First of all, I am assuming that you have access to service info on this radio from Riders or N.M. Beitman manuals if not an actual RCA Victor service manual (they are out there). Any of these would come with a schematic diagram and alignment information. I get my service info on older radios from Nostalgia Air online resources. They have scanned in all the Riders manuals.
There are a number of areas at the front end of the receiver that could be causing the Broadcast Band to be inoperative. Since the Short Wave band is functioning you can rule out the 12SA7 converter tube itself as faulty as well as the 12SK7 IF stage and the IF transformers and all the stages that follow.
From the schematic diagram and alignment info I can only say that this is a (2) band radio covering the medium wave Broadcast Band (AM) and one Short Wave band. I checked to see if it has a phono input jack but I didn't see one. The antenna input terminals need a closer look on this radio. There is a link to close on the antenna terminals if you are going to use the internal loop antenna only. You might check on that. More to the point on an inoperative band is to check if the oscillator stage is functioning. You check for that by seeing if you can pick up the oscillator stage signal on another receiver. The oscillator should tune from about 990 kHz to 2055 kHz on the Broadcast Band for an older radio with a 455 kHz IF frequency.
Another way to check to see that the oscillator stage is indeed oscillating is to use a high input impedance voltmeter and measure right at the control grid G1 (pin 5) of the oscillator stage to see if there is a least a few volts negative present. This indicates oscillation. If you don't have such a meter, insert a 5 Meg Ohm resistor in series with the negative lead of your DMM and use this as a probe to check for the voltage. Ground the positive lead of you meter to B- which appears to be the chassis ground on this radio. You can reverse the test leads connections if you like in which case the meter will show the voltage as being negative if any is present. By the way, you should be using an isolation transformer when you work on any AC/DC set like this.
If you have picked up the oscillator signal on another radio then you know the oscillator stage is functioning. If not then you need to focus on that area of the set such as checking the oscillator coil and possibly substitute another coil for it. A replacement coil will be way out of tune at first and will require a careful adjustment to get it to where it needs to be (aligned) to bring in stations at the proper points on the dial. Another point to check is the bandswitch. They can give a lot of trouble if there is any corrosion present on the various contacts-enough to make the set inoperative. Spray cleaners may not be able to remove this corrosion. In some cases it has to be scrapped off carefully with a small screwdriver. If the oscillator stage is functioning there could still be a problem between the antenna coil, the bandswitch and the connections to the converter stage's signal grid
#G3, pin 8.
As far as doing the RF alignment part goes, lacking a signal generator, you could hunt for (2) broadcast stations in your area at around 600 kHz and 1400 kHZ to use as signals to do the required alignment at the low end and the upper end of the Broadcast Band.
Gotta go for now. Hope this gives you something to go on.
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March 10th 2019, 4:05 pm