Hameg HM203-6 weird display problem [solved]

[Edward]

a couple of years ago i purchased a Hameg HM203-6 of eBay and it has developed a strange screen problem where the trace is squashed in the middle of the screen. the horizontal length of the trace will sometimes flicker. I don't even know where to start to fix it. I have checked the voltage rails on the test connector and all seems fine. I have included a link to a GIF showing the screen flicker (http://gif-creator.ru:9080/gif.xhtml?id=75395). any suggestions of how to fix this would be greatly appreciated. Thanks in advance.

[oldway]

I have only schematics of the 203-7, but it's probably the same or with minor upgrades.
You said you have checked the voltage rails and all seems fine. Have you checked ripple too ?

You will need another working scope to repair this one.
I would first check waveforms of the X final amplifier (L26 and L27).
You could also make a test in XY mode to check the X final amplifier. (Lissajous waveform)

I think this problem is a time base fault.
Check the input waveform of the X final amplifier on R2076.

[Edward]

Thanks for replying. I have found a HM203-6 schematic and i have located the input to the x amplifier i have probed this with my DSO nano but I don't really know what I am looking for. I have also noticed that the waveform is not uniform, it appears squashed at the edges. I will include the pictures and schematic. Once again, Thanks for helping
https://www.dropbox.com/sh/4zgh3sxurm0bxwh/AADKApB5qRuNFV5hd0OXdeS9a?dl=0

[oldway]

The blue trace waveform is OK, the red one isn't.

[Edward]

The red waveform is just on the screen of the scope and is not part of the measurement. I don't know how to turn it off.

[Edward]

Thanks for the help so far. I have tried to draw a circle by putting the cope into xy mode and putting 2 1kHz sine waves with a 90 degree phase difference into the inputs, the output is definitely not a circle. I have attached pictures. I have also tried to check for ripple on the supply voltages, the 5, 12 and-12V rails appear smooth but when looking at the schematics it looks like the x amplifier is powered by the 150V rail, and i cant test this with my DSO nano because the maximum voltage is about 80Vpp, Thanks

[Edward]

Thanks for the help so far, it has been really useful. I have probed across the x deflection plates with my dso nano with 10x attenuation. I have attached the pictures of the traces and oscilloscope screen. the first picture is what I assume it is supposed to look like with a smooth sawtooth wave, the second is the problem, the wave has less amplitude and a weird shape. I would be very appreciative of any ideas on how to fix this, I believe it is a problem with the x amplifier but i have no idea what could cause this.

[tautech]

Get the Tek PDF in the first post in the Repair board for assistance in CRO fault finding.
Ensure all supply rails are at correct voltages and ripple is is within spec.
Your waveforms indicate the problem is in the output stages of the plate amps but you must ensure PSU is to spec first.

[Edward]

I have checked the voltages on the test connector (12, -12,5 and 150v) and it all looks spot on, i have also attempted to check ripple and it looks smooth.

[oldway]

I don't understand....You show 3 pictures of waveforms and you said you have probed the 2 X deflection plates...where is the third coming from?

[Edward]

I connected the probe across the deflection plates with the ground clipped onto one plate (the scope is battery powered so it is floating) and the probe tip to the other plate. the 6 photos show the display of the hameg and the corresponding waveform across the x deflection and 3 different times to show how the problem changes over time.

[Edward]

After sitting on the shelf for a few years I decided to have another go at fixing this scope. I had recieved a message from another user suggesting that C741 and C731 could be faulty. I found that C741 had already been removed and replaced on the underside of the board. I tested both C731 and C741 and both were faulty. From the schematic I saw that both of these capacitors were in parallel with a 10uF 35V electrolytic (C732 and C739) when I removed C732 I found that this was also faulty (measures between 6 and 18nF). After replacing these four capacitors the scope is working again

[tautech]

Congratulations.

Can I ask whom the message was from ?
Was it by any chance oldway ?

[Edward]

Thanks
The message was not from oldway, I was in contact with two users both of which registered an account immediately before messaging me and have not made a post since

[tautech]

The message was not from oldway, I was in contact with two users both of which registered an account immediately before messaging me and have not made a post since

OK thanks.

I hope those anonymous new members become active participants as we need expertise on Hameg products now oldway has gone. 

[Calambres]

From your first shot (C741-C739_Closeup_s.jpg ) it seems that R902 is burnt, right?  (just right of the missing C741)

I have that very same oscilloscope!

[Edward]

From your first shot (C741-C739_Closeup_s.jpg ) it seems that R902 is burnt, right?  (just right of the missing C741)

It does look like that resistor has got hot, however it seems to be ok as the scope is working now. I measured the resistor at around 6.4Ω, the schematics specify a 6.8Ω resistor, interestingly this resistor seems to have a colour code of blue - violet - gold which would be 6.7Ω.

I noticed the discolouration but i decided to leave it alone as the scope is working now.