Performa 575 Whining/Squealing noise, analog board

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lsi-aigisx86

New Tinkerer
Feb 10, 2026
8
1
3
Hey ya'll.

I had recently acquired a Performa 578 off of market place and brought it home with me, it does function, and despite being a little bit dirty, it was in otherwise good condition. While it did work and boot to MacOS 7.1, it made this awful squealing noise the entire time it was powered on. I did open it up and take a listen, and it appears to be coming from the section of the motherboard where two transformers live (pic related). I have recapped most of the motherboard as some of the capacitors were just starting to leak. I did all of them apart from the capacitors under the RF shielding.

However the machine still makes the noise, and I'm unsure of what is causing it. The squealing starts from the moment I hit the soft power on the keyboard and seems to vary with system load. Like if I opened a large image, I could hear a pitch shift difference between the CPU processing data and the CPU waiting for data to load in.

I've attached an image of the board (shamelessly stolen from this thread https://tinkerdifferent.com/threads/mac-lc-575-analog-board-recap-guide-photos.4463/) with the red marking where I have yet to recap, and the blue marking where I heard the noise with the case apart. Along with a video with the noise.

Any ideas? I'm lost and unsure of where to go or what this means. Any help or advice would be appreciated.
 

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M

Mark_W

Tinkerer
Feb 2, 2026
36
29
18
Ah yes I had the same issue on my LC520 (shares the same chassis as LC550/575 other than the caddy CD drive). Got the machine in non-working state and after recap it boots up but has the same buzzing/sizzling sound as yours, but much louder. Found the source to be the switching mosfet and transformer (main transformer that provides 5v, +-12v), yellow one in the photo.

Everything I've tried:
- Full recap of the power supply section - no change
- Replace IRFBC40 (switching mosfet), TDA4605 (switching ic) and opto-coupler CNY17G - no change
- Reflow all soldering joints - no change
- Add some hot glue to secure the transformer/capacitors - no change

Put the LC520 aside for 2 months and next time I tried to power it on, the machine turns on and the sizzling sound comes back, but power will cut out a few seconds later. Before power cuts out, it outputs +5V/+12V normally. Poked around and found that if I push the yellow transformer towards CRT, the sizzling sound is gone and the machine boots up normally, but it will come back as soon as I stop applying pressure, and will also shut down randomly afterwards. Did some research and I believe the switching transformer has gone bad, power cuts out because it triggers the protection mode in TDA4605. Ordered a replacement board on ebay and did a recap, no more sizzling noise.

Well you can try what I did before, see if it works for you. I kinda gave up and just swapped out the analog board at the end, but let me know if you have any findings, the original analog board I have is in very good condition and I still want to get it fixed..
 
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L

lsi-aigisx86

New Tinkerer
Feb 10, 2026
8
1
3
I pushed on the transformer inside the computer and I noticed that it shuts up if I do so like you mentioned, pushing towards the CRT. I will continue to investigate, as mentioned the computer does appear to run fine, however I haven't tested extensively.
 
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M

Mark_W

Tinkerer
Feb 2, 2026
36
29
18
Have you reflowed all solder joints yet? That will rule out connection issues. if so then I guess you have the same bad transformer as I do.

From my previous research, the transformer probably has a loose or cracked core which creates harmonics in the audible range, making the sizzling sound. it may get worse over time due to vibration, eventually overload the primary switching MOSFET and cause the shutdown.

Anyway, I don't think we can find nos replacement transformers so keep an eye on scrap analog boards on eBay, just in case the transformer quits working one day
 
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