Mackie Powered Subwoofer

[davidfinch]

I'm trying to repair a Mackie HD1801 that has no sound and the thermal and limiter bulb are both lit. Here's what I've checked and they are all good.

The speaker
Tried measuring TP27 and TP29 and got +.4v/-.4v

BR1
D1-D4
D15
D5-8
D16

If anyone has any help, please let me know.
-Dave

[Enzo]

Disconnect the speaker. Does it still act like that?

Be a mensch and link us to a schematic.

[MarkusBass]

Isn't it a class-D power amp? A schematic would help.

Mark

[Jazz P Bass]

Yup.
Class D it is indeedy.

[davidfinch]

Yes, I'm testing it with no speaker. I can't upload the schem, it's from Mackie's login site. It is a class D. I can't figure out why the Voltage rails are equally low. I tried swapping the bridge rectifier and it didn't change.

[tedmich]

in a pinch you could "print screen" an image of the schematic from your desktop and save in an image program, or WORD

[Jazz P Bass]

in a pinch you could "print screen" an image of the schematic from your desktop and save in an image program, or WORD

Or you could "print "the screen to a pdf file by using CutePDF.
As to the amplifier: isn't the power supply a "Switching" power supply?

[tedmich]

Class D amp, SMPS, SMD components... a real nightmare I imagine

Hey people sell THUMP repair service on ebay for a flat $150...isn't that just a solder reflow?

[Enzo]

The power supply is an SMPS, it will shut itself down when there is trouble.

Two approaches. First, look at the second page, check diodes D23-27 for shorts. One of those secondary voltage rectifiers shorts, and your SMPS will shut right down. Second, disconnect the load to see if the SMPS comes back up. A shorted output stage in the amp will cause the SMPS to shut down just as easily as a problem in the SMPS. So unplug the power connections to the amplifier board. That includes the small connector with the low voltages in addition top the main power wires. Now are the SMPS voltages present?

[davidfinch]

Thanks guys...

So, I already checked D23-27 - Seem OK. Checked secondary voltages with amplifier disconnected - no change. Have 120AC on primary. DC resistances on windings on secondary seem OK - DC resistance ~ 250K. D1-4 and filter caps on secondary seem OK also. I'm suspecting T1, but no shorts from primary to secondary. Going to reflow power supply board to omit any bad solder joints.

[Enzo]

ANything is possible, but the last thing in the world I'd suspect in one of those switchers is a bad transformer.

[lowell]

Okay I just read a rework bulletin for this thing. Apparently C6/7 often require resoldering w/ added solder and silicone to keep 'em tight. So maybe that'll fix it, worth a shot.

Question: Is a switching supply used to eliminate the need for a bulky PT? Finally is the basic function like so: Rapidly changing current in the inductors increase voltage across said inductor? Or at least am I on the right path with this?

[Enzo]

A switcher eliminates the need for a bulky, heavy, and EXPENSIVE PT.

I think you can find the basics of switchers, also referred to as switchmode power supplies or SMPS, with a google search easy enough. Read up on the general principals. Also companies that manufacture commercial SMPS probably have an overview of the technology for reading pleasure.

You start with a DC voltage - typically the mains voltage is rectified and filtered. Then that DC current is switched on and off at high frequency through the primary of the SMALL transformer in the SMPS, then on the secondary side, the resulting choppy waveform is rectified and filtered for use. NO smooth sine wave here, so the filters will generally have caps and inductors.

The transformer and caps can be a lot smaller than a linear supply, because the filter caps are charged so frequently. Most circuits draw off the power supply filter caps and hte rectifiers keep the caps charged. In a linear supply, the demand can be high, but the caps are recharged only 120 times a second. The caps have to have enough charge to last 1/120th of a second. In a SMPS, things are operating at 50kHz or faster. SOme SMPS even work in MHz speeds. SO the filters only need to hold up 1/50,000th of a second or less.


It isn't about stepping anything up, we are just switching the primary current off and on. There is no need for the primary to have nice pretty clean sine wave current, and the very idea of these things is they operate at those REALLY high speeds.

It is tempting to compare this to the superficially similar auto ignition coil. But that coil is designed to step voltage way up. The secondary of the SMPS can be wound to make pretty much any voltage you want, just as a linear transformer could be. IN a subwoofer like this, we might be making +/-80v or 100v or whatever, but we also see SMPS in some tube amps, where the SMPS makes not only +/-15v for op amps but also 6v for tube heaters and 400v for B+. And a bias supply thrown in as well.

[davidfinch]

What's w/ all the AC grounds in the PSU? Why AC ground and not earth ground? Curious about this.

[Enzo]

This is why SMPS are EXTREMELY dangerous to work on if you don't know what you are doing.

The SMPS rectifies the mains DIRECTLY. There is nothing between you and the mains. That is something like 340 volts of high current DC, just waiting to stop your heart.

That AC ground - better to refer to it as common rather than ground, so it is less likely to confuse - is not earth. It is the negative side of the 340VDC. You cannot connect that to real ground or WHAM you just burned out your rectifiers and who knows what else. You MUST use an isolation transformer to work on these. If you clip your scope ground to that "AC ground", you will cause things to burn.

All those control circuits are referenced not to earth, but to the negative side of the mains DC.

Allow me an analogy. In an airplane, the systems may be "grounded" to the airframe, but that frame is not earthed. Whatever altitude the plane flies at, the systems are referenced to the plane, not the ground. If the floor of the plane is at 10,000 feet, then the stewardess' head is at 10,005 feet. But we say she is 5 feet tall, not 10,005 feet tall.

The switching circuits do nothing more than pulse current through the primary. That primary doesn;t need to be grounded to the rest of the system. And it is not. The secondary side circuits ARE referenced to earth.