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Hameg HM-404-2 oscilloscope focus issue

I have a problem with the focus on this oscilloscope, it gets worse after about 1/2 hour of running and the focus knob is at it maximum.
Unfortunately there are no longer any service departments in Sydney, Australia to get this serviced.
Anyone with any hints or knowledge of this problem?
I have downloaded the manual and I am an electronics engineer.
 

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John Samperi
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Don't know what happen to my comments from yesterday but thank you everyone.

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mosomedve
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I think the focus circuit parts are bad. The resistor "longer" when the temperature increase. But bay be the scope tube is to old.

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John Samperi
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The picture is very sharp when it is cold and the scope is only maybe 12 YO and with very little use.

I did check some of the focus resistors a while ago but then I needed the scope so did not investigate further, I will have a bit more time in the next month.

The power supply caps mentioned below is a possibility as well as some of the high resistors going higher. The other thing it could be is the actual voltage going to the focus circuit in page 27 not being stable.

Thanks everyone for the ideas.

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SzBálint
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**

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dbreeze
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Power supply filter caps?

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John Samperi
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Interesting thought, I'll do a bit more digging along those lines.

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