Mackie SRM450 v1 - No audio output

[beavernuggetz]

Hello everyone,

I'm troubleshooting a friend's speaker which no longer produces sound.

Already checked/confirmed/ruled out:
The speaker itself has been tested and works.
There are no shorts (that I could spot with a DMM & thermal camera) on the board.

Please see attached schematics for additional information. Specifically, the last page shows where to check voltages.
+45/-45 volts at D56 are present.
+15/-15 volts at Q34/Q35 are there too.

Anomalies:
+80/-80 at D55: reads as +96/-96 volts instead.
R254/R255 have +15/-15 across but are reaching +50 degrees Celsius when checked with a thermal camera (this could be normal, I suppose)

Any suggestions on what could be checked next would be greatly appreciated. I own the necessary equipment to troubleshoot, but simply lack in-depth knowledge on how to troubleshoot something like this.

Thanks again for any help.
PS: @AnalogSteph, if you are still around, thanks for helping me before and perhaps you'd be kind enough to take a look at this one too.

 

[LTig]

Hello everyone,

I'm troubleshooting a friend's speaker which no longer produces sound.

Already checked/confirmed/ruled out:
The speaker itself has been tested and works.
There are no shorts (that I could spot with a DMM & thermal camera) on the board.

Please see attached schematics for additional information. Specifically, the last page shows where to check voltages.
+45/-45 volts at D56 are present.
+15/-15 volts at Q34/Q35 are there too.

Anomalies:
+80/-80 at D55: reads as +96/-96 volts instead.

I haven't found D55, but the V2 voltage is +/- 95V as shown in the power supply schematic.

R254/R255 have +15/-15 across but are reaching +50 degrees Celsius when checked with a thermal camera (this could be normal, I suppose)

Yep, these are 2W resistors for a reason.

Any suggestions on what could be checked next would be greatly appreciated. I own the necessary equipment to troubleshoot, but simply lack in-depth knowledge on how to troubleshoot something like this.

Could be that both power amps are muted, for a reason. They have a mute input.

 

[beavernuggetz]

LTig, thank you for taking the time to go over the schematics and pointing me to the proper voltages.

Pardon my ignorance, had never heard about power amps and muting.

How would someone troubleshoot to confirm that to be the problem?

Looking at the board, I see 1 or maybe 2 components labelled: JRC4560 - Dual Operational Amplifier

I presume this is the part in question here.

Again, than you for your input and time.

 

[LTig]

What type of DMM do you have? What are the specs regarding AC voltage measuring?

 

[beavernuggetz]

I got a Fluke 117 and AC Volts resolution is as follows:
6.000 V / 0.001 V
60.00 V / 0.01 V
600.0 V / 0.1 V

 

[LTig]

Good one. You can feed a sinus test signal like 500 Hz (there are apps which generate test signals) to the input and follow it through the different stages with the Fluke. DISCONNECT THE DRIVERS BEFOREHAND!

 

[beavernuggetz]

Appreciate the suggestion. To be honest, that's where I run into trouble due to lack of proper knowledge.
I understand the hot/cold side of the PCB; no problem.
The issue for me is not knowing much about the different stages which you mentioned as well as which components to probe and where.
Can an oscilloscope achieve the same as the DMM? I know enough to realize not to probe the hot side with this tool.

 

[LTig]

A scope would be perfect. But without knowing how to read and understand the schematics (and find the points to test on the PCB) further probing is almost impossible.

My approach would be to feed a test signal and check the output of each opamp with a scope. There is a lot of them, each triangle in the schematics is one.

 

[beavernuggetz]

Thank you so much for your suggestion. I do see approximately 20 small chips; will look at their respective datasheets to figure out which pins to probe.
Will come back here with an update if any relevant discoveries are found.
Again, your help is much appreciated.

 

[AnalogSteph]

In cases like this you need to identify and follow the signal path and look for suitable points to split it or monitor the signal.

What I noticed is that this one has a green Signal LED, alongside a red PEAK indicator. If you cannot get the former lit up, for example, you have already narrowed down the problem area substantially, and could continue checking at TP2 and TP1.

As an aside, use all your senses in general. The presence of hiss in the tweeter (or lack thereof) could tell you a fair bit already.

 

[Speedskater]

Does he have two loudspeakers?
If so just compare the good with the bad.

 

[beavernuggetz]

Here is an update.
Tested all opamps (close to 20) and they are all getting the +/- 15 volts: NJM4560M & NJM2068.
Same goes for both LM339 & SSM2164.
All measurements were done with reference to ground.
Next, I'll try to understand how to feed a test signal and follow the path.
Thank you all for sharing your knowledge; I shall continue trying to revive this thing.

 

[beavernuggetz]

I may have found something.
There are approximately 12 electrolytic capacitors marked 47µf 25v which have an ESR of 2.2 Ohms which is too high. Tested a few others I had laying around and they are reading 0.2 which is night and day difference. Kind of odd that all would go bad at the same time.
@AnalogSteph and or @LTig, could this be related?

 

[LTig]

Are those 47µf 25v caps in the signal path (coupling caps for the audio signal) or at the supply voltages? Anyway this would explain hum or not enough bass but not no output at all.

 

[beavernuggetz]

These are in the signal path. I'll get clarification from my friend as to what the exact symptoms are with this speaker.