R390A's made after 1954 and those with "mod 2" stamped on the IF chassis have a field change installed to the mechanical
filters. Filter input and output trimmer capacitors have been added. The 4 input trimmer capacitors are found by removing the 2" square can on top of the IF chassis using the single nut in the top. The 4 output
trimmer capacitors are located behind 4 holes in the left hand side of the IF deck. The IF chassis will need to be loose but still electrically connected to complete these procedures.
An accurate counter, analog VTVM and a signal generator capable of outputting 455.00 KC, is required for these alignments.
Connect the VTVM to the "Diode Load" bus on the rear and configure it to read a
negative voltage of approximately -7vdc. Set the receiver "FUNCTION" control to "MGC," "BFO" to "OFF" and the "LOCAL GAIN" control to a comfortable level. Lift and tilt
the IF deck resting the front captive (green) screw over the front panel. You should be able to gain access to the mechanical filter trimmer capacitors through the large ventilation holes in the main chassis.
Locate
the cable running from the rear of the IF deck to the "IF OUT" BNC connector in the rear panel, upper left side. Unplug the cable from the IF deck only. Also unplug J-513 and the one next to it. Plug the cable
running from the rear "IF Out" jack into J-513 on the IF deck. Connect the output of your 455.00kc generator.
Mechanical Filter Alignment:
Set the .455kc generator output level to cause the VTVM to read approx- 2.5V.
With the If deck oriented so that the bandwidth control is towards you, set the bandwidth to 2Kc and align 1 of the 4 the top trimmers.
Adjust C-569 which is at 9 o'clock for a peak on the VTVM. Next, align the output trimmer in the left side of the IF deck, labeled C-567
Set the BW to 4KC
Align the top trimmer, C-568 located at 12 o'clock.
Align the side trimmer, C-566 located at rear, bottom.
Set the BW to 8kc
Align the top trimmer, C-570 located at 6 o'clock.
Align the side trimmer, C-565 located at front, top.
Set the BW to 16kc
Align the top trimmer, C-571 located at 3 o'clock
Align the side trimmer, C-564 located at front, bottom.
IF Transformer Alignment-
Select the 16KC filter. Set the generator frequency to .467kc. Note: The generator output should be increased until the VTVM indicates approximately -2vdc. Do not be alarmed if that level is over .1 volts.
Adjust the top slug (secondary) of T-501 for a peak reading on the VTVM
Adjust the bottom slug (primary) of T-502 for a peak reading on the VTVM
Set the generator frequency to .443kc. Note: The generator output should be increased until the VTVM indicates approximately -2vdc. Again, do not be surprised if that level is over .1 volts.
Adjust the bottom slug (primary) of T-501 for a peak reading on the VTVM
Adjust the top slug (secondary) of T-502 for a peak reading on the VTVM
Decrease the generator output and adjust the frequency to .455kc Note: The generator output should be decreased until the VTVM indicates approximately -3vdc.
Select the 4kc filter then peak top and bottom of T-503 only
AGC Alignment-
Verify the generator is still outputting 455.00kc. Set the "FUNCTION" switch to AGC and connect the VTVM between the AGC bus on TB-102 3 & 4 located on the rear panel & ground.
Adjust the generator output for a reading of approximately -5vdc on the VTVM. Peak Z-503.
BFO Alignment-
Verify that generator is still at 455.00kc
Turn on the BFO and exactly zero beat it against the 455.00 generator frequency. Loosen the bristol spline socket on the BFO shaft coupler, verify that you still have exact zero beat. Set the
"BFO Pitch" control to indicate exactly "0." Then, tighten the bristol socket on the non-mar clamp on the BFO shaft coupler
The filters have been aligned to 455 kc. When a station broadcasting in AM is zero beat, the carrier will be in the center of the filter selected.
Re-install the IF chassis in the receiver.
