MCI JH-500 Racking

[msheriff123]

hi all! i’m new into this forum and i’m currently working on racking up a few channels of MCI 500 pres/EQs

this is the first racking up project i’ve taken on and i’m wondering what type of line input transformer was used on the original console. If it’s a 1:1 transformer, what was the ohm rating for it? and if i were to use a transformer with a higher ohm rating (still 1:1) would it be an issue?

thanks!!

 

[jensenmann]

IIRC the line in tranny is 4:1. I still have a few of these collecting dust.

 

[abbey road d enfer]

this is the first racking up project i’ve taken on and i’m wondering what type of line input transformer was used on the original console. If it’s a 1:1 transformer, what was the ohm rating for it? and if i were to use a transformer with a higher ohm rating (still 1:1) would it be an issue?

Looks like a 2:1
+4dBu is attenuated by 9.5dB for nominal operating level (-5.5dBu). 2:1 gives 6dB attenuation and there's a 3dB pad after the secondary (330r/820r). Taking into account losses that seems right.
Reflected Z at input would be about 5k.
It would nominally be 5k:1.2k, but I suspect the spec would be something like 4k:1k or 10k:2.5k.
A decent xfmr accomodates easily +100%/-50% mismatch.
Using a 1:1, you'd have to change a couple of things: R71 from 330r to 1.6k and R70 from 9.1k to 27k.
That would be a 10k:10k.

 

[jensenmann]

They are 4:1, I measured them.

 

[abbey road d enfer]

They are 4:1, I measured them.

I believe you, but it's not consistent with the schemo I have, which may be a different version.
Are you sure we talk the same language? I mean voltage ratio, perhaps you mean impedance ratio, I don't know, just trying to find an explanation...

 

[jensenmann]

I measured voltage ratio. They came out of a cream colored desk, so I suppose that it must be an early version.

 

[abbey road d enfer]

My schemo is from 1976...

 

[porkyc]

My schemo is from 1976...

I thought they were a 10dB drop, but they might have been -12dB. Whatever, they weren't -6dB.
Later, the TX was dropped and a diff amp put in place.