Mackie 808S power mixer with no monitor output!

[Ampnostalgi]

I have owned this Mackie 808S FR series power mixer for a long time and I have used it occasionally (maybe once a month). It quit giving me the ouput signal for monitor section during a show today. There was no LED level indicator, just dead silent. The output to the mains worked normal. What could it be? Hope someone can identify the problem and give me some hints what to look for. Thanks.

[mozwell]

i hope these help, inject a signal into a channel (a cd will do, or a MP3 player output), then follow it to the monitor section, see where it fails to show up

[Ampnostalgi]

Thanks Mozwell, I got the signal output from the monitor out jack now, but I am having this annoying hum from the monitor speakers, the louder the volume, the greater hum I got. My amp that drives the monitor speakers works fine. I did not have this hum before!

[Ampnostalgi]

Forgot to mention a call to Mackie (Loud Technologies) and they sent me the schematic for my mixer (the same as the one mozwell posted).
I am still looking for the cause of that hum on the monitor speakers, my buddy said the only thing he did when set up for the show last time was accidentally using a speaker cable instead of an instrument cable to hook up from the monitor output of the mixer to the amp that drove the monitor speakers, but only a short while before he realized that he made a mistake. Wonder would that cause the hum I got now?

[Enzo]

Follow the signal path through the monitor section. YOu may have fixed part of it, but apparently not all. First thing I would check is to make sure each IC has both power supplies present. The board may have both power supply voltages, but that doesn't mean they reach every chip. Next I also would go down each relevant IC and check each output pin for unwanted DC offset. When ICs go to DC on their outs, it often causes hum as well.

[Ampnostalgi]

Thanks Enzo. I will follow the schematic to check the signal path for the monitor out circuit as you suggested. I will report what I found. BTW, it's great to have your help with this problem, I read somewhere in this forum that you were out sick at some point of time. Hope you are well.

[Ampnostalgi]

It seems that I will have to live with that hum on the monitor out for a while! I check for unwanted DC at the ICs' output but found none! Every relevant IC gets power in OK. What else could cause that hum only on the monitors, not the main speakers?

[Enzo]

Oh that was just one quick preliminary test.

So isolate the problem. Hum out the monitor speakers. Are you using the internal amp or an external amp? Sounds like external. Looks like the right channel can be selected as monitors. Set it that way and connect a speaker. Does it hum on that internal amp?

Does turning the monitor volume to zero make the hum disappear? The monitor signal runs through the monitor graphic. Does moving the 63Hz band up and down affect hum level? Does it make any difference to turn ALL the channel monitor sends to zero? The monitor and main each have a low cut button, does turning that on reduce the hum any?

Your exterenal power amp might work fine, but that doesn't mean it is not involved in a ground loop.