What would cause a ticking sound on a SMPS based circuit board?

[pickleit]

I am trying to fix a Wakins PN 1182201 REV E board. It's from a IQ2020 spa control system (approx 2008 era).

The system came in for repair as the system was accidentally sprayed with a jet washer. The main IC (just below the transformer) let out the magic smoke, as did the PCB fuse. Also faulty were the 4x 1N4007 diodes that make up the bridge rectifier. I have replaced 4x diodes, fuse (T5A 250V) and the IC (TOP-255YN). Also I replaced most of the electrolytic capacitors around the transformer and the TOP255YN IC. The only capacitors I have changed in that area are the 400V capacitors near the bridge rectifier.

When you switch on the system the screen on the control head flickers (as if it is turning on and off) and you can hear a ticking sound.

The power supply appears to be SMPS design and I have ~312 volts DC +ve and 0 volts -ve going into the transformer, but an unstable (almost zero) voltage coming out.

I have measured voltages (DC volts) on the pins of the TOP255YN IC:

V = 2.99
X = 1.024
C & F = 5.8 (pins linked)
D = 312.3
S = 0

I have measured voltages at various points around the control board (after the transformer) and the only voltage I get oscillates from ~60mV +ve to ~60mV -ve.

The ticking sound happens approx every 300ms for 2-3ms.

Questions?
What would cause the ticking sound?
What would cause the oscillating DC voltage?
FYI: I don't have and can't find schematics for this board.

[feedback.loop]

Ticking means that it tries to start, but some sort of protection kicks in or something is wrong in feedback, so it shuts down, and after a while it tries again.

[Wallace Gasiewicz]

Those black box things look like relays.You may be able to find the one that is turning on and off by putting your finger on it.

[Jeff eelcr]

I have seen capacitors cause this many times.
Jeff

[floobydust]

It sounds like the new IC is stuck in auto-restart (hiccup) mode. This is due to a perceived or real overload by the IC.
Some neighbouring parts can get damaged when the old IC blew. Check the little 47uF cap on the Control pin 3, it is critical.
I would next look at the opto-coupler as these can get taken out too. Then it would be to look for secondary-side shorts like the rectifier diodes etc. It seems to make +18V for the relays, and something else for the MCU.

[MathWizard]

What's the controller chip ? It should have a pin that indicates when something goes wrong and it resets. If it's a fine pitch pin spacing on the controller, I'd just trace back from the pin and solder onto that, and not the pin. No way would I try probing it directly, too easy to short the pins.

I'd get the controller datasheet, and see what it's doing, see what's out of spec. If it was me, I'd map out a lot of the circuit in LTspice.

Ok it's the TOP255
https://octopart.com/datasheet/top255yn-power+integrations-7156213

That's a pretty straight forward SMPS, not that much to go wrong. I'd start with it pin's.