Mac Classic making loud sizzling/frying noise & random reboots

[Diego-R]

Hi all, first post here!

I have a Mac Classic that's making a loud sizzling/fryinng noise, but works fine otherwise. The noise disappears when I disconnect the logic board. The machine is 110v and the analog board is the 820-0525-A Hitachi version.

Repair background:

• Fully cleaned and recapped logic and analog boards, except the RIFAs (the logic board had some cap leakage but no bad corrosion)
• Replaced CNY17G, TDA4605, DP3, DP4 diodes in an attempt to fix wobbly display and low/unstable voltage (didn't help initially)
• Flushed the PP1 voltage adjustment with contact cleaner and that fixed the display/voltage issues (voltage now snaps straight to 5v / 12v as as soon as I flip the switch - before it creeped up slowly and was very unstable)
• Reflowed all cracked & high stress joints and cleaned all contacts

The machine seems to work fine, except for the noise which was present from the beginning. The Dead Mac Scrolls guide mentions the sizzling and mentions bad grounding, but I checked it and it seems fine.

Any ideas what I could try next?

I thought about using an old PC microphone and headphones to try to pinpoint the component that's making the noise, but maybe having a mic wired straight to my head, near high voltage might not be such a good idea

Thanks in advance!

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UPDATE: I spoke too soon, upon further testing the machine is not stable at all, it randomly reboots and when it does it gets stuck on a chime loop or in a gray screen, so I have to power it off/on again for it to boot back up. Also the screen is a bit too flickery/shaky in the middle (not wobbly though). Voltage is still stable though.

 

[phunguss]

I thought about using an old PC microphone and headphones to try to pinpoint the component that's making the noise, but maybe having a mic wired straight to my head, near high voltage might not be such a good idea

Use a piece of rubber hose, a couple of straws taped together, or a cardboard tube from a paper towel roll (cut it lengthwise and roll it smaller then tape). All non-conductive.

 

[Diego-R]

Use a piece of rubber hose, a couple of straws taped together, or a cardboard tube from a paper towel roll (cut it lengthwise and roll it smaller then tape). All non-conductive.

Great idea!

I used a rubber huse and the noise is definitely coming from the SMPS transformer (157-0130). I assume it's just physical vibration and not high voltage arcing, so safe to keep using as is?

I'll definitely try to fix it or find a replacement as I was planning to use this machine for writing and the noise is kinda annoying.

Thanks for the suggestion!