[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...

[supertech]

got an input jack put in the 38-690. Sounds very good with a cd player but not full volume. Should a preamp of some sort be used?

[David]

How did you combine the 2 channels of the CD player into the input jack? David

[supertech]

I am just using a portable cd player,and a mono cord.

[David]

Might need something like this?      

[supertech]

Great idea...I can probably buy one of these....thanks

[David]

   

Or this.

[MrFixr55]

I'm a little surprised that you do not get full volume from the CD player.  If you put the input jack just before the volume control, there should be plenty of gain for a "line level" input.  Are you using a CD Deck or the headphone output of a portable CD Player?  If you have access to a component preamp, this may be useful. A "Mono" cord? If the 3.5mm plug is only "TS" (tip and shank or tip and shield, then this type of cable will short one channel out. This may lower the sound and damage the CFD Player. Either of the 2 diagrams (placed there while I took a break from typing this) will work. The second one is easier and cheaper to build. In the diagram that uses resistors, values for RMix 1 and RMix 2 of 75-150 Ohm will suffice.

I did this to a Crosley 02CA.  The Customer liked to play jazz guitar and liked the sound.  Of course, he also used this as a radio.  Yes, granted that he could not duplicate Jimi Hendrix or (Lord help us) Led Zeppelin.  The "mod" done to this unit was to make it like the similar 02CQ which had a switchable phono input.  The input was put between the detector and volume control.

I don't have a great home transmitter.  I have a Talking House transmitter and a Vectronics Kit transmitter.  I don't expect full HiFi but I have never been as impressed with AM reception from these transmitters as I have been with direct connect.  My guess is that the relatively low Broadcast frequency, the low IF frequency and the distortion of a detector subtract from signal quality.