Mackie SRM450 v2 burnt cap

[nosaj]

i have a mackie srm450 v2 with a burnt cap c156 on the tweeter amplifier just curious what would cause it to burn up.
There is no sound at all from the amplifier but it does power up.
Thanks,
nosaj

area of interest page 5 lower left quadrant of schematic.
Mackie SRM450 actspk ver 2_.pdf

[The Dude]

Looks like just a noise filter on the negative rail. Sometimes they just go for no apparent reason. Is the -38V rail voltage there and approximately the correct voltage?

[nosaj]

No there is not a -38v DC it's like -5v that is also the leg c156 is on. i will pull the TDA and measure -38v rail again.
nosaj

[Jon Snell]

I would check T5-B for dry joints. If T5-5 goes open circuit, (quite likely with heavy copper in a vibrating box), the ground will float, causing spurious voltages and HF amplifier and power supply failure. C156 may have just failed or someone has been in there before you and damaged it.
Measure 78volts between the rails on the HF amp.If not there, check the TDA by removing it but you may have to rebuild the power supply afterwards and that can be fun.
Being a SMPSU and it employs a PFC that delivers 380volts DC, watch out!!!

[nosaj]

Ok just went ahead an reflowed T5 joints. Removed TDA voltages at that point now show -40 and +40v rails. Snipped old one off daughter board and soldered in a new one.
Now occurs to me I should check voltages on LF AMP.
Whats the consensus should i remove HF AMP daughter board to check the LF Rails?
Thanks nosaj

Once i hear on the daughter board I'll just check all the rails/
Also Can I do all this without the PCB connected to the heatsink with no signal applied?

Thanks again.

[The Dude]

The LF amp power rails are independent of the HF rails and those 2 supplies are derived from different transformer windings. I don't see why it would be necessary to remove any part of the HF amp to check LF amp rails, unless maybe you can't get to parts without removing it? I'm not sure which parts of the thing are on daughter/separate boards. I wouldn't run the amp at all without heat sinks connected. Even idle current can get out of control and burn up parts without proper heat sinking.

[nosaj]

All rails are good. Powered up runs great. Thanks guys I really appreciate it.

Another question how was it possible a shorted HF amp made LF amp not work?

Thanks,
nosaj

[The Dude]

Hard to say for sure now that you have it working, but I'm going to guess the shorted HF IC caused the power supply to go into OCP shutdown also killing the LF amp supply.

[Jon Snell]

If you follow the muting circuit, it monitors both HF and LF amplifiers.