One of my two Mackie HR624 powered studio monitors failed. Upon opening it up, I found an ash-filled crater in the circuit board where C52 and C53 used to be. They were SMD caps mounted adjacent and very close to each other, so it's possible that one of the two failed and took out the other. Their twins, C50 and C51 are mounted further away and are fine.

I've ordered some replacement components (through hole rather than SMD - trying to repair the traces on the board is beyond me) but I wonder if the evidence suggests more than just a simple component failure and I should check out anything else to avoid a fireworks show.

Kudos to Mackie for making their schematics available online. The speaker was set and operating on 120v.

Thanks,
Mac

 

In an event like this it is highly probable that other components were stressed. It is unclear if this stress will precipitate a subsequent early failure. Your decision to repair or replace that equipment depends on how critical the next failure will be. If it was me and the failure would negatively impact my revenue stream, I would definitely go for the replacement.

 

In an event like this it is highly probable that other components were stressed. It is unclear if this stress will precipitate a subsequent early failure. Your decision to repair or replace that equipment depends on how critical the next failure will be. If it was me and the failure would negatively impact my revenue stream, I would definitely go for the replacement.

I recently attempted a similar repair on an old PBX where some caps made fire.

Replacing the caps just made another fire.

Edit: I'm drinking tonight. I thought sharing this story would be funny.

 

By all means, replace the blown capacitors. But that does not mean that the problem is resolved.
Before testing with sound input/output, I would disconnect the loudspeakers and check for DC balance at the output stage.

 

I recently attempted a similar repair on an old PBX where some caps made fire.

Replacing the caps just made another fire.

Edit: I'm drinking tonight. I thought sharing this story would be funny.

Certainly, an apocalyptic tale.

 

Certainly, an apocalyptic tale.

You don't know the half of it!

 

You don't know the half of it!

More's the pity.

 

Yes, it is safe to try just replacing components.

There might be a problem with D24, I would check it with a DMM in the diode test mode before replacing the capacitors. Also any charred bits should be removed.

 

>>There might be a problem with D24, I would check it with a DMM<<
Thanks. All 4 diodes in the D24 assembly check fine. Waiting on parts to try the replacement.

 

Before testing with sound input/output, I would disconnect the loudspeakers and check for DC balance at the output stage.

Would I do this by grounding the inputs and checking that the outputs are within shouting distance of 0v DC?
Thanks.

 

If it was me and the failure would negatively impact my revenue stream, I would definitely go for the replacement.

Ah, it's been a long time since these were involved in a revenue stream and, if I were to do a clear-eyed assessment, they've probably never been involved in a profit stream. They have been pressed into duty as home entertainment system speakers.

 

Would I do this by grounding the inputs and checking that the outputs are within shouting distance of 0v DC?
Thanks.

You don't have to ground the inputs. Just turn the volume level to the lowest setting.
The first thing you should check is the power supply output voltages, +41V, -41V, +15V and -15V.

Then check the output voltage to the loudspeaker (with the loudspeaker disconnected). You are looking to see 0V at the output.

 

It is a bit puling because C52 and C53 are across half of the 24 volt power transformer. As I examine the circuit I see noway a believable failure of any component could have caused them to fail. And unless they have damaged the transformer by excess current. So I suggest first clearing away the damaged capacitors and then checking to see that the transformer is still able to deliver the original output voltage. Possibly, if the amplifier voltage input switch was set for 120 volts mains and the power applied was 340 volts, that might cause the problem.
I also suggest checking the two fuses, F1 and F2, a shorted circuit probably took one or both fuses out.

 

yup... i would start by unplugging secondary from the rest, then checking fuses and transformer. check both for correct voltages as well as any leakage path between primary and secondary.

 

I did check those - fuses good and transformer voltages nominal. I don’t know how to check for a leakage path between the primary and secondary windings - ohmmeter?

 

With the opening statement that the amplifier failed, I am assuming that for some reason there is no longer an audio output when there is an input that should cause an audio output. and given the location of the two chip capacitors as shown So at this point if nothing else has failed the amplifier should function correctly, although the capacitor melt down could have damaged the trace on the circuit board.
Chip capacitors seldom fail for no reason. so after cleaning the PCB and removing the remains of the caps, does the amp work???

 

With the opening statement that the amplifier failed, I am assuming that for some reason there is no longer an audio output when there is an input that should cause an audio output. and given the location of the two chip capacitors as shown So at this point if nothing else has failed the amplifier should function correctly, although the capacitor melt down could have damaged the trace on the circuit board.
Chip capacitors seldom fail for no reason. so after cleaning the PCB and removing the remains of the caps, does the amp work???

The obvious symptom of failure was that no sound came from the speaker. Also, the LED power indicator wouldn’t illuminate (this monitor was normally run in an auto-power on mode based on sensing an input signal) but even in the manual power mode, the LED indicator wouldn’t respond to pressing the on/off button. When I saw the schematic, I assumed one of the internal fuses had opened but they were fine. I was looking for the power test points when I discovered the blown caps. They definitely damaged the board - both the substrate and the traces and pads associated with the failed caps. Not sure about traces on the other side - it would be a big job to get a look at the underside of the board.

To answer your question, I haven’t replaced them yet - waiting on the parts. I ordered through-hole because there are no traces/pads left to mount SMDs. I selected 200v/high temperature tolerant components (the schematic calls for 100v), and will have someone hold my beer.

 

Killing 0.1u 100V is not easy and they are being used as simple high freq filter caps. IMHO, the only way they "toast" is overvoltage caused by a primary to secondary short in the transformer. As others suggested, measure resistance between primary wires and secondary. And be aware, transformer winding shorts can come and go due to heat. So the short may only happen after a period of time while under load.

 

Indeed! The cause of the failure of the capacitors is a puzzle to me. Checking the transformer output, and checking for mains voltage present between secondary and safety ground connection is in order. But with the amplifier non.functional it might be that the capacitor failing broke a trace on the circuit board.That couold be repaired with a jumper wire.

 

Killing 0.1u 100V is not easy and they are being used as simple high freq filter caps.

I've been thinking about that. I can't see any reason that this failure would, on its own, stop the amp from working. More precisely, stop the power on circuitry from powering on, including lighting the Power LED. I'd post the schematic of that section but it's in sections distributed over a couple of pages. In case anyone's interested, this link will download the full schematic: link

I'm beginning to believe that it's very unlikely replacing those capacitors is going to revive the unit.