pucho812

Web info on testing transformers...
« on: March 31, 2005, 02:37:51 AM »
Testing audio transformers
© Brian A Clarke
1 Test equipment required
o   Multimeter ? analog or digital
o   Audio signal generator ? fixed frequency or variable
o   Variable resistors, 1off 25k to 50k and 1off 10 Ohm to 500 Ohm
o   Connecting leads
o   Audio voltmeter- preferably calibrated in dBm, sensitive down to ?60dBm
2 Identify windings
o   Use the multimeter to identify continuity between the various connections.
o   For an output transformer, the high resistance winding, single ended or center-tapped is the primary
o   For a microphone, line or an inter-stage transformer, the higher resistance winding is usually the secondary
o   For centre-tapped windings, the two halves may not be exactly the same resistance, especially on E+I cored transformers
o   There may also be a screen winding, which may or may not be connected to the frame.
3 Measure dc resistance
Use the multimeter to measure the dc resistance of each winding; write this down.
4 Measure impedance
4.1 Output transformers
1.   Connect the 100 Ohm variable resistor across the low resistance winding; initially, leave one wire open;
2.   Connect the 25k variable resistor in series with the high resistance winding and the output of the signal generator; set the resistance to 0 initially;
3.   Set the signal generator to 1kHz and near maximum output, which may be say, 20V rms
4.   Use the audio Voltmeter to measure the input and output Voltages; write these down
5.   Calculate the turns ratio = VIN/VOUT
6.   Connect the 100 Ohm resistor in circuit
7.   Reduce this resistor till the output Voltage falls 3dB
8.   Increase the 25k resistor till the output Voltage falls another 3dB
9.   Without disturbing the settings of the variable resistors, disconnect one lead from each
10.   Use the multimeter to measure the settings of these two resistors ? these are the impedances of the primary and secondary ? you may need to replace the 100 Ohm variable resistor with a lower value to get an accurate measure
11.   Compare the turns ratio with the impedance ratio ? do they agree? Allow for the dc resistance of the windings
4.2 Microphone, line and inter-stage transformers
Follow the procedure for output transformers, except:
1.   Connect the 25k Ohm variable resistor across the high resistance winding; initially, leave one wire open;
2.   Connect the 100 Ohm variable resistor in series with the low resistance winding and the output of the signal generator; set the resistance to 0 initially;
3.   Set the signal generator to 1kHz and low output, which may be say, 20mV rms
Warning: do not exceed 20mV rms for microphone transformers
4.   As above
5.   As above
6.   Connect the 25k Ohm resistor in circuit ? it may have to be a higher resistance;
7.   As above
8.   Increase the 100 Ohm resistor till the output Voltage falls another 3dB
9.   As above
10.   As above
11.   As above

5 Measure frequency response
Note: this section can only be done with a variable frequency signal generator.
1.   Reconnect the variable resistors without changing their settings
2.   Connect the audio Voltmeter across the secondary
3.   Reduce the frequency till the output falls another 3db compared with steps 8 above; this is the lower 3dB point
4.   Increase the frequency till the output falls another 3db compared with steps 8 above; this is the upper 3dB point
5.   Write this information down ? perhaps attach it to the transformer.

6 Measure power response
If you want to measure power response of output and inter-stage transformers, you will need a very good quality power amplifier, high power non-inductive load resistors and a CRO.
1.   In place of step 1in section 4.1 above, use a variable high power load resistor to achieve the value found in step 10
2.   In place of step 2 in section 4.1 above, use a high power variable resistor to achieve the value found in step 10
3.   Connect the audio Voltmeter across the primary;
4.   Set the input power to about 1W
5.   Connect the CRO across the secondary. Set the signal generator to 1kHz
6.   Increase the power input until the image on the CRT just starts to flatten ? this is probably 3dB over the rated power of the transformer
7.   Reduce the power input
8.   Change the input frequency to the lower 3dB point identified in step 3 of section 5 above
9.   repeat step 6 at this lower frequency ? the low frequency 3dB point may be at a much higher frequency than the lower 3dB point at low power
You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is.


Web info on testing transformers...
« Reply #1 on: March 31, 2005, 12:11:13 PM »
WAY NICE!!! THANKS
I need help with  RCA BA-43/ BA-45. RCA BA-25a, RCA BC3 mic pre's, Gates Level Devil, Federal AM-864u, DIY la2a stuff, Altec 436b

saint gillis

Re: Web info on testing transformers...
« Reply #2 on: December 05, 2017, 12:52:13 AM »
Sounds great!
And how to know if an unknown transformer is line input / mic input / interstage or output ?


 

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