Tektronix 2465 - Beam Intensity too bright when more than 1 channel is active

[Terraqua]

Hello EEVblog folks and Tek 24xx owners

My Tektronix 2465 is having a problem with the beam intensity.

If I adjust the intensity to a normal brightness when 1 channel is active, it becomes too bright when I activate a second channel. If I activate a third, it remains the same, and when I activate all 4 the intensity becomes really bright...

All PSU voltages are within spec, DAC ref in spec. I replaced U650 (just to check).... the problem is located somewhere else 

Does someone have an idea where I should start the troubleshooting? My old tekky and I will be gratefull if you could point us in the right direction...

Here a video of the problem (brightness adjusted to 4 active channels, if I disable a channel, I can't see any traces (unless I increase the intensity)

https://youtu.be/n9u6L1K3P8Q

[bdunham7]

Can you post a photo of the entire front face of the scope showing how the controls are set?  Is this something that just happened suddenly after working normally or is the scope new to you?

[tggzzz]

Those are sensible questions!

If you need to go inside the scope, the traditional places.to start are with the PSU voltages/ripple, SMD caps on the control boards, and voltages near the DAC. All are well described on this forum and elsewhere.

A good 2465 is a delight to use

[Terraqua]

I have this scope for a while now, but it was in storage in a box with silica gel granules to keep it dry (don't know if that's the right name in English for that stuff)

Now I have a seperate room for my hobbies, and more space, so I thought to recomission my old scope again.

It still works all right, apart from the brightness problem... the PSU voltages and ripple are within spec, that's the first thing I checked...

It seems cumulative. If I adjust the brightness when 1 channel is on, it is too bright when 2 or 3 channels are active, and blasts away the fosfor on the screen when I activate all 4 channels...

Here a picture of the settings on the front panel: