MCI 500 OT, phase, and the good old API output attenuator

[substitute]

Hey everyone, I'm working on racking some MCI 500 modules, I also have the original output iron from the mother board.  It's going pretty well so far, but I have a question about phase and the OT.  I'm taking my output from the line out.

https://www.dropbox.com/sh/2cuxpxiuprace4c/AAB6zmfOGL8krJUVuqgfDId4a/JH500c%2010-25%20-%20MotherBoard%20Mid.pdf

It looks like what would normally be the +, hi, or pin 2 connection is grounded on the primary side while the -, lo, or pin 3 connection feeds the OT.  This seems backwards to me, it's not that I don't trust MCI, could some one explain this a bit?  Now on the OT's themselves there's not much in the way of labeling, I know they're 600:600 so I guess it doesn't matter.

Now, this afternoon, I hadn't yet seen the above diagram, I hooked up the OT with a 1k pot after it like the schematic Jeff posted here...
http://groupdiy.com/index.php?topic=31472.0

It "worked", but I'm confused a bit as to how.  For starters, how exactly does the API output attenuator work?  It seems like it blends the out of phase signals together to achieve it's attenuation, right?
Secondly, with the output volume all the way up, I noticed the signal is only on pin 3 of the XLR connector I have wired up, this doesn't seem right.
Can anyone shed some light on how to wire this up so the phase is correct?

Thanks

 

[radardoug]

I think you can assume MCI knew what they were doing, so wire it the way they do.
But you need to learn how to phase test.
The simplest way is a two channel scope. Put one channel across your input, one channel across your output.
But first make sure by testing what you are seeing.

I get the impression you are not very technical. So first you need to read and learn.
From your description of the output pad on the API, it is clear you do not understand.
That pot is a VOLTAGE DIVIDER across the transformer secondary. Look it up.

Learn grasshopper!! And it will all become clear.

 

[substitute]

I've been using my scope for signal tracing, checking out the eq and generally figuring out how to get the signal to and fro.  But, I've never used the second channel, that's a big help, thanks.

 

[Bill Wilson]

Hi Substitute: Check the phase of the transformer as Radardoug described. The output transformers MCI used used the bifilar winding technique. They will handle a level of +30 dbm with no problem. The frequency response is flat to approx. 1 mghz, with a -3db point at 3mghz.; I know this as I measured several myself. The transformers were manufacture3d by Southeastern Transformer Co. in Miramar,
Fla.; they are no longer in business.

I know this as fact as I ran console checkout at the MCI factory when we were shipping the 416 consoles; later I was service mgr. for the MCI dealer in Nashville, before opening my own business there.

 

[substitute]

Hey Bill thanks for the stats.  Sounds like a cool gig!

 

[squib]

I've used dozens and dozens of MCI JH-500 output transformer in lots of projects, including many pairs of racked JH-500 modules.

i reckon they are possibly the best 1:1 output transformer ever made .... big call i know ....

 

[substitute]

Big words, but why not?  Even just futzing around saying "check check" into a 57, there was a noticeable change when I added the OT.  Squib, if you don't mind me asking, what is your basic approach to racking the 500 channel strips?  I'm going into the mic pre (I'll add a switchable line pad) jumpering from the mic pre insert over to the EQ assign switch, switch in channel position, then from there over to the ACN in and out the line out.  Seems like it'll work fine, but I haven't tested this on a cut module yet.

 

[squib]

Perhaps email me [email protected] and I'll see what info I have on file