Hameg HM203-5 scope retrace fault

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Discussion Overview

The discussion revolves around a retrace fault observed on a Hameg HM203-5 oscilloscope, specifically when displaying a sine wave. Participants explore potential causes, troubleshooting methods, and maintenance suggestions related to the issue, which appears to be linked to the scope's age and condition.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes the retrace line is visible only on the left 1/4 of the screen when handling a sine wave and suggests that the blanking signal may be fading too quickly.
  • Another participant recommends using a second oscilloscope for troubleshooting and expresses confidence that running the faulty scope will not cause further damage.
  • A later reply indicates that the retrace line changes with the sweep rate, becoming shorter on faster sweeps and potentially showing a vertical line at the start of the sine wave.
  • One participant points out that the vertical line appearance may depend on the trigger settings and emphasizes the importance of proper blanking during sweeps.
  • Concerns are raised about using contact cleaner on the input circuitry, with a warning from Tektronix regarding potential degradation of the PC board material.
  • Another participant suggests a cautious approach to using contact cleaner, advocating for leaving switches alone if they are not noisy.
  • The original poster acknowledges the finicky nature of certain switches and considers a careful application of contact cleaner while protecting surrounding areas.

Areas of Agreement / Disagreement

Participants express a mix of agreement and differing opinions on the troubleshooting approach, particularly regarding the use of contact cleaner and the implications of running the oscilloscope in its current state. No consensus is reached on the best course of action or the exact cause of the retrace fault.

Contextual Notes

Participants discuss the potential risks associated with using contact cleaner and the variability of the retrace line based on sweep rate and trigger settings, but no definitive conclusions are drawn regarding the fault's cause or resolution.

Guineafowl
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This circa 1990 scope has started to display a retrace line when handling a sine wave. It’s only visible on the left 1/4 of the screen. The fault doesn’t appear when the square wave cal is used.

Looking at the block diagram, and from initial thoughts, I’m guessing the blanking signal must be present, but is fading out too quickly and letting the beam through just before the start of the next sweep.

Since the scope is quite old, I think I’ll give it a service soon, including:
1. Contact cleaner for all switches.
2. General cleanup and inspection of soldering and connectors.
3. Recap.

If the fault persists after the service, I’ll start looking into it then. Is this a sensible approach, or does someone know straightaway what might be wrong? I don’t know how common a fault this is. Could the scope be damaged by using it in the meantime?
 
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Best bet is another scope to troubleshoot the one you have. Looks to me like an excellent opportunity to gain some troubleshooting skills. I doubt you will damage anything running it this way. Does it make a difference what the sweep rate is?
 
Averagesupernova said:
Best bet is another scope to troubleshoot the one you have. Looks to me like an excellent opportunity to gain some troubleshooting skills. I doubt you will damage anything running it this way. Does it make a difference what the sweep rate is?
Yes, the retrace line alters according to the sweep rate - shorter on a faster sweep. At some speeds, there’s a vertical line at the start of the sine, as if the sweep is jamming at the start.
 
That vertical line will likely change based on how you have the trigger set. It is supposed to hold until the trigger initiates a sweep. If it is not blanking you will see something vertically because you have a signal in the vertical section.
 
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Guineafowl said:
1. Contact cleaner for all switches.
Be cautious about using contact cleaner at the input circuitry. Tektronix specifically warns not to use contact cleaner there because it degrades the PC board material used.
 
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Tom.G said:
Be cautious about using contact cleaner at the input circuitry. Tektronix specifically warns not to use contact cleaner there because it degrades the PC board material used.
I tend to go: If it ain't broke, don't fix it. In other words, if none of the switches seem noisy, leave them alone.
 
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OK, thanks. The switches are a bit finicky, especially the component tester toggle and on/off. I was going to spray in a tiny amount, under guard of a paper towel, and work the switch.