DSO 6054A DC offset problem

• DSO 6054A DC offset problem
  Posted by maxtee on 21 Nov, 2017 18:32
• I have a DSO6054A, with DC offset issues. The picture is attached. The problem is on all 4 channels. No matter what I try the signal trace can't be centered using the DC offset control. The signal trace is railed to the bottom of the screen. Any help will be greatly appreciated.

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• #1 Reply
  Posted by Neganur on 21 Nov, 2017 18:46
• Is that without anything connected to the inputs?
• #2 Reply
  Posted by EPTech on 22 Nov, 2017 00:01
• Hi there,
  
  Do you have experience with this scope, is it your own? On some scopes you need to press the trace positioning wheel. in order to bring the trace to the center of the screen.
  Then again it is also measuring a large negative value. If it is a general problem, It mail be a faulty supply rail or some reference voltage to the ADC. Take it apart!
  
  Happy hunting,
  Pascal.
  
• #3 Reply
  Posted by Jwalling on 22 Nov, 2017 11:40
• Have you tried running the user calibration?
• #4 Reply
  Posted by HighVoltage on 22 Nov, 2017 11:45
• I have seen this problem a lot on older scopes, never on a modern DSO
  
  What happens if you press DEFAULT setup?
  What happens if you apply a signal at the inputs?
  Is this problem right away after a startup?
  How did this problem come about? Did you overload the input?
  
  
• #5 Reply
  Posted by maxtee on 07 Dec, 2017 15:41
• Thanks guys for your response. The following steps have been tried without any success. I have no experience with this scope. I did use the repair manual to check for voltages and they seem to be OK. The self-tests results do show pass. Since applying input signal on any of the 4 channels have no effect, I suspect something wrong with the front end section. Perhaps one of the voltage is not correct. I have verified Three of the Hybrid power rails 5V, 12V and 3.3V to be within specs.
  
  - Default setup. No effect.
  - User cal. User cal starts but abruptly quits within seconds.
  - Applying signal doesn't show anything on the screen. That is true for all 4 channels.
  - The scope had a damaged 12V LDO which supplies12V to the input hybrid chips for all 4 channels. I replaced the LDO.
  - Yes, the problem starts right away after startup.
  - I don't think overloading since the problem is present on all 4 channels. What is the likelihood of all 4 channels getting exposed to overloading?
  - trace positioning wheel has no effect on the DC offset.
  - Yes, this without any signal present at the input.
• #6 Reply
  Posted by Neganur on 07 Dec, 2017 16:35
• Yup, sounds like the offset DAC is not doing its job. No idea if that's inside the ASICS or just a matter of another rail being blown too
• #7 Reply
  Posted by maxtee on 11 Dec, 2017 03:07
• Any idea what power rail that might be or which chip controls the DC offset?