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I have a mixer with a TNY288 based SMPS which wasn't turning on. I've found the problem in a shorted schottky diode on the secondary going to the -12V regulator, unfortunately can't recall if I've had to swap the 7912 too. I did get the mixer (and the SMPS) back to life by replacing the diode.

There's a hissing sound now that seems to be coming from the transformer, that grows over time and gets pretty loud as the SMPS warms up. Since there are not many components I though I might go for exclusion and went on to replace the TNY288 controller.

Nothing changed so I've went to see how much power the mixer was drawing, 0.45A on the +12V and 0.15A on the -12V. If that's 7.2W it shouldn't be too bad for the TNY288 to handle.

If the noise is generated by high flux density in the transformer, what could be the issue causing it? I've tested a couple of capacitors and they measure well. Could the PC817 be the issue?

I'm attaching a simplified version of the schematic since I don't have the original. The secondary has taps for -15,+15 and +50V. The LED side control circuit of the PC817 is on the +15 side.

schematic

(Image source: TinySwitch-4 family datasheet)

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  • \$\begingroup\$ Have you checked capacitors (electrolytics especially)? \$\endgroup\$ Commented May 7, 2023 at 13:58
  • \$\begingroup\$ Noise in a transformer or coil is caused by lack of mechanical rigidity in some component(s). For example loose wire, loose shim in an airgap, etc. \$\endgroup\$ Commented May 7, 2023 at 15:53
  • \$\begingroup\$ From what I've seen on YouTube videos, shorted diodes on the secondary can result in damaged components on the primary side. Have you checked the primary-side components after replacing that diode? \$\endgroup\$ Commented May 7, 2023 at 17:56
  • \$\begingroup\$ I've replaced the "box" capacitors on primary side (before the diode rectifier and between secondary and primary ground) and checked Vf on rectifying diodes and the two diodes for flyback suppression between the primary coil. There isn't much left to test beside the primary's 3x parallel electrolytics but I don't have a replacement yet and they were at the bottom of the list for suspiciusness. \$\endgroup\$
    – Alex P
    Commented May 7, 2023 at 18:11
  • \$\begingroup\$ the noise is similar to this, it starts relatively quiet and becomes louder with time. photos.google.com/share/… \$\endgroup\$
    – Alex P
    Commented May 7, 2023 at 18:13

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