Faulty T-Pad Attenuator?

[JCN1218]

Hi everyone,

I'm currently working on a stereo 1176, but I'm running into all sorts of problems right at the finish line. I'm trying to get one working channel and it seems like I'm pretty close to achieving just that. Unfortunately I'm having a problem with my T-Pad attenuator.

This is the attenuator in question: http://www.hairballaudio.com/catalog/parts-store/attenuators/bi-600-t-pad-attenuator
I have one wired up for each channel at the moment. On channel 2, the one that's closer to working, I'm noticing some odd behavior. The volume spikes dramatically within the first 1/4 of the attenuator's rotation. I hooked it up to my DMM and while turning it slowly I saw the VAC start at around 0.02, but once it hit 0.03 it quickly spiked up to 0.1 VAC. For a very short portion of the rotation, VAC would drop from 0.1 to about 0.8 before returning to 0.04. This doesn't seem like normal behavior to me, and it's preventing me from continuing with the calibration process. Is this attenuator busted, or is there a problem elsewhere? Keep in mind that I'm measuring my values directly from the output of the attenuator, so before the input transformer. Could the cable's proximity to the main power wires be causing this?

 

[ruffrecords]

How have you wired it up?

Cheers

Ian

 

[JCN1218]

Hi Ian,

I have it wired according to the 1176 wiring guides which is simply straight from the input XLR into the attenuator.

 

[mjrippe]

Are you driving it from a balanced source? 

 

[ruffrecords]

Hi Ian,

I have it wired according to the 1176 wiring guides which is simply straight from the input XLR into the attenuator.

Sorry, I meant how did you wire the three pots of the attenuator together or are they already wired together?

Cheers

Ian

 

[JCN1218]

Are you driving it from a balanced source? 

I'm feeding it the test tone from my UA apollo line out. I'm almost 100% sure it sends out balanced signals.

As for the wiring of the attenuator, I have it set on a little PCB provided by Hairball. All center pins are along the same trace. The left pins (left if looking at the shaft end of the attenuator) are as follows:
1st pin - unconnected
2nd pin - connected to 'input -' and 'output -'
3rd pin - unconnected

And the right pins:
1st pin - connected to 'input +'
2nd pin - unconnected
3rd pin - connected to 'output +'

Let me know if that clears anything up. This is a three gang pot, so I'm looking at the pins in their common rows, so for example, the center pins are all of the wiper pins I believe.

 

[RuudNL]

Does this help?

 

[JCN1218]

Yes! Thanks for that. That's exactly how I have it wired up