Mackie RCF ART300A Loud Speaker Hum

[Budsta]

Hi, was hoping some of you may be able to lead me in the right direction. I have a RCF 300A where the tweeter is working fine. the speaker blew quite some years ago but have been running it in a nightclub along with other rcf speakers only with the tweeter (in this unit) working.

I've since brought a new speaker for it. When i hooked it up the speaker went to full excursion with a loud hum. switched of immediately. Speaker is still ok. ran a multi meter on the 2 speaker wires. without even any line in signal plugged in the speaker is getting 83v sent to it. would you have any idea what to look at for this?

Regards,

Chris Budde

[Enzo]

Yep, loud hum and cone at extreme is classic DC on output. Your amp is "blown". Two reasons mostly. One is a shorted output transistor, easy to check with an ohm meter. the other is good outputs but some part earlier in the power amp is telling the outputs to turn on fully and stay. Another possibility is one blown power supply fuse - "rail" fuse. If one side blows its fuse, then all the circuits snap over to the other side voltage.

This is more complex than many amps inthat it has two sets of power supply rails for the power amp. 40v and 80v. It mostly runs on the 40v, but the 80v ones switch in for peaks. Class G or class H, I forget which. SO with 80v on the speaker, the high voltage is turned on, but as I read the schematic, putting 40v on the output would by itself turn on the 80v. There are MOSFETs for each polarity which turn the 80v on and off. I don't know whehter we have a blown MOSFET or not, but can't rule it out. We check them too.

Fuses F1, F2 are 6A slow blow and need to be checked. Pull them from the clips and use an ohm meter to test them. Don't test them while clipped in.

here is the service manual.

http://bmamps.com/Schematics/Mackie/...ice_Manual.pdf

[Budsta]

Yep, loud hum and cone at extreme is classic DC on output. Your amp is "blown". Two reasons mostly. One is a shorted output transistor, easy to check with an ohm meter. the other is good outputs but some part earlier in the power amp is telling the outputs to turn on fully and stay. Another possibility is one blown power supply fuse - "rail" fuse. If one side blows its fuse, then all the circuits snap over to the other side voltage.

This is more complex than many amps inthat it has two sets of power supply rails for the power amp. 40v and 80v. It mostly runs on the 40v, but the 80v ones switch in for peaks. Class G or class H, I forget which. SO with 80v on the speaker, the high voltage is turned on, but as I read the schematic, putting 40v on the output would by itself turn on the 80v. There are MOSFETs for each polarity which turn the 80v on and off. I don't know whehter we have a blown MOSFET or not, but can't rule it out. We check them too.

Fuses F1, F2 are 6A slow blow and need to be checked. Pull them from the clips and use an ohm meter to test them. Don't test them while clipped in.

here is the service manual.

http://bmamps.com/Schematics/Mackie/...ice_Manual.pdf

Thanks for the reply Enzo. Ive checked fuses they seem fine. would the output tranistors be the ones located in the manual on page 20 "the only green page" on say Q12? what should i be looking for when running the multi meter over it as it is if i put say the black lead on the left pin, red lead on the far right i would get "931" and if i put the black lead on the center pin and red lead on the far right i get "631" Any other combination doesnt get any reading..

I've also read somehwere this "Change Q12 and Q18 before even thinking, 70% of the problems of this speaker are those Q's"

EDIT: So testing the Q18 using black probe on center pin and moving the red between the left and right pin. the readings are very close to the same 839 & 871. could this be causing the Open circut to send 80v to my speaker? It does appear to me that q18 is part of the high Freq amp tho. which isnt an issue..

Chris.

[Jazz P Bass]

The low frequency output section can be found on page #18 of the attached schematic.

The output transistors are Q9 & Q10.

Q12 & Q11 are the 'Rail Switching' MosFets.

[Budsta]

The low frequency output section can be found on page #18 of the attached schematic.

The output transistors are Q9 & Q10.

Q12 & Q11 are the 'Rail Switching' MosFets.

Thanks Jazzy P

[Budsta]

Hey guys, Ive replaced both the output transistors and one mosfet, the unit wont fire up now. and keeps blowing the IEC glass fuse. im going to double checck the wiring from the input power to the board(see photo attached) to make sure ive connected all wires to correct spots.

Any other ideas on what it may be?