T15 noise? Radio interference? (Solved)

[rhythminmind]

EDIT: Solved - Human error, J1 & J2 jumpers where set vertically as opposed to horizontally.


I just assembled 8 T15's over this past weekend. They all pass audio & sound great when used at normal levels. But i can tune in a strange noise at really high gain levels. I made a MP3 of the noise (the music you hear is from my stereo in the other room) .
Any ideas what could be causing this?

http://www.rhythminmind.net/temp/Random%20Audio/t15noise.mp3

EDIT: ok well i'm picking up a radio station now.
I think it's just an artifact of the high gain. What is weird is it's triggered by audio. It will cut out with a DC offset sounding pop then if i talk into the mic it will pop back in.
http://www.rhythminmind.net/temp/Random%20Audio/t15radio.mp3

[tpryan]

Are you absolutely sure you installed all of the ceramic capacitors in their proper places?

Are you testing with the chassis cover attached?

What happens if you use different / shorter cables?

Do you get the same interference with only a 150 ohm resistor across the input?

Does the squeal change when phantom or polarity is switched?

Are you confident the chassis is properly grounded?

Have you cleaned the paint from the top cover countersinks and front panel mounting holes? You should see a very low resistance (nearly 0) between the switch bushings and AC ground pin.

[rhythminmind]

Are you absolutely sure you installed all of the ceramic capacitors in their proper places?

I am.

Are you testing with the chassis cover attached?

At the time no.

What happens if you use different / shorter cables?

I will have to try.

Do you get the same interference with only a 150 ohm resistor across the input?

Added to the test list

Does the squeal change when phantom or polarity is switched?

Yes. It seems to change the threshold of what triggers the noise.

Are you confident the chassis is properly grounded?

i wired the PSU ground wire but i will check with a DMM tonight.

Have you cleaned the paint from the top cover countersinks and front panel mounting holes? You should see a very low resistance (nearly 0) between the switch bushings and AC ground pin.

Will have to check.

[tpryan]

The only way I can induce any kind of radio interference is to open the chassis and poke the input coupling caps with my finger. Obviously, that's not normal operation. I can't induce oscillation on any of our test modules.

Absolute worst-case noise output with a 150 ohm load is about 5mV in a 30KHz bandwidth. That's not quite -50dBu at 70dB of gain, which gives an EIN of 120dB.

[rhythminmind]

I did my test with a mic pluged in. My noise floor is really low @ full gain without a mic. That isn't a problem. It was only happening when i had a mic in the path.

[tpryan]

Solved - Human error, J1 & J2 jumpers where set vertically as opposed to horizontally.

OK, that explains the RF interference. J1 and J2 connect the cable shields to ground, and an ungrounded shield is basically the same as no shield as far as high frequencies are concerned.