PSU repair: Works but screeches when plugged in

[Spitko]

My 68k finally arrived in the post.

Plugged it in... nada. No big surprise there; I ripped the PSU out, replaced every damn cap, and zd31 since I had it handy. Every cap was leaking other than the line filter cap.

When not plugged into the 68k, the PSU fan instantly turns on when power is applied. I measured voltages, and everything looks to be in spec but the yellow wire (1.2V, but I'm not sure if this is due to it being disconnected since the schematic labels it POWER_CONTROLE.

Figured I'd take a shot and plug it all in. The 68k instantly turns on when plugged in, and happily boots to disk. However, there's this constant, hideously loud screeching coming from the PSU. It's like coil whine but 20x louder, and at a lower pitch.

At this point I'm hesitant to go further until I have an idea of what's causing that noise. Any ideas? I'm thinking the Sub_Power circuit is unhappy given the low voltage but I'd like a bit more information on how much of what I'm seeing is normal before I go chasing wild geese.


Edit 1: A bit of sleuthing has uncovered https://gamesx.com/wiki/doku.php?id=x68000:soft_power_switch_troubleshooting_and_repair which will help me chase down the power button issue. Still no leads on the noise though, someone in another thread had identical readings to my yellow wire on a working unit.

Edit 2: Thought it might be the speaker, since it's right next to the PSU. Unplugged, no change. Also tried unplugging the floppies and SUB_POWER connector, seems it'll do it with just the 6 pin connected.

Edit 3: Was going to check the bottom board (IO board?) but I can't seem to figure out how to remove it. I removed all of the screws from the bottom but the plate doesn't seem to want to budge from the rest of the shell. The wiki's disassembly instructions also don't mention how to access this board.

[JulBS0]

Hi,

Which kind of X68k do you have? The original one?

If it's the case, in addition to the five screws under the computer, you have to unscrew two vertical brackets, which will require you to remove the side panels.
There is one shared with the bottom of the floppy drives assembly, and another one on the motherboard side (near the connectors side IIRC).

[kamiboy]

As for the screeching PSU, going to larger capacity caps for the big filters on the PSU did the trick for me.

[Spitko]

This is an Expert.

All of the caps were replaced with recommended values on the wiki, which are generally already larger capacity. What size did you use?

[kamiboy]

I don't remember, about 1-2000uF larger than before.

[Spitko]

Update: I ignored the noise for a bit since I was busy doing some hardware development. Suddenly I lost video while it was on, and the squealing went away.

Now it no longer boots, and you can hear the PSU fan slow down significantly while its doing the boot floppy seek. I'd guess the 12v rail isn't putting out enough juice; any common suspects for this failure?

[kamiboy]

I think some diodes are common points of failiure.

[vanpeebles]

I reckon you might be better off throwing the old PSU into a ditch, and using an ATX one. Far more reliable and up to date 

[Spitko]

Fixed it. R40 resistance had gone up to 18k (rated 2k) which was preventing the soft power circuit from doing its job.