I have mackie mr8's my left one, when powered on makes a loud buzzing sound...i have disconnected power and pre amo...reconnected and it still happens...whats wrong?

I assume that you switched L and R monitors to verify the problem and so I would say that you have a amplifier failure issue. Knowing Mackie, they're probably using an output IC for amplication which although it took some time, has now completely died.

Me being an old-timer I can honestly say....Oh how I miss the discrete transistor and "thru-hole" component mounting days.

Open the unit and remove one wire from one of the woofer terminals and using an ohmmeter check the resistance between the terminals. It probably should be between 6 and 16 ohms... they don't give the specs on the speaker... if it is very high or infinite resistance, the voice coil is blown and you be re-coning or buying new speaker. These are close in speakers and intended for a small room for studio listening while mixing. If the speaker is OK, then the bass amp in the cabinet has a failure... pray, as that is cheaper to repair than a speaker.

If this monitor is humming without nothing connected to it, maybe the electrolytic capacitors from its power supply are bad - they are responsible for filtering the fluctuation at the output of the internal voltage rectifiers that transform the AC input voltage into a DC voltage to feed its internal amplifier.

When they fail, the supply voltage for its internal amplifier that should be flat, continuous, now presents a pulsating profile, according to the mains frequency (generally 60 or 50 Hz, depending on the country you're in). So, the output of the amplifier will be varying at the same rate, producing the humming.

Another possibility is that one internal cable connection that should be shielded is now disconnected or loose and is picking the hum from the internal power supply.

Anyway, maybe you should take it to a service center and have it checked.

Hope this info may help you!

The woofers were likely the problem in the first place. DO an ohmmeter check on them.

Note that the ohmmeter often doesn't work for testing piezo tweeters, but does work for the woofers with voice coils.

From a site:

Order new replacement tweeters from Mackie by contacting Mackie at: LOUD Technologies Inc.

1. 16220 Wood-Red Rd NE
   

2. Woodinville, WA 98072
   

3. Phone (800) 258-6883
   

Ask for the tweeter for the MR5 & MR8 the part number might be 0030616. The price is about $31/tweeter

What cables are you using? I have found brand new XLR cables that were mis-wired causing hum.
Check that the signal wires connect to pins 2 and 3 in the connectorss and that the shield is tied to pin 1.

Find images of the pin positions here:

http://www.google.com/images?hl=en&expIds=17259,27740,27760,27796,27824,27867&sugexp=leprodsca4&xhr=t&q=xlr+pins&cp=7&wrapid=tljp1291152977062014&um=1&ie=UTF-8&source=univ&ei=V271TMPdLoj2tgOK2PG7Cw&sa=X&oi=image_result_group&ct=title&resnum=3&sqi=2&ved=0CDgQsAQwAg&biw=1233&bih=570

Make sure the speakers are powered from the SAME receptacle as your mixer !!!!!

If you are using 1/4 inch connections they MUST be TRS type using BALANCED lines to avoid hum.

If nothing is pluged into the speaker and it makes that noise. It is not a ground problem. You either have a blown amp or remotely some bad connection at the large filter caps.

This is just a hint as I was unable to find the schematic. Transformers VERY seldom fail, if you are getting anything out and the fuse is holding with no heating, the voltages are probably right.

Common voltaages seen after the rectifier and filter and regulators are +/- 15 volts and sometimes +5 volts. For the power amp they are usually between +/- 35 and 60 volts unregulated.

For small amps using the TDA2050 type amps, you will see aproximately +/- 24 volts DC unregulated.

Hi there. Disconnect all auxiliary inputs. Still buzzing? Internal problem. Not buzzing? External problem.
Thanks
Paul

I built a web page on how to replace the tweeter on the MR8. Lots of good photos to help you with this problem. You can find it at: http://www.idahomojo.com/IdahoMojo/Mackie_MR8_Tweeter_Replacement.html Hope it works out for you!