Dead Mackie HR624 MKII measurements... check my work?

[boinzy]

I have a dead Mackie HR624 MKII part of a pair I got for free. The other one powers on, but emits only static.

I want to use this as a learning opportunity to do test measurements and diagnosis and hopefully fix the problem.

Schematic: https://www.audioservicemanuals.com/m/mackie/mackie-hr624/4208089-mackie-hr624-mkii-main-schematics

I am starting with the dead monitor first. Like I said, dead, no power light, no sound. Fuses checked and are good. A visual inspection does not reveal anything obvious. no leaking, bulging capacitors. no shorts or burnt parts of the board. the grounding wires look ok at the connection point, no corrosion, but maybe they could be cleaned up anyway.

I did a Transformer AC Output Test and got ~31V AC from the center wire to each outer wire and ~64V AC between the two outer wires, so it seems power is being correctly delivered to the main circuit board.

Next I did a Main DC Power Rail Test and measured 0V DC on both test points.

So that led me to the bridge rectifier. I got the correct forward voltage drop (~0.6V) and an open circuit reading in reverse.

So I did some point-to-point continuity tests

• AC Path (Transformer to Rectifier): PASSED. There was a good connection.
• Ground Path (Center Tap to GND): PASSED. There was a good connection.
• Positive DC Path (Rectifier + to Capacitor C39): FAILED. Showed "1" (no connection).
• Negative DC Path (Rectifier - to Capacitor C48): FAILED. Showed "1" (no connection).

I'm guessing there is a physical break in the copper traces on the PCB? If so, reflowing that is beyond my knowledge and skill probably. But I could do small jumpers?

Or am I misunderstanding the result of the measurements?

I can share pictures if anyone can help me see the issue the measurements are pointing towards. But now i feel a little over my head.

Thanks.

 

[mhardy6647]

Is there AC from the PT secondary winding(s)?

Usually heat is the killer. The victims are usually capacitors. Next on the list, rectifiers or voltage regulators.

 

[boinzy]

Is there AC from the PT secondary winding(s)?

Usually heat is the killer. The victims are usually capacitors. Next on the list, rectifiers or voltage regulators.

Thanks for responding!

I agree about capacitors being usually the issue. My early research (youtube videos) showed that C65 is the common failure, but when performing a continuity test, it passed.

But to answer your question, I did confirm AC from the transformer. Readings were ~31V AC from the center tap to each outer leg and ~64V AC across the entire secondary winding.

 

[boinzy]

I think the BR component itself might be okay, but the solder joint or a PCB trace right at its output is broken. Does this sound like a plausible diagnosis to anyone?

my next plan is to try re-flowing the solder on the four legs of the bridge rectifier to see if that fixes the broken connection.

 

[AnalogSteph]

I think the BR component itself might be okay, but the solder joint or a PCB trace right at its output is broken. Does this sound like a plausible diagnosis to anyone?

Absolutely.

my next plan is to try re-flowing the solder on the four legs of the bridge rectifier to see if that fixes the broken connection.

Sounds like a plan. Do the capacitor as well.

If this doesn't help, employ major magnification and look for any lifted pads or broken traces. Maybe watch out for signs of corrosive "killer glue" as well just in case.

 

[DVDdoug]

I'm guessing there is a physical break in the copper traces on the PCB? If so, reflowing that is beyond my knowledge and skill probably. But I could do small jumpers?

Reflowing the connections just means heating the connections, and maybe adding a little solder. You don't have to re-solder the whole board.

It's not that unusual to have an open trace BUT power traces are usually thick so that's rare. It's also rare that both would be open. And, I'm assuming there's no sign of burning?

But yes, you can "bridge" the connection by soldering wires in parallel with the traces.

 

[boinzy]

Hey everyone,

Wanted to post an update and say a huge thanks for the support here.

I focused on the connection between the bridge rectifier and the main caps. First, I reflowed the solder on the four legs of the rectifier. Afterward, I re-tested continuity from the rectifier's DC outputs to the main filter caps and this time i got a perfect connection, reading .000 on the meter. it was a simple open circuit on the board.

Then a power-on test. The instant I plugged it in, the power ring flashed red once and then went dark. The unit wouldn't power on after that.

As I understand it, the red flash means "Overload Protection." Since there was no audio signal, it had to be detecting a dead short somewhere downstream.

Following that lead, I tested the resistance to ground from the output pin (Pin 14) of the two main power amplifier ICs (TDA7293V):

* U1 (LF Amp): Read as an open circuit (the correct reading).
* U2 (HF Amp): Read as a dead short (~0Ω).

Soooo, I guess the U2 amp failed and shorted out, which then caused the massive over-current that burned out the PCB traces??

I have a replacement TDA7293V on order to replace the faulty U2.

But now I am wondering what is behind the U2 failure? Natural end of life? Craked too loud for a party? A shorted tweeter?

Later today I will test the tweeter. Unless anyone has any thoughts?

Thanks again for all the help!