trouble in paradise

[wertt]

so...

i built a couple of c84's about a month ago. the first one went off without a hitch but the second had a little bit of trouble (see the last couple forum posts). mysteriously, the second pre started working and i used it for a choral recording 2 weeks ago without any problems.

i finally made room in my rack for my sca chassis and was using the first channel for a vocal overdub the other day when i noticed that the meters on my a/d converter were lighting up for the second channel (which didn't have anything plugged into it). after the dude left, i listened to the second channel and, though protools was metering a good level pretty consistently, i couldn't really hear much other than an occasional scratching sound (i'm guessing the pre was passing something really low/high?).

i took the chassis apart to see if anything looked out of the ordinary and nothing did. now when i plug the pre in, it's quiet but it doesn't pass any mic signal. i can hear the pre turn on and a soft click when i adjust the gain, but nothing from the input. i reran the tests in the build instructions and everything is as it should be. i also tried swapping power connectors with the working channel, but the problem persisted.

any advice on how to start diagnosing this? i could go through and test various solder spots to see how they compare between the 2 preamps, but it seems like this might not be the smartest idea.

[wertt]

so i spent some time this afternoon cleaning up a few slightly questionable solder points and hooked the unit back up...still no luck. on a whim i decided to play with the phantom and polarity switches and, lo and behold, if i flip phantom on the pre works just fine. i'm definitely using a dynamic mic, so i'm not being a complete moron. does this provide any insight to anyone?

[tpryan]

Can you carefully measure and post the voltages at U1 with the phantom switch in each position?

[wertt]

yep. it's -18 / +18 both ways. or did you want me to measure every pin?

i also double checked to verify i'm getting phantom when the phantom switch is on and it's working correctly.

[tpryan]

Every pin.

[wertt]

going around clockwise, i get 4.3 / 12 / 12 / -18 / 12 / 12 / -4.4 / 18, both with phantom on and phantom off.

[tpryan]

What revision of the board do you have?

[wertt]

1.9

[wertt]

it turns out i'm an idiot. i had the board out of the chassis and was looking at it upside down.

it's not the phantom switch that makes it stop passing signal, it's the polarity switch. if the polarity is flipped, it works. if it's normal, it doesn't. the voltages at u1 are the same as above with the switch either way.

this explains why it worked for me before, as i was using it with the switches both to the flipped position for my last recording (i was only using the 2 mics and i wasn't 100% sure which was was flipped so i left them both in the flipped position). hope this makes the answer easier to find.

[wertt]

so if i understand the schematic correctly (and i am an electronics noob, so i could be wrong), there aren't a whole lot of possibilities here, are there? i just double checked all my solder points and cleaned up a couple of them to be extra sure, but no change. when you flip the phase switch, it goes from passing audio to a faint white noise. i'll record it if you'd like me to, but there's not much to hear. do i just have a bad switch?

[tpryan]

What voltages do you measure at R31 and R32 (both ends)?

What voltages do you measure at R6 and R7 (both ends)?

Are you absolutely sure that you've installed the right value resistors at these locations and that the solder joints are good?

[wertt]

i put the chassis back together to use for my session this evening, but i'll take it apart afterwards.

[wertt]

r6 - 11.6 (connector side) / 0
r7 - 10 (connector side) / -17
r31 - -6.2 (top) / -7.6
r32 - -16.3 (top) / -17

i think there must be something weird and intermittent somewhere. i pulled the channels out of the chassis and plugged them back into protools and they're both working regardless of the switch position. i'm going to do some more testing in the morning, but hopefully these numbers will reveal something. i tested my good channel and the r31 and r32 values seem way off. i also got 10/0 on r6 and r7.

i tested the impedence on all 4 resisters and they're definitely correct.

[wertt]

and it's back to being weird this morning. i posted a wav of the noise the preamp's passing to www.elmstreetrecording.com/c84/ . it's clipping the meters and lighting up the paz analyzer somwhere really low (below 8hz), but there isn't really any audible sound.

[wertt]

and, awkwardly enough, i can't recreate the -17 at r7 that i measured last night. now it's bouncing around in the low negative numbers.

r31 and r32 are measuring -16 on both sides.

i'm using a pretty cheap multimeter, but it measures consistently on the good channel.