I'm trying to troubleshoot the above scope which has a dim screen and loud high pitched whistling when on.

Would a faulty backlight cause this whistling or the drive circuitry?

 

I have looked through the service manual but it only goes as far as checking the voltage at the backlight connector with it connected and removed. No further info. other than to replace the entire power supply board.

I'm aware that there could be in excess of 1kV at the backlight connector.

I did try disconnecting the backlight and the whistling was still present.

Attached is a photo of the power supply board showing the backlight circuitry.

I know @Delta Prime has some experience repairing backlight issues?

 

The whistle will be the Yellow Transformer vibration at high frequency, it looks like a two transistor oscillator feeding AC through the Blue capacitor.

 

Any ideas what might cause the dim screen? Would it be the oscillator circuit?

Attached is a photo of the markings on the blue high-voltage capacitor. The capacitance is too small to measure with a multimeter 18nF?

 

it could be the screen putting heavy loading on the oscillator causing it to whistle.

 

I did try disconnecting the backlight and the whistling was still present.

The large red Wima capacitor (0.68) in circuit measures a short, however that may be because it's across the primary windings of the transformer? I'm not sure if it's for the oscillator circuitry?

What's the best way to remove the clear potting compound and desolder the capacitor?

 

you can try and see if it will melt with a soldering iron or heat gun, or cutting off with wire cutters, the capacitor is for tuning, have you tried pressing on the transformer with a piece of plastic to see if it changes or stops?