overseas

Variable IF Reference & Stage Gain

Part of the secret behind the R390A's incredible selectivity and its immunity to near RF fields is found in the two tracking IF sections. The First Variable IF operates while receiving frequencies below 8 Mhz and is tuned to pass IF signals from 17 to 25 mhz. This represents the sum of the actual received frequency that is amplified by V-201 and then applied to the grid of the First Mixer, V-202. The 17.0 Mhz output from the 1st Crystal Oscillator, V-207 is also applied to the cathode of the same tube. The signal represented by that sum is directed to the tracking coils Z-213-1, 213-2 and 213-3 which are kept tuned to the correct frequency within the 17 - 25 mhz IF frequency range by the cam system. This signal is then known as the First Variable IF Frequency.

During reception of frequencies 8 Mhz and below, the composite output of the First Variable IF is routed through S-208 (front), then C-286 and is applied to the grid of the 2nd Mixer, V-203. For reception of frequencies 8 Mhz and above, the amplified recived signal signal from V-201 is routed around the First Mixer by S-206 and applied to the grid of the 2nd Mixer through S-208 and C-286. The output of the 2nd Crystal Oscillator, V-401 is also applied to the cathode of V-203. The resultant output is the 2nd Variable IF Frequency which is always between
2 and 3 Mhz. That difference is applied to the tracking coils Z-216-1, 216-2 and 216-3 which are also kept tuned to the correct frequency within the 2-3 Mhz IF frequency range by the cam system.

The 2nd Variable IF frequency is applied to the grid of the 3rd mixer, V-204 where it is mixed with the output of the PTO (V-701) applied to
pin 7. The PTO tunes from 3.455 to 2.455 Mhz . The difference frequency between the 2nd Variable IF Frequency and the output of the PTO is always 455kc. This difference frequency is applied to T-208 which is tuned to 455kc. The result is the 3rd IF Frequency which is then routed from the RF deck to the IF sub-chassis through P-213 and P-218.

In analyzing a failure in the variable IF system, it can be seen that a signal related failure in the First Variable IF will result in low gain during reception of only the frequencies below 8 Mhz. A like failure in the 2nd Variable IF or 3rd IF would affect all frequencies.

Here is a quick test to see if the Variable IF stages are generally working.

Initial checks:

First Crystal Oscillator output as measured at pin 7 of V-202: Normal is approx 4V p-p
Second Crystal Oscillator output as measured at pin 7 of V-203: Normal is approx 3.2V p-p
PTO output measured at pin 7 of V-204: Normal is approx 6V p-p

First Variable IF Gain Test:

Tune the receiver to 00 500, AGC OFF, VTVM connected to read voltage at the rear Diode Load point.

Inject a 17.5 Mhz signal at approximately 15uv through an RF coupled probe to E-210. This should result in a reading of approximately -5V at measured at the diode load point. This bypasses the coils Z-216-1, 216-2 & 216-3 and can be used as a reference.

Inject a 17.5 Mhz signal at approximately 40uv through an RF coupled probe to pin 1 of V-202. This should yield approx -3.5V at the diode load point.

Discussion: An indication at the diode load point of approximately 1.5V of loss is nominal. If there is loss which cannot be corrected by proper alignment of coils Z-216-1, 216-2 and 216-3 or no signal at all, change the injection point coil to coil to moving "away" from the 2nd mixer to find the failure.

Second Variable IF Gain Test

Tune the receiver to 01 000+, AGC OFF, VTVM connected to read voltage at the rear Diode Load point.

Inject a 2.0 Mhz signal at approximately 15uv through an RF coupled probe to pin 1 of V203. This should result in a reading of approximately -2.2V at measured at the diode load point. Again, this bypasses the coils Z213-1, 213-2 & 213-3 and can be used as a reference.

Inject the 2.0 Mhz signal at approximately 15uv through an RF coupled probe to pin 6 of V-204. This should yield approx -4.2V at the diode load point.

Discussion: An indication at the diode load point of an increase of approximately 2V is nominal. Again, If there is loss which cannot be corrected by proper alignment of coils Z-213-1, 213-2 and 213-3 or no signal at all, change the injection point coil to coil to moving "away" from the 2nd mixer to find the failure.

To align the variable IF stages, I use a different method from that found in the various military manuals. My technique yields better results by taking a systems approach and also not loading down the stage to be aligned. Its a bit lengthy to explain here but that procedure, along with many other techniques are demonstrated in the 7 hour long, R390A video tape series
available from Hi-Res Communications. Don't forget to ask about the new 4 hour addendum to the 7 hour series..

Page prepared by R. Charles Rippel

E-mail to: Charles Rippel
URL: http://www.R390A.com
Revision: 04 December 24, 2001
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