Tek TDS 540 (no suffix) Power Supply issue

[Marco_Mar]

Hi,
I bought a completely dead Tek TDS 540 (no suffix).
When plugged, it tripped the RCD, due to a leaky CY cap.
After replacement, no more RCD tripping, but the scope won't start.
Following service manual troubleshooting hints, I'm testing the power supply (119-3371-03) with no load.
The attached pic shows what I see across C12: don't be fooled by the Fluke readouts: the voltage is downscaled 100-1.
The average voltage is correct (around 410 V) ma where are those tremendous spikes (200 Vpp) coming from?
I'm stuck: please any help will be highly appreciated.

Thank you

Marco

[TERRA Operative]

First up, replace C17 with a new Low-ESR capacitor (yes, it does need replacement! 47uF and voltage rating greater than 80V) and check the associated parts as per the image in this post:

https://www.eevblog.com/forum/repair/tektronix-tds500-tds700-power-supply-models-psu-repair/msg5165748/#msg5165748

[Marco_Mar]

Hi Terra,
thank you for answering me.
I followed your recommendations replacing the other 3 RIFAs (C4 was already replaced) and C17.
No change: spikes again.
How could I check VR5 and VR4 (TVS Diodes)?
Attached, is a screenshot of +15V BIAS from STANDBY SUPPLY.
The average voltage is ok, apart from the same spikes as for + BULK.
Thank you

[TERRA Operative]

Ok, preventative maintenance is done.

To check the TVS diodes, they would need to be pulled and tested to their working voltages on a current limited power supply similar to how you'd test a zener. i.e. don't worry too much about it. They protect the transistor highlighted in the linked image, and if that was dead due to dead TVS diodes, you would get no life in the PSU at all.

Besides the voltage spikes you are seeing, what are the symptoms shown by the PSU? Is it not starting up? Bad output voltage? etc?

[Marco_Mar]

Hi,
well, when I power on the scope with the front panel button I can hear a click inside but nothing else happens.
I pulled out the PS and checked the outputs: nothing besides spikes!
I noticed that  U5 (PWM) on A18 is permanently switched on and off (pin 10) at the same rate as, guess what!, the spikes...
Many thanks

[TERRA Operative]

Ok, so it seems your standby power is operating (hence the click).

I would start by checking the transistors in the main converter and boost converter sections (in the schematic page you posted above), I've had bad transistors there before.
It almost looks like the PSU is trying to start over and over but is hitting some sort of self-protection each time.