Repairing a power supply of a Class-D amplifier

Thread Starter


Joined Jan 18, 2016
Hi guys,
I've got a RCF art735-a in for repair which went through a power surge. The half bridge driver IR21531 was fried with the top chipped off. One of the mosfet was shorted along with few other components.

Luckily I had the exact same board with the amplifier section fried. So I started replacing the power supply components from it.
Here's what's happening now- The power supply is not starting up. There's no voltage at VCC pin of the IR21531.

First question- Is it okay to turn on a class D amplifier without a load connected? I'am just trying to check whether the power supply works, and I do not have an input signal applied.
I have attached a diagram of the effected area/ and noted the components changed. I also changed Q6 as it was shorted too (shown in the picture), not sure how its used.

What I have noticed is on start up the supply voltage is dropped across 'the start up resistors and there is no voltage at VCC. Why could this be?
I checked across VCC and COM (of the chip) with diode mode and I get a reading of 0.2V! with probes either way. Could this be a problem?

Thanks in advance for the help
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Last edited by a moderator:


Joined Jun 22, 2012
The chip has an internal 15V zener across the Vcc and Gnd pins, so it can run on High voltage with a series Resistor, if you're not getting 15V , then either the resistor is open circuit, the Zener is shorted, or the Capacitor is shorted ..


Thread Starter


Joined Jan 18, 2016
Thanks everyone, I would have to order in a new IR21531. However I was able to get a IR2153 pdip package, I'm thinking of soldering some wires onto it and check whether the power supply works.