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Last updated 13th March
2003
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Troubleshooting
The I-F Amplifier Stage
Function of the I-F Amplifier
Stage:
To accept the intermediate
frequency signal from the converter, amplify it and
pass it on to either, the next I-F stage (if the receiver
has one), or to the detector stage.
Theory of Operation:
Refer to the schematic diagram
below.
I-F transformer T-4 couples the I-F stage to the
preceding converter stage. The output I-F transformer T-5
couples the I-F stage to the following detector stage. In
our study of the converter stage, we learned that the
output of the converter contained four different
frequency components; the incoming or tuned signal, the
oscillator signal, the sum of the two signals, and the
difference of the two signals. The difference of the two
signals is the intermediate frequency and is the
one accepted by the I-F stage, as that is the frequency
to which it is tuned - in this case 455 KHz. This signal
is applied to the grid of the 6K7 tube, the signal is
amplified and appears on the output of the tube which
feeds output transformer T-5. T-5 tunes the
output of the I-F amplifier to the intermediate frequency
and the signal is passed on to the next stage.
Other receivers may operate with different intermediate
frequencies. Some early sets of the late 1920's and into
the 1930's often used lower frequencies such as 175 and
260 KHz. In almost all cases, the intermediate frequency
will be noted on the schematic.
Troubleshooting The
I-F Amplifier Stage
A dead I-F amplifier stage will
result in the signal from the converter stage not being
passed on to following stages, thus no output from the
receiver. Other failures may include, weak output
producing low volume, or oscillation of the stage
producing squeal in the output.
A quick check can be made by applying a modulated signal,
at the intermediate frequency, from a signal generator to
the grid of the preceding converter stage. If the I-F
stage is operating, the signal will be amplified and
passed on to the following stages and the tone will be
heard in the receiver speaker.
Refer to the charts below and the
schematic diagram above. Assume all other stages are
functioning properly.
Service
Data Chart For Improperly Functioning I-F Amplifier
Stage
Symtom |
Abnormal reading |
Check for |
Inoperative
|
Plate voltage (pin 3) = 0 |
Open I-F transformer
winding L10.
Open plate resistor R-25
Shorted by-pass capacitor C-25 |
Screen
voltage (pin 4) = 0 |
Open screen
dropping resistor R-24
Shorted screen by-pass capacitor C-24 |
Cathode
voltage (pin 8) = 0 |
Shorted
cathode by-pass resistor C-23 |
High cathode
voltage |
Open cathode
resistor R-23 |
All voltages
normal |
Defective
tube
Shorted trimmers in the I-F can
I-F transformer secondary open
Open AVC by-pass capacitor C-28 |
Weak signal |
All voltages
OK |
Weak tube
Open AVC by-pass capacitor C-28
Open cathode by-pass capacitor C-23
Open plate by-pass capacitor C-25
I-F circuits out of alignment |
Noise |
All other
checks OK |
Noisy tube
Corrosion in the I-F transformer windings |
Squeal or
oscillation |
All other
checks OK |
Open screen by-pass
capacitor C-24
Improper ground of tube shield
Open AVC by-pass capacitor C-28
Open plate by-pass capacitor C-25
Incorrect routing of wiring |
Typical
voltage readings
Circuit |
6K7 Pin No. |
Volts |
Plate |
3 |
245 |
Screen |
4 |
100 |
Cathode |
8 |
8 |
Input I-F
Transformer Color Code
Plate lead
|
Blue |
B plus lead |
Red |
Grid lead |
Green |
Grid return |
Black |
©Bill
Harris 1997
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