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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.
 

Comments
John Samperi
John Samperi picture

Hello all, after 2 years of inactivity, mainly due to not being able to get parts, I'm back at trying to fix this.

1) I have replaced all electros in the power supply. No change crying

2) The schematic available (as above) is a bit different than the circuit in my scope. The above schematic is from around 1998, my power supply is April 2000. Also the circuit is for a power supply made in Germany, the one in my scope is made in China. I have searched for about a week for a diagram but not found anything. cryingcrying

3) I have made some progress identifying the area that is sensitive to heat (about 25mm x 30mm area) where the focus circuit is on the power supply. If I heat it up the focus is bad, freezing it with spray brings the picture back to being sharp as a knife edge. Unfortunately I can't identify any part, mostly 1M high voltage 0805 resistors (all perfect) 1x10nf 2kv cap also perfect, 1 x BF820?? sot23 transistor that measure OK. I will try to get some new transistor and caps, mostly available for about 2 weeks delivery. I'm attaching the power supply bit of the cct. keeping in mind that is not exact.

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John Samperi
John Samperi picture

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

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mosomedve
mosomedve picture

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
John Samperi picture

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
SzBálint picture
****

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dbreeze
dbreeze picture

Power supply filter caps?

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John Samperi
John Samperi picture

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

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