Need help SB104 mic

[JF]

KG7FS wrote:

> Dear Friends.  I inherited a SB104 and power supply.  Everything
 > works fine except there is no mic or mic jack.  Just the blue and
 > black wire for the mic.  I have no manual.  Can anyone tell me how
 > to wire it up to a 1/4 SwitchCraft Drake Shure 444 I have?
 > Of course chassis is ground?
 > Rich KG7FS in Michigan.

You mean the front panel MIC jack is gone from the left front panel,
next to the PHONES jack, and there's just a hole there?

There's a schematic (SB-104.PDF, about 1.5 MB) of the 104A that you
can download from      http://www.qsl.net/k5bcq/sb104/sb104.html
by a hot link about 5 paragraphs down:  "SB-104A Schematic".
   (actual link is   http://www.qsl.net/k5bcq/sb104/SB-104.PDF )

If there's a standard 4-pin female plug on the Shure 444's cable,
put a standard 4-pin male mic panel jack in the 104A's MIC hole.
You'll have to first pull out all the boards and remove the chassis
side panel to get easy access to the area. If you use a Radio Shack
mic jack, it's not quite long enough (because of front panel
thickness) to use both a lockwasher and nut on the back. It would be
better if you could find something like a slightly longer U.S.-made
Amphenol or Switchcraft jack. Otherwise don't use the lockwasher.

Then connect the unshielded blue wire from inside the chassis to
MIC jack pin 3 - it's the PTT line, on which a ground is supplied
via the mic when the mic is plugged in and its PTT button is pushed.
The blue wire should go to pin 8 on the "B" board, the TX
Audio/Regulator board (verify this).

Connect MIC jack pin 2 to a nearby chassis ground.

MIC jack pin 1 connects to the center conductor of the small black
coax that ends up going to pin 16 on the "B" board board (verify).
The coax's shield should go to a nearby chassis ground (which may be
on the PHONES jack).

On the Shure 444 desk mic there's a switch under the base plate
labeled NORMAL-VOX: put it in the NORMAL position in order to
use or test it's PTT button. Before plugging the mic into the
radio, first verify with an ohmmeter that on the mic's 4-pin (male)
plug, pin 3 makes continuity with pin 2 when the mic's PTT button
is pushed. If it doesn't, you'll have to rewire the plug to match
the jack you just put in.
Again, for this test make sure the Shure mic's NORMAL-VOX switch
is in the "NORMAL" position.
You should also open the mic's plug to verify that the audio lead
goes to its pin 1 (standard).

The original SB-104 series had 2-pin rather than 4-pin MIC jack's,
but the 2-pins are hard to find these days. If you want it to be
original you'll have to scrounge around for 2-pin mic jacks and
plugs - the PTT line in that case goes to MIC jack pin 2, and
pin 1 is the mic audio. The mic makes a ground contact through
the plug shell when it's connected to the 104's chassis jack.
The unamplified Shure 444 microphone works very well with the
SB-104A.

73

John F.  KE5ZB

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