Hello all, new here. I need some suggestions on a powered mixer repair I am in the middle of. Read on to see where I stand with this. Thanks in advance!
Background:
I bought an EMX5014c. Long story short, I was stupid and didn't fully test it before buying it. A previous owner blew up the power amp then internally unplugged it (just the ribbon, not the high voltage) so the unit would still power up and mix just fine, just no output (all line-level outputs work except 'ST OUT').
Symptoms:
-ST-OUT Line-level out does not work.
-If you plug in PA section internally, the unit will power on for 1-3 seconds then shutdown.
Troubleshooting so far:
So, what I know so far is that I have at least two problems. Mixer and PA, PA being what I am looking for help with the most. I just don't have enough experience in these.
Mixer:
- The line level ST OUT section is shot. Everything works on the mixer up to and including the "ST SUB OUT" which is parallel to the ST OUT, so it is just the last section that is dead. I have a replacement OP-AMP on order, which I am hoping will solve that. Either way I can work around this for now but using AUX1 & 2.
PA:
- Based on the schematics and tests I have done, it appears that there is voltage on the 'DC' line that goes from the PA back to the power supply. This causes the power supply to shutdown.
Connector on PA side:
On power supply side:
Does anyone know what the specs on this should be? What blew up that is allowing this DC voltage?
I have done BASIC testing with my DMM on the transistors, diodes, caps and some resistors and everything has looked OK, but I am hoping to have some guidance on which part of the circuit to look.
It appears that at some point the PO plugged something live into channel A (1/4" port tab on left has burn and char marks) that should not have been there...
I have no problem replacing SMD parts and I have most electronic test equipment including good meters and o-scopes (I have an ESR meter that should be here on Friday) so I am not afraid of tackling this, I am just looking to talk it out with more experienced audio folks.
Let me know your thoughts!
Thanks!
Brandon
Background:
I bought an EMX5014c. Long story short, I was stupid and didn't fully test it before buying it. A previous owner blew up the power amp then internally unplugged it (just the ribbon, not the high voltage) so the unit would still power up and mix just fine, just no output (all line-level outputs work except 'ST OUT').
Symptoms:
-ST-OUT Line-level out does not work.
-If you plug in PA section internally, the unit will power on for 1-3 seconds then shutdown.
Troubleshooting so far:
So, what I know so far is that I have at least two problems. Mixer and PA, PA being what I am looking for help with the most. I just don't have enough experience in these.
Mixer:
- The line level ST OUT section is shot. Everything works on the mixer up to and including the "ST SUB OUT" which is parallel to the ST OUT, so it is just the last section that is dead. I have a replacement OP-AMP on order, which I am hoping will solve that. Either way I can work around this for now but using AUX1 & 2.
An externally hosted image should be here but it was not working when we last tested it.
PA:
- Based on the schematics and tests I have done, it appears that there is voltage on the 'DC' line that goes from the PA back to the power supply. This causes the power supply to shutdown.
Connector on PA side:
An externally hosted image should be here but it was not working when we last tested it.
On power supply side:
An externally hosted image should be here but it was not working when we last tested it.
Does anyone know what the specs on this should be? What blew up that is allowing this DC voltage?
I have done BASIC testing with my DMM on the transistors, diodes, caps and some resistors and everything has looked OK, but I am hoping to have some guidance on which part of the circuit to look.
It appears that at some point the PO plugged something live into channel A (1/4" port tab on left has burn and char marks) that should not have been there...
An externally hosted image should be here but it was not working when we last tested it.
I have no problem replacing SMD parts and I have most electronic test equipment including good meters and o-scopes (I have an ESR meter that should be here on Friday) so I am not afraid of tackling this, I am just looking to talk it out with more experienced audio folks.
Let me know your thoughts!
Thanks!
Brandon
So You have DC. I would bet on the power stage. Blown drivers perhaps.
Is there any output protection relay ? Does it click ? Measure for DC before the relay.
Is there any output protection relay ? Does it click ? Measure for DC before the relay.
No output protection relay. Only an input soft-start relay. I am guessing thats why this circuit is there, it just shuts down the whole system.
It only runs for a few seconds, but if I measure the DC line to the system ground (this should be the same measurement that the protection circuit does) I see -1.54V for just a moment.
Based on my reading this should be 50mV or less, correct? And this would be why it is shutting down.
Should I just try to order the whole output stage worth of transistors and just replace them all? Is there a better way of testing in-circuit?
Or could this be a capacitor/resistor issue?
Thanks!
1.54 V is high. Have You Checked the output transistor insulation between chassis / heatsinks ? Was the unit serviced before ?
You should test the output and driver transistors first, but it can be some other component like a leaky cap. Shorted Diode etc. Or remove the board and power it externally and test it. .. it depends on connections... I mean if You have only power rails and input output connections this should be easy.
Is the st-out broken circuit sending DC to the PSU ? Disconnect that wire to determine or remove the op-amp.
You should test the output and driver transistors first, but it can be some other component like a leaky cap. Shorted Diode etc. Or remove the board and power it externally and test it. .. it depends on connections... I mean if You have only power rails and input output connections this should be easy.
Is the st-out broken circuit sending DC to the PSU ? Disconnect that wire to determine or remove the op-amp.
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At the time I tested it, the PA board was removed from the chassis.
How would you best recommend testing the transistors? IN circuit they have all given me acceptable readings.
The ribbon cable provides +/-15V, feedback for the clipping circuits, channel a/b signal, DC feedback, and temperature feedback for fan speed.
Then there is 5 wires that provide large voltages for the drive circuits (+/- 48V, +/- 90V).
I have disconnected the ST-OUT from the PA and it will still shutdown, I have replaced the op-amp and I still get no output signal, and nothing on the VU-meter when it is set to monitor the ST-OUT.
I have a new discovery, I found 6 or 8 mylar caps that show an infinite ESR (they were open). I have removed those from the board and I was going to plug it in, but I left the ribbon cable at home, so I will have to check it tonight.
Will I damage anything running it with no load and a few caps removed? My initial guess is no, it would just sound like crap. I wont run it long that way.
I am guessing there are caps that are bad on that board as well, but I will have to dig into it after I get the PA working.
How would you best recommend testing the transistors? IN circuit they have all given me acceptable readings.
The ribbon cable provides +/-15V, feedback for the clipping circuits, channel a/b signal, DC feedback, and temperature feedback for fan speed.
Then there is 5 wires that provide large voltages for the drive circuits (+/- 48V, +/- 90V).
I have disconnected the ST-OUT from the PA and it will still shutdown, I have replaced the op-amp and I still get no output signal, and nothing on the VU-meter when it is set to monitor the ST-OUT.
I have a new discovery, I found 6 or 8 mylar caps that show an infinite ESR (they were open). I have removed those from the board and I was going to plug it in, but I left the ribbon cable at home, so I will have to check it tonight.
Will I damage anything running it with no load and a few caps removed? My initial guess is no, it would just sound like crap. I wont run it long that way.
I am guessing there are caps that are bad on that board as well, but I will have to dig into it after I get the PA working.
So, Sub and St-out are in parallel and with op-amp replaced still no sound ? components around ok ? Where does that wire after the op-amp (Chinese characters) go ? Touching input of that op-amp or small signal after the pot nothing on the outs ? Is the jack eventually shorted ?
The only reliable way of testing the transistors is to take them out of the circuit.
Ig the caps are in the loopback path You could get oscillation. If they are in the signal path NO SOUND. Yes and those caps depending what they do can produce offsets on the output and trigger protection.
My believe is someone plugged in a high voltage on the outs and many components got zapped..
Can You upload the whole schematic ?
The only reliable way of testing the transistors is to take them out of the circuit.
Ig the caps are in the loopback path You could get oscillation. If they are in the signal path NO SOUND. Yes and those caps depending what they do can produce offsets on the output and trigger protection.
My believe is someone plugged in a high voltage on the outs and many components got zapped..
Can You upload the whole schematic ?
I have attached a link to the service manual, as uploading a pic of the schematic makes it hardly readable. Its a fantastic service manual:
yamaha_emx5014c_sm.pdf - Google Drive
It seems that ST-SUB-OUT and ST-OUT are nearly identical circuits in parallel, and replacing the op-amp on ST-OUT did not solve the problem. I will look at the jack and the smaller passives around it to see what I can find. The output capacitors are hidden and I will need to get some covers off to take a look. The output, I believe, goes to a connector then to the jack. I have not checked to see if the jack was shorted. I seem to now be able to hear some very faint music coming through the ST-OUT line-out when I hook my little headphones up. The ST-SUB-OUT still comes out loud and clear.
After I get the PA back together I will work on that. For now I just have the mixer set to send AUX1 and AUX2 (which still work) to the PA when it is connected.
Pulling those capacitors (I pulled C115, C116, C119, C120, C133-C136 as I thought they tested bad) did not solve the problem. Still shuts off. Turns out those are all fairly small and my ESR meter probably could not read them.
You are probably right that several components were zapped, but I will work on it a while longer before just calling it dead.
yamaha_emx5014c_sm.pdf - Google Drive
It seems that ST-SUB-OUT and ST-OUT are nearly identical circuits in parallel, and replacing the op-amp on ST-OUT did not solve the problem. I will look at the jack and the smaller passives around it to see what I can find. The output capacitors are hidden and I will need to get some covers off to take a look. The output, I believe, goes to a connector then to the jack. I have not checked to see if the jack was shorted. I seem to now be able to hear some very faint music coming through the ST-OUT line-out when I hook my little headphones up. The ST-SUB-OUT still comes out loud and clear.
After I get the PA back together I will work on that. For now I just have the mixer set to send AUX1 and AUX2 (which still work) to the PA when it is connected.
Pulling those capacitors (I pulled C115, C116, C119, C120, C133-C136 as I thought they tested bad) did not solve the problem. Still shuts off. Turns out those are all fairly small and my ESR meter probably could not read them.
You are probably right that several components were zapped, but I will work on it a while longer before just calling it dead.
It isn't going to be easy. The Amp has high and low rail voltages. It is not easy to follow unless You print it on a large sheet.
There are also switches in the path. Are they in the correct positions ?
The St-OUT is connected to the limiter and main amp afterwards.
First of all are You getting any signal at C619 / C616 ? TL544 543
Without main amp connected does St-out work ? C647 /648 also checked ? Now the amp...I would order a new board since it has many components that can fail.
There are also switches in the path. Are they in the correct positions ?
The St-OUT is connected to the limiter and main amp afterwards.
First of all are You getting any signal at C619 / C616 ? TL544 543
Without main amp connected does St-out work ? C647 /648 also checked ? Now the amp...I would order a new board since it has many components that can fail.