[supertech]

Wanting to install an input jack in my 38-690. Has anyone did this on a 690?

[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.

[supertech]

I totally agree with your comments. I may consider an AM  transmitter instead of a hardwired jack.

[MrFixr55]

The aux in jack will sound much better than any AM transmitter.  Besides, this radio would make for a rather mellow guitar amp.  However, remember that this is a very valuable radio and its use should not be limited to being a guitar amp..  Nonetheless, it would be easy to make the mod without actually drilling holes in the chassis or wood cabinet, and could be removed if any new owner objects to it.

RCA made an external audio adaptir sometime in the early or mid 1940s which would be a period-correct "mod"

[radiorich]

Hello guys,
Yes, adding an input to this set would sound amazing I have General Electric model L-916 Console with dual 6V6's in push-pull arangement this set has factory Aux already !
I plan to add one to my Philco 38-10T too.
Sincerely richard

[RGK20m3]

You would need a preamp for a guitar. And I beg to differ, streaming through an AM transmitter sounds fantastic on mine.

[morzh]

mr. Fixr:

One thing I wouldn't use it for is a guitar, unless acoustic only. Guitar amps are designed a tad differently, and clipping and distortion are intentional.


RSK20m3:

This is where I would agree with Mr. Fixr. Yes, AM sounds very good on 38-690, but, at least in theory, putting the same signal through mo-dem conversion vs amplifying directly, as a minimum, does not improve anything.
AM Radios never reached the quality of direct HiFi.
FM came close. But even then...