HELP : Interfacing XLR ( positive ground ) to Standard XLR

I'm converting a Scully 280 discrete transformer coupled head amp into a preamp , but it runs on a postive grounded supply 24Vdc. in other words the + side of the power supply is grounded to the chassis rather than the negative side. This head amp was used for the Scully reel to reel recorders , and this one came from a parted out 2 track deck.

The scully 280 has the following transformer compliment
UTC A18  15K:80K Line Input
UTC A39  600:2K Mic input
Freed  600:600 output

Question : What proper conversion must be done to the Scully's XLR mic and line input and outputs to ensure that standard XLR equipment like a condenser microphone will not be harmed when connected to the unit.

I'm guessing that changing the wiring positions on the connectors of the Scully to match EIA XLR standard must be done but I'm not quite sure. I rarely come in contact with equipment running on a positive ground.

I don't want to screw up any XLR gear experimenting either. I would like to get this right the first time.

Also, I made a slide in cassette power supply for this scully 280 by utilizing the compartament normally
occcupied by the oscillator board. This board is not needed for turning this unit into a mono preamp. Nevertheless, how do I tie the saftey ground on the center pin of a three prong 115Vac power supply cable. It would normally go to chassis ground but in this configuration....It would cause a short if it were connected to a positve ground. I'm thinking a safety ground on the power supply may not work in this case, any ideas ?

The Scully XLR connector is as follows
Pin 1 - positive chassis ground (+) zero reference
Pin 2 - Negative signal , Hot (-)
Pin 3 - Cold

The EIA XLR Standard is
Pin 1 - Negative Chassis Ground or Shield ( - )
Pin 2 - Hot (+)
Pin 3 - Cold (-)

Here is a input conversion cable I thought might fix this problem. The XLR output cable would likely follow the same scheme. Would I be better off with a polarity reversal switch than a conversion cable.

Can someone check my work ?

You don't have to care about the polarity of the inner voltages; the concept of negative or positive ground is a technical nonsense; ground is always zero. In most modern equipment there are positive and negative rails, how could it define a positive or negative ground?
The ground lug of the mains connector has to go to the chassis. You have to sort out the polarity issue, i.e. is pin one or pin two hot? You mention Pin 2 - Negative signal and Pin 3 - Cold, that just can't be: cold is same as negative signal.
I reckon the Scully is in accordance with the original standard of Pin 3 Hot. The actual AES standard (Pin 2 Hot) has been put in use in the late 70's/early 80's. So, yes you have to swap pin 2 and 3.

electrochronic said:

electrochronic said:

  cold is the same as negative signal

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Thats what I thought, they have the same potential.

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No, you have read something wrong; Pin 1 has always been ground

With regards to polarity, you really mean swap pin 1 & 2 on the Scully XLR inputs and outputs, since pin 1 is referenced as 'Hot' or positive chassis ground on the Scully unit ?

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Pin one can't be Hot, you must be mistaken. It's pins 2 & 3 you have to swap

On my 24 vdc power supply , the positive is connected to the Scully rear connector but is actually attached to the chassis inside the unit by means of a star type ground. The polarity positive)  is just the opposite of that which I am used to in all other gear ( negative) . and its confusing the heck out of me.

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As I said, this is the internal voltages; you don't have to care about them

We want to get to the EIA standard which makes pin 2 hot for interfacing with other EIA XLR standard gear
like microphones for inputs and compressors for outputs , right ?

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Yes

Since the inputs and outputs are transformer isolated there will not be any harmful voltages , yes ?

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Yes

Abbey ....,

Just want to make sure you understand that the power supply +24 Vdc leg is connected to chassis star and Neg
to circuit before continuing on.

electrochronic said:

No, you have read something wrong; Pin 1 has always been ground

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Yes, I undertand that pin 1 is normally negative chassis ground , but on this unit pin 1 is set up positive because the 280 circuit is running off a + 24VDC positive chassis reference from the power supply , my schematic shows this. And the connections on the 280 also confirm this. Its a strange polarity implementation of dc power , I know.

The 24Vdc power supply positive leg actaully attaches to the chassis like a star ground would, maybe this explains zero reference. So swapping the pin 1 XLR  so it is a negative ground and leaving the power supply alone even if positive is attached to chassis,  is OK ?

So when you switch the XLR to make pin 1 ground  does'nt that conflict with the power supply's  + 24Vdc also connected to chassis ground ? seems like a potential short circuit waiting to happen ? because the Chassis now
becomes the shorted path between pin 1 XLR (-) and + 24Vdc of the power supply.

Does this make any sense ? Or am I completely off track ?

to further understand my situation , if a said condenser microphone has a zero reference ground on the microphone body but the the Scully 280 has a zero referenced postive on its chassis , what is the consequence with mixing a NEG zero reference with a POS zero reference ?

Abbey.....,

I had to read your first posting over a few more times and now I understand your explanation

Scully pin 1 eventhough its positive chassis ground has zero reference.

Scully Pin 2 on the scully is actually hot

Scully Pin 3 is actually cold

and if 2 & 3 were not correct , they would be reversed as you suggested. Strange that a positive chassis ground could have zero reference.

so being that these connections appear reversed,  this is very unconventional non-sense like you described. but it operates like the conventional methods I'm used to. A mind opening experience.

electrochronic said:

Yes, I undertand that pin 1 is normally negative chassis ground , but on this unit pin 1 is set up positive

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[quote author=electrochronic]
what is the consequence with mixing a NEG zero reference with a POS zero reference ?[/quote]

[quote author=electrochronic]Strange that a positive chassis ground could have zero reference.[/quote]


Pin 1 is the zero of the balanced signal.
If you attach it to the ground of the gear A [which have "positive" ground], and then you connect gear A to gear B [which have "negative" ground],
the only interesting thing for gear "B" is how the signals on pin 2 and pin 3 are, in reference to pin 1.
The gear A and B doesnt know and doesnt care if the ground of the other machine is "positive" or "negative".
The mistake is to think at the ground like if it should have an absolute polarity, which can travel out of the machine.
[And anyway, as abbey road enfer said, its better to think at the ground as zero, not as "positive" or "negative".]

P.S. your quoting isnt correct.. it seems that you're talking only with yourself