help measuring transformer windings

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soundguy

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Jun 4, 2004
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Location
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I have two input transformers that Im trying to figure out, they were custom wound for a console and no data is available, came out of either a quad eight or electrodyne console, looks like reichenbach, cinemag has no data for this one. Can anyone help me interpret my findings:

there is a black lead which definitely was connected to the chasis ground.

I believe the primary to be

yellow
orange
brown
red

and the secondary

green
blue
grey
purple

between yellow/orange I measure 6.9 ohms
between brown/red I measure 6.9 ohms
with orange & brown connected, I measure 13.6 ohms betweeen yellow/red

on the secondary I measure .725K bewteen both green/blue as well as grey/purple.

If I connect blue & grey I cant get a reading between green & purple.

Im assuming that the input is selectable between 2 diffferent impedances. Im pretty sure this was a mic input transformer (although not positive) so perhaps it could be strapped 150/600? does that seem right based upon my readings?

Where I am confused is with the secondary, I cant combine them to add so perhaps this was wound to provide two outputs?

My understanding of ohming out a transformer is primitive at best, any help on interpreting this data (or suggestions for any other tests I should conduct to determine what I have) is greatly appreciated.

CJ, where you at?

dave
 
13 ohms seems a bit low for a 600 ohm winding (at least in my limited experience.) Maybe 50/150?

Tom
 
It sounds like a dual-primary, dual-secondary transformer. Apply a 1kHz sinewave at a given level across either of the secondaries and measure the level across either of the primaries. I'm telling you to inject the signal "backwards" like this because if you injected at the primary and measured at the secondary, the shunting effect of your AC voltmeter could give a false reading. The level you read at the primary may be rather low if it's a high-ratio transformer. See if the ratio is the same from either secondary to either primary, etc. Measure the inductance of the windings (especially the primary) if you have the equipment to do so.

If you're pretty sure they were input transformers, then the turns ratios, primary inductance, size of xfmr and type of core/shielding will give you a pretty good idea of what's up. For example, if you find the ratio to be 1+1:5+5 and the primary inductance somewhere on the order of 6H, you're most likely looking at a mic input xfmr that could be strapped for 150 or 600 to 3.7K or 15K.
 
I take it you are using a d.c. multimeter rather than an a.c. impedance meter? You won't get much correlation between d.c. resistance and the designed operational impedance, since it will depend on wire gauge as well as number of turns (length of wire at d.c.).

It's odd that you can't get a reading when putting the two secondaries in series. Have you tried the other combinations of connecting secondary colours?

You really need to try it out at a.c. I suggest putting out a 1KHz test tone though a headphone jack and potting it down using something like 10K Ohm and 10 Ohm. Connect the transformer primaries in turn across the 10 Ohm resistor and plug the secondaries into your Hi-Z mixer inputs. Take the primary signal to another channel. Adjust the channel gains so the 1KHz appears equally loud on the two channels. With the primary winding connected one way round, the mixer output will be loud and the other way round will be quiet as the signals from the two channels add or subtract respectively. In the subtract position, adjust the gain of one channel until there is no mixer output at all. The difference in gain of the channels (in dB) when expresed as a linear quantity is the voltage gain (turns ratio) of the transformer. The impedance transformation is the square of the turns ratio.

Example: channel 1 (connected to primary) has 55dB gain. Channel 2 (on the secondary) has 21dB of gain to null out the signal. Difference is 34dB which is a ratio of 50:1. So the turns ratio of the selected windings is 1:50, giving an impedance transformation of 2500:1.

Repeat with the primaries paralleled and in series.
 
untwist the splice on the sec and connect one of those wires to any other wire for a splice.
try a pri signal again and see what happens.
 

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