ms. The flicker is even worse.

- From 14" to end, TB in 10ms. The flicker goes wild.


Any ideas?

[StillTrying]

As the trace bright times appear to be 10ms long, at least on the vid, and not synchronized with the X position nor the 100Hz rest signal, I would guess it might a problem with the -2 Killer Volt HV generation.

Try with the scope trigger set to AC Line to see if the brightness is then X synchronized.

If you're not very used to working near such voltages one mistake could be your last.

[Calambres]

As I said in my first post, no matter what trigger option I select (or any other option for that matter), I cannot get the wave to be steady in those particular TB settings. Also all TB settings starting 1ms and faster are rock solid. Is it possible that a CRT-HV failure shows only at certain timebase settings?  If I switch the TB for settings slower than 10ms (20ms, 50ms, 1s, 2s) the trace then moves ostensibly from left to right but with NO flickering at all.

Besides, I have neither experience with such high voltage nor test gear (i.e.: HV probe) to make it safely.

To make things worse, the image in my youtube is worse than reality but I am not able to make it better, sorry. A camera artifact, I presume.

Thanks for your help 

[newbrain]

I'll go out on a limb, and say that what we see is normal.

At 2ms/div, when retrace and trigger times are included, the redraw frequency is less than 50Hz, at 5ms/div less than 20Hz and at 10ms/div less than 10Hz.

Even 50Hz, depending on the persistency of the CRT, can be slow enough to perceive flicker.

This, in the assumption that the chopping in the video be only due to the interference between the redraw rate and the camera shutter.

[Calambres]

Are you saying that any analog scope displaying that signal (Sine, 100Hz) at those timebase settings (2ms, 5ms, 10ms) are going to flicker?

This is my first analog scope. I have an old Tek TDS-320 (CRT DSO) that is absolutely steady at any TB setting but this is a Digital Storage Oscilloscope and it is supposed to do so. My other scope is a semi-toy JYE DSO-150 "Shell" model that behaves the same as the Tek. You may be right but I remember using analog scopes a looooong time ago when I was studying EE at the university and I don't recall seeing such behaviour.

On the other hand, you may be perfectly right. In fact, what you see in the video is much worse than what you actually see in the old Hameg. I've tried with several recording methods (compact digital camera and smartphones) to no avail... the chopping is always present.

[newbrain]

Exactly, unless it's equipped with P7 phospor (long persistence).
Your Hameg is 99% not, see also this thread:
https://www.eevblog.com/forum/beginners/monitoring-audio-oscillators-on-analog-oscilloscopes/

[Calambres]

Very interesting reading. Thanks a lot for the reference 

It seems everything is probably OK with my scope.