mastech hy3005d-3 linear power supply making loud sounds

[jmaja]

This PSU has been fine for several years, but today it started making strange noises. I was charging a lead acid battery (12 V 80 Ah) and heard rather loud noise, much louder than the normal relay clicking, but a bit similar sound. The same sound repeated a few times in an hour, sometimes even louder. I disconnected the battery after ten hours and turned the PSU off. It still made the same sound a few times in an hour. I got worried and took the power cord off.

What could make such a sound?

[alm]

A relay with its control circuit oscillating?

If you were not using a series protection diode between the power supply and the battery, then any fault could have shorted the battery through the power supply (what is the short-circuit current of that thing?). For example if an overvoltage crowbar circuit tripped. That could easily destroy something.

But it could also be something simple like a dried out electrolytic cap that messes up the tap switching.

[jmaja]

Note that the sound continued while the PSU was switched off. So it can't be the relay. It only stopped when I pulled the cord off.

[Armadillo]

Check all electrolytic capacitors. Measure the ESR.

[Rasz]

something got hot and started cracking while thermally expanding? would crack again when shrinking , for example transformer

[jmaja]

something got hot and started cracking while thermally expanding? would crack again when shrinking , for example transformer

Probably, but why. Hasn't made any sounds before although I have been using it at full power (2*5A). I just put the power cord back to see, if it starts making sounds again even while off.

[Kleinstein]

I see 3 possible sources for noise:
The two more obvious sources are the fan and relays switching fast. Fast relay switching could be due to a problem with the tap switching and maybe a relay. If it happens the relays might get damaged over time.

Another possible problem is a broken diode in the rectifier. In this case the rectifier would work as single puls more and the transformer would see a significant DC current. This would cause large current pulses in the transformer as it would go hup to saturation. Through magnetostriction this can get quite audible - not just the "normal" 100 / 120 Hz hum, but also quite a lot of harmonics.  It might be worth checking the diodes / rectifiers.

[Armadillo]

Note that the sound continued while the PSU was switched off. So it can't be the relay. It only stopped when I pulled the cord off.

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