002 rack unit repair (monitor section)

hotel valkenburg · Jun 27, 2013

I have a recent issue with the monitor section on my 002r BLA. with monitors hooked up to mon out L&R and the mute switch off-it passes a fairly quiet sporadic static sound on the left channel, mute on mutes the static. when i switch the mute on and off i get a loud dc sort of pop only on the left channel. the right channel will produce some audio very quiet with no noise or distortion. I wish i had a schematic or something to go off of, where should i start? check for cold joints? start replacing caps? some of the surface mount caps look like they've seen better days.

i looked around on the board where i would imagine the audio path to be, for something scorched but found nothing. some of the solder joints do look a bit corroded and i plan on getting some isopropyl alcohol or contact cleaner, maybe it will help, but that doesn't seem like it should be the problem.

I tried reseating the ribbon connections.

audio will pass through the main outs 1&2, ive just gotten used to adjusting the volume on the front of the unit.

help? thanks!
J

jwhmca · Jul 1, 2013

Do you have a scope?

I would start by checking the power supply voltages.

hotel valkenburg · Jan 19, 2014

ok my job has finally slowed down and it looks like ill have more free time.

the voltages at the regulators look fine to me +19.97 -17.90 not exactly +-18 and im not sure how to dial them in better.
but for now that shouldn't be the issue?

the voltages at the ne5532 opamp for the monitor section read the same on the corresponding Vcc pins. within spec for operating conditions but certainly higher than the data sheet recommended +-15

there is -16.91v seen at the output of the opamp. i am having a hard time figuring out where this is coming from
i replaced the opamp with the same chip and get the same readings.
according to the data sheet this thing can handle Vcc at the output? so the chip is fine?
is there something at the output drawing too much?
is there something before the opamp supplying too much. I just dont know enough to know

I do have a scope, its a cheapy all in one DMM.

thanks
~Jordan

jwhmca · Jan 20, 2014

So it doesn't pass audio at all or at normal levels in the monitor section?

Spino · Jan 21, 2014

hi valkenburg,

i don't know if this might help (or if it's even in topic), but both 002 and 003 are known for producing noise on the left channel only. this is usually when turning slowly up or down the monitor volume pot with the mute off. people say its digidesign's choice of cheap potentiometers. sporadically people say they got rid of it by just spraying the pot, but it doesn't seem to work for everybody. people seem to solve it temporarily also by rotating a few times the pot, but that doesn't work for me.

hotel valkenburg said:
I have a recent issue with the monitor section on my 002r BLA. with monitors hooked up to mon out L&R and the mute switch off-it passes a fairly quiet sporadic static sound on the left channel, mute on mutes the static. when i switch the mute on and off i get a loud dc sort of pop only on the left channel. the right channel will produce some audio very quiet with no noise or distortion. I wish i had a schematic or something to go off of, where should i start? check for cold joints? start replacing caps? some of the surface mount caps look like they've seen better days.

i looked around on the board where i would imagine the audio path to be, for something scorched but found nothing. some of the solder joints do look a bit corroded and i plan on getting some isopropyl alcohol or contact cleaner, maybe it will help, but that doesn't seem like it should be the problem.

I tried reseating the ribbon connections.

audio will pass through the main outs 1&2, ive just gotten used to adjusting the volume on the front of the unit.

help? thanks!
J

hotel valkenburg · Jan 21, 2014

if i send 1k signal in and look at the output from the monitor section (mon out L + R) i get no 1k waveform and a -16vDC.
its weird somehow that 16 volts is being fed to the opamp inputs and i have no idea where from. with the multi layer pcb an ribbon cables
its like wack a mole, i think ive given up. maybe ill just work on a monitor controller and use up two different line outs.

thanks