Author Topic: Hameg HM 203 double trace problem  (Read 23062 times)

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Offline BFX

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Hameg HM 203 double trace problem
« on: July 23, 2014, 08:24:14 pm »
Hi,
I try to repair this nice analog oscilloscope but is still there one problem.

Have anyone any idea where can be problem?
All electrolitics capacitors are changed.
 

Offline miguelvp

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Re: Hameg HM 203 double trace problem
« Reply #1 on: July 24, 2014, 06:53:41 am »
Not triggering right and TR (Trace Rotation) needs adjustment?
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #2 on: July 24, 2014, 08:10:19 am »
Thank you.
Yes TR will be adjusted during calibration.
I'll check trigger circuits and trigger signal amplifier.
« Last Edit: July 24, 2014, 08:13:35 am by BFX »
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #3 on: July 24, 2014, 09:50:41 pm »
Some news.
I found bad zener diode in trigger circuit (Z400 in scheme) but after replace without change.

Here is video (sorry only from hands).


And trigger signal screenshot from oscilloscope (IC400 pin 9)
Scheme of timebase and trigger at the end.
Any another suggestions?

 

Offline Andy Watson

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Re: Hameg HM 203 double trace problem
« Reply #4 on: July 24, 2014, 10:00:59 pm »
What are you using as a signal source? It looks as if there is another asynchronous signal, perhaps mains, superimposed on the squarewave. Notice how the height of the displayed waveform varies - if it were just a trigger problem it would just show as left-right jitter.
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #5 on: July 24, 2014, 10:34:08 pm »
My homebrew generator  8)
But here is another video.

In this case I use calibration output of Owon oscilloscope.
 

Offline Andy Watson

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Re: Hameg HM 203 double trace problem
« Reply #6 on: July 24, 2014, 10:47:10 pm »
Whooa! Took me a while to work out why the signal was apparently retracing backwards! I assume that's a trick of the camera and what you saw was two relatively steady traces?

I still think you have a problem with mains corrupting the signal. Ground loop? - does the Hameg's cal output work. Is there anything producing a magnetic field near the base of the tube? Is there ripple on the scopes power supplies?
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #7 on: July 24, 2014, 11:20:17 pm »
No it's no trick it's real  :D (I think it's depends on temperature)
First pic is ripple of 140V that's used for X and Y final amplifier.
Second pic is ripple of -2kV but I'm not sure that is correct measurement.
I used 22nF/2kV capacitor between test pin and scope probe as is recommended in the service manual.
I am using an isolation transformer to power tested scope.
Last pic is power supply scheme.
« Last Edit: July 24, 2014, 11:41:09 pm by BFX »
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #8 on: July 25, 2014, 08:49:22 pm »
Ok next day without progress :-//
At all low voltages (+5V,+24V,-12V) is ripple about 10 - 20 mV and 140V, 260V about 40mV.
Some pictures again. Ramp output and output of Y final amplifier.
Any hints? Or suggestions what can I measure?
 

Online tautech

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Re: Hameg HM 203 double trace problem
« Reply #9 on: July 25, 2014, 09:06:50 pm »
If you study the PSU schematic you will see a ripple spec for each of the 5, 12, and 24 V rails.
In all cases it is better than 5 mV p-p. (1, 3, 5 mV respectively)
Get the PSU to meet those specs first.

You said you replaced the Ecaps, but maybe lower ESR versions are needed.
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Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #10 on: July 25, 2014, 09:59:38 pm »
Hmm but now is really problem to buy low esr axial capacitor.
There are currently used radial 105 C. I think there may be problem long capacitor wires.
 

Offline David Hess

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Re: Hameg HM 203 double trace problem
« Reply #11 on: July 28, 2014, 05:14:44 am »
The problem is not because of the capacitor lead length when you swap radial for axial capacitors.

Try setting the trigger to line input and sweep to about 5 ms/div to see if the noise is line related.
 

Offline oldway

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Re: Hameg HM 203 double trace problem
« Reply #12 on: July 28, 2014, 07:45:07 am »
This problem seems to come from an unregulated -2Kv power supply with high ripple.
Look for T800/T801 shorted or with leakage.
A good way to test de -2KV is to vary the main voltage of+/- 10% with a variac.
Pattern would not vary at all if regulation is ok.
Remember that deflection factor (V/mm) depends of accelerating voltage.
http://www.jmargolin.com/xy/xymon.htm
 

Offline Andy Watson

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Re: Hameg HM 203 double trace problem
« Reply #13 on: July 28, 2014, 10:22:30 am »
This problem seems to come from an unregulated -2Kv power supply with high ripple.
Look for T800/T801 shorted or with leakage.
I agree, the -2kV is interesting. It might explain the weird Z-modulation in the video. Bear in mind that the reference for the 2kV appears to be derived from the +5V supply - any disturbance here is going to be magnified on the -2kV output.
 

Online tautech

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Re: Hameg HM 203 double trace problem
« Reply #14 on: July 28, 2014, 10:45:18 am »
This problem seems to come from an unregulated -2Kv power supply with high ripple.
Look for T800/T801 shorted or with leakage.
I agree, the -2kV is interesting. It might explain the weird Z-modulation in the video. Bear in mind that the reference for the 2kV appears to be derived from the +5V supply - any disturbance here is going to be magnified on the -2kV output.
Exactly
The OP has stated his V+rails ripple spec is 4-10 times higher than that specified in the schematic.
Also:
Quote
All electrolitics capacitors are changed.
As the -2KV is derived from a dedicated ~470V AC winding then multiplied, that being regulated by circuitry supplied from the 12 & 5 V rails, any ripple on those supplies could be imposed on -2 KV
Not to mention any other affect the ripple might have on other CRO circuitry.

Get the LV rails ripple to spec.
« Last Edit: July 28, 2014, 10:52:02 am by tautech »
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Offline oldway

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Re: Hameg HM 203 double trace problem
« Reply #15 on: July 28, 2014, 01:03:14 pm »
Have a look for ripple upon C817 ! What is the frequency of this ripple ? 50 or 100 Hz?
With Hameg scopes, a very common fault is a rectifier bridge (BR802) with an open diode.
Then ripple is 50Hz instead of 100Hz and the input voltage drop too low for IC804 (5V voltage regulator) to do his job.
« Last Edit: July 28, 2014, 01:34:43 pm by oldway »
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #16 on: July 28, 2014, 09:20:27 pm »
Thank you for answers T800/T801 seems to be ok also BR802.
I try to measure ripple of 5V but it's difficult because ripple when I turn on the scope and asign probes (both) to GND ripple is more than 10mV  :palm:

I'll try to assign HV control voltages from battery, to inspect HV modulation ripple.
 

Offline Andy Watson

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Re: Hameg HM 203 double trace problem
« Reply #17 on: July 28, 2014, 09:54:18 pm »
I try to measure ripple of 5V but it's difficult because ripple when I turn on the scope and asign probes (both) to GND ripple is more than 10mV  :palm:
Are you saying that your measurement scope is creating a ground loop and introducing >10mV? Do you have a multimeter that measures AC? - If it will not directly measure the ripple on a DC supply, use  a capacitor to remove the DC component - like you are doing with the scope. Be careful, neither your measurement scope, nor your DVM will like having 2kV "spikes" pushed up their inputs!

Also, remember that whatever you measure on the 5V line is multiplied by ~400 on the 2kV. - You need to get the 5V correct.
 

Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #18 on: July 28, 2014, 10:40:38 pm »
When I measure 5V output it's 4.961V 
and -12 is -12.037
measured by HP 3478A

I don't know how to measure ripple lower than 20 mV to get right accuracy using my equipment.
I try suggestion by David Hess "Try setting the trigger to line input and sweep to about 5 ms/div to see if the noise is line related."
Yes it litle betther but I'm not sure with result.
 

Offline Andy Watson

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Re: Hameg HM 203 double trace problem
« Reply #19 on: July 28, 2014, 11:09:12 pm »
measured by HP 3478A

I don't know how to measure ripple lower than 20 mV to get right accuracy using my equipment.
The HP 3478A has a 300mV AC range. Set the meter to measure AC and use the capacitor (as you did previously) to block the DC. But connect the meter and capacitor before you power-up the Hameg - otherwise you risk damaging the meter.

Quote
I try suggestion by David Hess "Try setting the trigger to line input and sweep to about 5 ms/div to see if the noise is line related."
Yes it litle betther but I'm not sure with result.
Your video shows that the trigger is working - it is locking to your test waveform. Something else is superimposed on your test waveform and it is drifting across the screen because it is at a slightly different frequency to your test waveform (or, actually, an integer frequency divisor of your test waveform). What David is suggesting is that if setting the trigger to "line"  causes the problem waveform to lock and your test wave to drift, then the problem is most likely due to mains/line related issues (like excessive ripple).

Edit: It might be clearer if you move your test waveform frequency away from being an integer multiple of your mains frequency - say 1005Hz.
« Last Edit: July 28, 2014, 11:33:20 pm by Andy Watson »
 

Online tautech

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Re: Hameg HM 203 double trace problem
« Reply #20 on: July 29, 2014, 02:29:14 am »
I don't know how to measure ripple lower than 20 mV to get right accuracy using my equipment.
I try suggestion by David Hess "Try setting the trigger to line input and sweep to about 5 ms/div to see if the noise is line related."
Yes it litle betther but I'm not sure with result.
Set you other scope input to AC coupling.
For a perfect DC source it will show a flat line trace irrespective of input voltage.
Turn any HF filters on. (optional)
Set V/div to 5mV or better.

Use Line triggering if available, this will set triggering to mains frequency and multiples of.
By using Line there is no need to adjust the trigger level to within the waveform.

Timebase, 5 or 10 mS/div

5, -12 and 24V share a common GND that is also common with mains earth.
This is perfect for your reference clip.

Connect to each rail in turn and note ripple.
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Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #21 on: July 29, 2014, 07:24:18 am »
Some ripple measurement of low volages by HP 3478A.


5V


-12V


24V


260V


140V

 

Online tautech

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Re: Hameg HM 203 double trace problem
« Reply #22 on: July 29, 2014, 08:22:48 am »
So for each rail:
5 V measured 0.591 mV and schematic spec is 1 mV  :-+
-12 V    "       0.890 mV         "                      3 mV  :-+
24 V      "      0.669 mV          "                     5 mV  :-+

For the 160 and 240 V there is no schematic spec to meet however these are nice and low for non critical rails.
It will be interesting to see if the problem is gone.

But before you put her back together, see if you can duplicate your findings using your other scope to measure the ripple, but you probably can't as it is so low.
Even if your most sensitive setting is 5 mV/div and using a 1:1 probe, these ripples are ~ 1/10 division and barely discernible.
But try the 160 & 240 V rails for a hopefully quite visible result.
Good homework, this is good basic scope usage that should give confidence in use of scope functions.
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Offline BFX

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Re: Hameg HM 203 double trace problem
« Reply #23 on: July 29, 2014, 05:28:27 pm »
Story continues  :)
Measurement of ripple 2kV ... hmm something wrong is there   >:D
From where ?  :palm:

 

Offline David Hess

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Re: Hameg HM 203 double trace problem
« Reply #24 on: July 29, 2014, 06:03:23 pm »
Is that ripple on the -1876 or -2000 volt supply?

The later is almost certainly the CRT cathode supply and must be tightly regulated because it directly affects vertical and horizontal deflection accuracy.  50 volts of AC ripple is about right to create exactly the effect shown.

It would be helpful to see the CRT schematic.  I am not clear about what the -1876 volt supply is for.

As others have pointed out, the -2000 volt supply uses the +5 volt supply as a reference so ripple on the later will show up on the former and the +5 volt supply regulation is critical.

I would consider getting a high voltage probe so you can accurately measure the magnitude of the -2000 volt supply directly.
 


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