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Last updated 13th March 2003

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

ęBill Harris 1997