Help, burnt rectifier in small rack mixer Power Supply

isophase · Feb 9, 2018

Hi all,
There was a very strong smell this morning in the thech room of the theatre where I work.
Indeed the rectifier bridge of the small mixer/monitor system has burnt.
Strange that there is no fuse in this machine (Australian Monitor model AMIS ZRM4)
And even more strange that the rectifier has blown as opposed to the regulators?
I have never seen a rectifier burn like this. All surrounding components "look" fine.
I am curious to know what could cause such a failure?
The machine is not very old but is used quite a lot as it serves as a backstage return system in the theatre, and from time to time it stay ON over the night.
It uses an external 20Vac power supply.
Does anyone have a clue what could have caused this?
Thanx
Jonathan

isophase · Feb 9, 2018

Trying to upload a photo in vain

JohnRoberts · Feb 9, 2018

The common failure mode causing a diode to short is over temperature. The silicon literally melts and become a simple conductor. The heat is typically caused by excessive current that can be caused by a shorted reservoir capacitor (driving a cap with AC will also draw a lot of current), so a shorted diode will also cause overheating after the fact (chicken/egg? ).

Checks the caps, replace the bridge.

JR

PRR · Feb 9, 2018

Lamp limiter.

isophase · Feb 10, 2018

Thank you,
I will take a closer look when i have the time, and try to fix it > change the bridge and check/replace the surrounding caps.

Lamp limiter? you mean the lighting circuit for the LEDs ? causing excessive heat?
thanks a lot

Khron · Feb 10, 2018

I second this.

In the sense that a crapped out cap can and will draw loads of current, which can burn rectifiers, melt / char / burn power connectors etc.

A few years ago i remember buying a faulty Focusrite Octopre Platinum that had such an issue. The rectifier was still alive, but the main filter caps had crapped out, and the connector coming from the transformer (as well as the PCB in that area) was "50 shades of" beige-to-brown-to-black.

JohnRoberts said:
The common failure mode causing a diode to short is over temperature. The silicon literally melts and become a simple conductor. The heat is typically caused by excessive current that can be caused by a shorted reservoir capacitor (driving a cap with AC will also draw a lot of current), so a shorted diode will also cause overheating after the fact (chicken/egg? ).

Checks the caps, replace the bridge.

JR

PRR · Feb 11, 2018

Lamp limiter!

isophase · Feb 12, 2018

right, got it!
Thank you PRR

CJ · Feb 13, 2018

plug that lamp limiter into a Variac and now you are cookin with gas,

this small variac at work, it starts into vibrating if there is an overload, actual resistance to turning the knob, and it has a push button breaker.

Help, burnt rectifier in small rack mixer Power Supply

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