SMPS noisy and overheating

[flyingtele]

Hi there,
I'm starting to repair a PSB subsonic 5i amplifier, and its a class H with a SMPS.The power supply is qiuite noisy with an audible high frequency hum, but of more concern the two transistors that are feeding the main transformer are getting very hot. This is with the amp at idle. They are on a small heatsink, but looking at the size of the heatsink they are not meant to heat up so much.

Any ideas what could cause this, and any tests I should take? I have little experience with SMPS so any help would be appreciated. I have looked but cannot find the schematics for the amp, and have emailed PSB for the service manual. If anyone can get the service manual I would be most grateful.

[savu]

when the amp is not idling, does the temperature problem persist?

[!]

What was the original symptom prompting repair (before you took it apart and noticed noise and heat)?

Determine if the PSU is bad or the load (amp board) and the PSU is simply responding to that. The hum might go away at normal load, or the transformer or an inductor might need some lacquer or other fixating material put over the coils to quiet it down - but the noise is a secondary concern that can be fixed later.

Generally speaking the heat would usually be either one (or both) switching transistors are shorted and need replaced, or something downstream is shorted although if something downstream is shorted it usually isn't long running before you see evidence of that like swollen capacitors, burn marks on the PCB or component which has failed.

However, you should disconnect the amp from the power supply and get it running with a minimum load on whichever power rail it uses to regulate. I do not know the specifics of this PSU so I can't tell you that, but probably the higher current circuit has feedback so you would place a power resistor or equivalent on that PSU output as a load to draw at least an amp or two "IF" the PSU does not have its own dummy load transistor onboard (which many do to prevent damage if powered w/o sufficient load).

In other words, maybe the amp itself is damaged and the PSU is just doing what it can to supply excess current, but in that cause you should have signs of amp shorting such as excess heat there with no audio input.