Re: 2465 repair attempt with (horizontal?) issues, help needed [Solved!!]

Tony Fleming
 

Thank you for detail information!

On Tue, Apr 30, 2019 at 8:08 AM satbeginner <@satbeginner>
wrote:

Hi all,

Here a solution I found for a horizontal problem I had on (one of) my 2465.

The problem was that all horizontal traces, readout, etc. were waaaay too
wide, also the time cursors were not shown.
Also when put in X-Y mode I still had a horizontal line, instead of a dot.


But the good news is: I solved it.

My assumption was that the horizontal shaped line must have been some kind
of deflection??
In a way it was, but not some hum or ripple, it turned out to be the
control of the last set of convergence grids.

On the A9 High Voltage board there is the Edge Focus potmeter (R1864),
that sets the voltage on the vertical convergence grid.
Amplifier U1890B and Q1890 are supposed to invert that voltage as an
inverting voltage follower to be used on the horizontal convergence grid.

After some searching I found that the voltage on the Quad Pole #3 (the
horizontal convergence grid) was always -285V, regardless of the setting of
the R1864 potmeter that did change the voltage on quad pole #4.
It was the €0,03 1MOhm feedback resistor R1891 that went all open :-), so
no feedback so the output went as negative as negative can be.

So, instead of extra focussing the beam it did exactly the opposite, it
spread the beam out completely, hence the line and the way too wide traces!

After replacing that resistor I have about the sharpest trace of all my
scopes, so I am a happy bunny!

Now I will put it all together again and start the calibration.

Un saludo,

Leo



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