----- Original Message -----
From: <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Friday, April 01, 2011 6:56 PM
Subject: Re: [Drakelist] Fwd: My R-4B audio?
Garey,
How would I check to see if that is the correct one in
there..
Can I just use a meter and check the resistance on the
primary & seconday?
If it is the wrong one...it could be the problem
73,
Lee
The DC resistance won't tell you much because the size
of the wires in the primary and secondary are probably
different. You can check the effective output impedance with
a varirable resistor and voltmeter. The resistor should be
perhaps ten or twenty ohms to cover the range of impedances
likely. Get a tone on the RX using the calibrator. With the
speaker and headphone jacks open circuited set the level of
the output to a convenient level. Now, connect the resistor
across the loudspeaker or headphone jack with the voltmeter
across it and adjust it for exactly half the voltage you get
when open circuited. Now measure the resistance of the pot,
that will be the impedance of the output. Since this is the
plate impedance reflected into the secondary it will be the
correct value of loudspeaker impedance to match the
amplifier.
If you have an audio oscillator you can measure the
turns ratio directly by putting a voltage across either
primary or secondary and measuring it at the other end.
Voltage varies directly with the turns ratio, impedance with
the square of the ratio.
Note that transformers do not have a characteristic
impedance, they have only a ratio. They are specified to
work at certain impedances because the inductance and
distributed capacitances will allow a specified frequency
range. A transformer can be used over quite wide range of
impedances but there will be a change in the low and high
frequency cut-off. A variable frequency oscillator and good
AC voltmeter can be used to measure the frequency response
at various impedances.
Output transformers are also designed to handle a
certain amount of DC current.
The transformer may be the source of the trouble but it
would have to be pretty far off.
--
Richard Knoppow
Los Angeles
WB6KBL
[email protected]
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