Power supply card

[mnhiphop101]

Are those gaps between the waves normal? This measurement was taken from the driver card pins with it powered up out of the amp.

 

[Perry Babin]

What's the DC voltage on terminals 3 and 4 of the driver IC?

 

[mnhiphop101]

Pin 3: 92mv
Pin 4: jumps to .5v at startup and then drops to 150mv

 

[Perry Babin]

The DC on pin 4 could be causing the deadtime. I'd expect it to be about 1/3 of that voltage from a generic diagram values.

 

[mnhiphop101]

How do I fix that? Is something out of tolerance on the card?

 

[Perry Babin]

Zooming in on your image, I think R27 (connected between pin 4 and ground) appears to be a 15k. That would make the voltage a bit higher than the normal 10k but 150mv still seems high. Check the value of R23 and R27 (pull them) and you may want to lift one terminal of C24 to see if that affects the voltage on pin 4 (with the resistors in the circuit).

 

[mnhiphop101]

With the capacitor pulled I get 69mv on pin 4 buy the waves still look the same

 

[Perry Babin]

Did your scope indicate that it had precisely the same duty cycle?

 

[mnhiphop101]

Yes

 

[mnhiphop101]

I've never seen this before and now the last 3 amps in a row all have this problem.

 

[Perry Babin]

Does it do this with the board in the amp?

 

[mnhiphop101]

Yes. And they draw current and the fets heat up

 

[Perry Babin]

I wouldn't expect this to cause the FETs to heat up. It's likely to have the opposite effect. Under load, I wouldn't be surprised if the primary filter caps would overheat.

The voltage on pin 3 of the 494 is another input that can cause the amp to have excessive deadtime.

If you connect pin 3 to pin 7 on the 494, does the pulse width increase?

What is the marking on R28 of your board?