Ribbon microphone, which transformer wire is which?

[sedrec1]

Hi,

I have an old Reslo RB ribbon microphone. The trouble is two of the three output wires are not connected to anything. How can I tell which wire is which?

I read that if there are three wires then one of them is a centre tap, but I don't know enough about ribbon transformers to know what to look for.
If I disconnect the primary from the ribbon motor, can I find out which wire is which by using a multimeter to measure the resistance between the wires of the secondary, and if so what resistances am I looking for?

Thanks

 

[Khron]

On old mics, there's a good chance the transformers have "high/low impedance" secondaries, and somewhat unlikely the two parts of the secondary are equal.

Resistances should in the tens of ohms for the "low" winding, and perhaps some hundreds(?) of ohms on the "high".

 

[mad.ax]

There are many versions of the RB. Some with a single impedance, some with dual... Varries from 30 to 10 000 Ω...
The complete reference RB/? will tell which model is yours.
Some info can be found here:
Reslo wiring guide

You could also consider swapping the transformer and perform the BBC mod...
https://groupdiy.com/attachments/1961-05-bbc-modifications-pdf.94676/

 

[sedrec1]

Thanks both, very useful.
The model is RBT/H. It states "30/50 Ohms & Hi-Z RBT/H".

I disconnected the primary from the motor.
Here is what I got at the secondary of the transformer.

White to Green - 1289 ohm
White to Yellow - 20 ohm
Green to Yellow - 1282 ohm

The readings seem to change slightly every time, so I think I need a better multimeter - but I guess this suggests that the remaining connected white wire, is the common connection. And the other two wires, which need re-connecting, are to select the required impedance, green = Hi-Z, yellow = 30/50 Ohms.

 

[sedrec1]

The mic I have has a metal ended lead that screws onto the mic. On the microphone end, one of the pins is connected to the metal bit by a small wire, and this presumably grounds the microphone when that is screwed onto the microphone body. I removed the existing cable and on the other end soldered a male XLR that will go to the preamp.

For the microphone end, I am not sure what to do. I intend to swap the transformer, but it would be great to first hear the mic with original transformer.

I could connect, via the connector, pin 2 of the XLR to the Yellow transformer wire to get the 30/50 ohm output. But for the white wire, should this be connected to ground (XLR pin 1)?

And, if my preamp has a balanced input, will the mono output will still work? - I suppose as there is no signal on pin 3 to cancel it out, it will work, but perhaps with a lower output?

Thanks again

 

[zebra50]

Hi!

Don't connect the third transformer wire to ground or you will get an odd frequency response. Insulate it and push it back inside the microphone.

Depending on the model, there should be a connection screwed to the shell of the Reslo plug. That should be connected to the cable screen, which in turn connects to pin 1 of the XLR.

S.