TDS220 power supply dead

[zmaster]

Hello, the other day I plugged in my good old TDS220 and it would not power on.

I opened it and verified that nothing comes out of the power supply (all supply lines are zero).
On visual inspection, nothing stands out. No burn marks, no leaking/swollen caps.

I checked all components till the main DC cap and they're okay. The voltage there is 322V.
I also did a quick on-pcb check of all caps with my multimeter capacitance function and they seem to have sensible values, though I believe I should probably desolder them to properly asses their status.

I'm not sure what my next step should be.
I only connect it to the mains when I use it, so it can't be damage from overvoltage or something like that.
And, as far as I know, semiconductors don't get old...

Suggestions would be welcome.

[inse]

I once fixed the power supply of my TDS3000.
There was a voltage dropper in the 100k‘s range to generate the startup supply for the controller, it had gone high ohmic.
Look for some larger resistors on the primary side, maybe it’s the same principle here…

[Miti]

Change the electrolytic capacitors. Most likely they are dry and one of them prevents the PS from starting.

Edit: Start with the one at pin 6 of the driver chip, about 47uF or so.

[zmaster]

So, after inse's reply and looking at some reference designs, I decided to check the KA7552 power supply.
While measuring the voltage, I checked again the outputs and realised the PSU had started working again... weird.

I checked the KA7552 supply capacitor, C6, having a nominal value of 22uF. It measured 15uF with my UT210T (a bit suspicious, but I wasn't sure how well it would measure while still on the pcb) and 5uF with my BM235 (now very suspicious).
I removed it and found a bit of leakage on one of its legs. So, I replaced it with a new one.
That was definitely an issue. Good guess Miti!

Unfortunately, I have another problem now. The display is glitching a bit.
I don't know if it's related or not.

I measured the AC component on the outputs from the power supply and there's virtually none (a few millivolts).
I don't have a second scope to properly check for glitches. 
I disconnected the backlight to check if it was related to the high voltage and it made no difference.

I found this video of a similar problem:
Mine it not this bad, but I'm seeing the same black horizontal bands.
I checked the electrolytic caps accessible on the back of the LCD module without taking it apart (presumably the ones the guy in the video had to replace) and they measure 3.5uF, which appears to be okay (text on them is "3.3 35V").
I also tried to heat up the LCD module and PSU a bit with a hairdryer and it didn't seem to make a difference, though I did not want to use too much heat.

I'm undecided if I should replace those caps on the LCD module, the remaining caps on the PSU or look for another scope to borrow to dig deeper...

[TERRA Operative]

If one cap is bad in the PSU, replace the lot.
Be sure to use low ESR on the larger values at the DC output.

You aren't the first to find worn out capacitors in one of these scopes.

Once you are sure the voltages are stable, then go deeper if needed.

[zmaster]

I replaced the caps on the display and that fixed the glitches. Contrast now looks much better too and no "waves" on background 
I had not realised it degraded this much over time.

I didn't get to replace all the other caps on the PSU but that's something I should do.

Thank you everyone for the advice!

[Miti]

I removed it and found a bit of leakage on one of its legs. So, I replaced it with a new one.
That was definitely an issue. Good guess Miti!

That is not a guess, that’s science. 

I didn't get to replace all the other caps on the PSU but that's something I should do.

Change them all. My favourite type for power supplies is UPW series from Nichicon.