[MrFixr55]

I have not done this on a 38-690.  I don't own one but in general, the best place to add the input is right at the volume control.  This would allow for a "line level" signal such as the output of an MP3, Player, CD Player, iPhone, etc. If using a turntable, it must contain either a ceramic pickup or a magnetic pickup with a preamp.  The pic below shows the section of the 38-690 detector and volume control circuit:
   

The blue X shows where to "break into" the circuit.

The second pic shows the modification, in blue.  
   

The switch is shown in the radio position. The dotted blue line is the switch in the external audio position.

Use a SPST switch.  The common connects to Cap 73, one leg connects to the junction of 70D, 71 and 72.  The other leg connects to the tip of the RCA jack or other connector.  The outside ring of the jack should be grounded to the same ground as the volume control.  If the radio can be heard at low volume when the aux input is used, tune the radio to quiet place.  A somewhat fancier technique is to use a DPDT switch and add connections to ground the junction 70D, 71 and 72 to prevent radio signal bleed into the aux signal.

The above instructions are basic to adding a jack to any radio, only the part number designations change.  The biggest issue is to not excessively "molest" the radio, as this represents the Zenith (pardon the pun) of Philco technology.  This radio can be seen as a major competitor to the Zenith "Stratosphere", and I believe that these sets are worth quite a bit of money.  Consider putting the components into a metal box that can be screwed into existing chassis holes to minimize any drilling.  Use separate shielded cables for the radio to switch and switch to Audio in.

Judging by the circuit, any MP3 or other aux signal would make this thing ROCK.  I can just picture listening to Big Band music piped directly into this radio from an Aux Input.

Hope this helps.