Apple Multiple Scan 15AV CRT monitor repair

[tony359]

Hello all,

This is my first post here!

To start: I (hope) I know what I am doing inside a CRT, that I need to discharge it and ground it before touching anything.

I've got a number of these Apple monitors. They're somehow brand new but likely DOA or used by service centres as there are RMA numbers on the boxes. I was trying to make one or two working out of a pile I found
I brought home a couple, one unfortunately had a broken neck.

The other one would make a fast clicking noise when powered up. The Power LED also flashes. I know this usually points to the flyback but it looks like the PSU section to me. I do not see arching and the clicking noise comes from the PSU (I've confirmed using a plastic hose!). I've monitored one of the test points mentioned by the service manual for voltage and it goes on and off with the clicking.

So I swapped the boards between the two monitors and... same clicking. Now, I cannot be sure these monitors ever worked. Chances are those boards have the same fault on them. Again, both the boards and the tubes are spotless.

I took a look around, I could not find anything shorted around the PSU area and my thermal camera does not show anything warming up.

Unfortunately I do not have schematics - only a service manual which is pretty generic.

The question I have for you is: can a faulty CRT cause this kind of issue? When I swapped the board, I swapped the neckboard with it so there is nothing left in the monitor!

I have a feeling this is a "simple" SMPS issue but it's not easy with no schematics. The board is also pretty much inaccessible - I guess I would need to take it out and power it up on a side to test something, but still pretty difficult without schematics.
The board is manufactured by LG and it's PN 6870T024A

Any pointers would be welcome! Thank you!

Service manual can be found here: https://1drv.ms/b/s!AukePwK7XyEPhOZL...X5v0Q?e=64MaJY

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