Mackie SWA1501 Got Me Stumped. Help!

[county]

Hi there. I have a Mackie SWA1501 on my bench and I'm at a block right now with it. It came in because the woofer voice coil shorted and took out the amp section. I replaced those parts, but the Mosfets Q12 and Q16 blew right away. I replaced those, along with most of the other semi's because I had them on hand. Now the amp can power up and is stable at idle, but the power rails are unbalanced and won't switch. At the bridge D35, I'm getting +24V and -115V. I've checked and double checked everything and I haven't found a fault yet. I know there's been a few threads on these beasts here, I've read through them and they've helped in checking things out, but the problem persists and I could really use an extra thought or two on tracking the issue. I've taken a couple other measurements that might help.
Q12, and Q15 both have -12V at the gates. The drain source drop at Q12 is 12V, while the drain source on Q15 is 24V. The voltages at U1 are as follows:
1 0.5v
2 -0.4v
5 -98v
6 -114v
7 -113v
8 -98v

Could it be that U1 is faulty and not switching? I'm not familiar enough with this circuit to say for sure. Any advice may save this thing from being "fixed" with a hammer!

Thanks in advance!

SWA1501_AMP_board.pdf

[52 Bill]

Welcome to the place.

At the bridge D35, I'm getting +24V and -115V.

How are you reading these voltages, referenced to ground? There is no direct center tap on the power transformer, so there will not be two equal sides.

[county]

Ah, yes, that makes perfect sense. In my head I guess I was thinking the rails should be balanced at the respective d,s of Q12 and Q15. From reading the other threads on these units, I understand the theory of operation, but diagnosing it is another story. I guess the voltages around U1 also can't be ground referenced? Am I correct in assuming that the gate voltages of Q12 and Q15 seem okay?

[nickb]

Can't say I'm too familiar but I can fumble my through some ideas and see if it helps.

First don't connect a load until you are 100% certain that all is good.

With it at idle, check the voltages on the collectors of Q17 and Q14: should be +/- 25V.

The voltage from nodes 3 to 4 should be small<< 1V as there is no demand for main power supply yet i.e. the d to s voltage of Q4 should be zero. You should be able to drive close to a ~48Vpp swing on the output and remain in this state. Check it.


Once you pass this drive level their will be voltage across nodes 3-4 to turn on Q4. The ground switching FETS will come on dependant on the output swing polarity. I can only imagine using a scope to check this dynamic operation region.