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SBM-1 7-pin to coax mod instructions

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Brian Skalinder:
Thanks to Wayne Brissette for this info!

On-board 7-pin female plug
Remove the 7 pin connector from the header.

The pinouts of the header you need are:

Pin 5 - Transmit / SPDIF out
Pin 6 - Ground

Looking at the header on the PCB straight on, the pinouts are:

1 2 3 4 5 6 7 8
        X X      

Coax cable pin assignments:

Tip - Transmit  (pin 5)
Shield - Ground  (pin 6)

7-pin male dongle plug
Grey 2 conductor wire (red brown1 wire wrapped with shield)
The red brown1 wire is your Transmit or SPDIF out, the shield you need to strip off.

Grey 1 conductor wire
This is ground.

Also ground.

The rest of the wires on the 7 pin connector you can cut off.  Recommended:  TEST the mod to ensure it's working before lopping off the extraneous wires!2  Just make sure you don't short any of the wires.  I used glue on the tips of the wires I cut off just to ensure they wouldn't short.

Edited by Brian Skalinder:  1updated red/brown per Todd R's comments below, 2added recommended test.

Todd R:

There is a mistake in the directions for modifying the 7-pin dongle.

To attach a coax output onto the 7-pin dongle cable of the SBM-1, you need to use the grey 2-conductor wire that encases the BROWN wire wrapped in shield.  Otherwise, the directions are the same:  brown wire is SPDIF out and goes to the center of the coax connector, outer shield of the coax connector is connected to the shielding wire of the grey 2-conductor.

This is opposite from other applications with the 7-pin cable because the 7-pin cable is typically connected to the DAT deck, not an outboard piece of equipment.  So typically, grey 2-conductor with internal red wire is for transmit, grey 2-conductor with internal brown wire is for receive.  But transmit/receive is from the deck's perspective.  You want to transmit a signal from the SBM-1, but the signal is really to be received from the DAT deck.  So in the case of modifying the 7-pin cable of the SBM-1, you want to use the brown wire that is for receiving a signal (at the DAT deck), and is connected to pin 5 of the 7-pin port.

Explanation may be confusing, but trust me, it's the grey/brown two-conductor wire you need to use.  


Todd R:
Info on a coax jack mod pulled from another thread:

(Pictures of this coax mod available in this thread:;action=display;threadid=11892  )

--- Quote from: nickgregory on April 02, 2004, 01:38:19 PM ---how difficult is the coax on the unit mod vs the one where oade just modifies the pigtail?

--- End quote ---

It really isn't too difficult to do, just tedious, and requires working in very tight quarters.  I put the one in for Tim next to the existing RCA analog input jacks.  There is enough room in there to put in another RCA jack for the coax out, but not very much.

You'll need to get an RCA jack with a nut for securing it to the SBM1 chassis.  (There really will be so little room that using the nut will be difficult--I tried doing it just by screwing the jack into the plastic of the SBM wall, but Simpy proved that wasn't the most robust design.)  You'll have to drill out a hole for the jack in the side of the SBM1.  There will be very little margin for error as to where exactly the jack will need to go, so plan accordingly, including making sure there will be room for the securing nut above the circuit board.  Then disassemble the whole SBM1 to get the circuit guts out of the case, so you can drill a hole into the case to fit the jack (you'll need the right diameter drill bit).  To get it completely apart, you'll need to take off the volume level knobs.  To do this you need to take off the rubber gasket that goes around the knob to help provide grip.  Underneath the rubber gasket is a screw that will release the knob from the post.  You might be able to do the drilling without pulling everything apart, but be very careful the drill bit doesn't rip into the circuit board if you choose to do it this way.

Then you'll need to put guts of the unit back in place.  Unfortunately with this mod, the coax jack will need to be the last thing put into the SBM1 case.  If you need to pull out the circuit board to do any kind of service or modification of the line input stages or whatever, you'll need to pull apart the soldering of the coax jack and remove it.

With everything back in place, there will be very little room to work (have I mentioned this before?  starting to get the picture?).  First, secure the RCA jack into the hole you drilled with the nut.  Now all you have to do is make the solder connections.  You just need to follow previous directions on replacing the 7pin tail with a coax.  The SBM uses two circuit boards sandwiched together, which seemed too difficult to want to pull apart.  Unfortunately, the points on the board where the 7pin tail connects are between these two boards and you can't get at them without pulling everything apart.  So I found it easier to hack down the 7pin tail to just an inch or so, and use the existing wires to make the connections.  Find the brown wire with the ground shield wires in the wire bundle.  The ground shield wires go to the outer ground portion of the RCA jack.  The brown wire gets connected to the inner portion of the RCA jack.  Soldering in these tight quarters will be difficult. 

You will probably need to rotate the jack around to get at the tab for the ground connection, make the solder connection, and then tighten down the RCA jack to rotate the jack around so that the ground tab is down at the 6 o'clock position and the center tab of the RCA jack is now available to get soldered.  You probably won't be able to turn the securing nut to tighten things, so you will need to turn the jack into the nut to secure things, and when it is almost completely tightened, stop turning so that the ground tab is up.  Make that connection and then tighten it the rest of the way so that the ground tab is down.  Then make the final solder connection of the brown wire to the center.  Then you're done.  Well, after you put it all together.  Be sure when doing this that you get the slider switches on the outer case (for 32k-44k-48k and whatever else) aligned with the actual switches on the circuit board.

It really isn't technically difficult, just very tedious due to the small spaces you need to work in, and time consumed to pull everything apart and put it back together.  You also have to have a steady hand for soldering in the tight spaces required.

Good luck!

thanks to Wayne Brissette and Todd R for the great info above.

photos of the recent coax mod for Simp-Dawg's sbm-1

1. circuit board showing the 7pin dongle > 8pin harness:

2. showing 8pin male harness:

3. desoldered 8pin harness:

4. desoldered 8pin harness #2:

5. pinout for the Tx and ground connections (pin4 is Tx and pin3 is ground):

6. prepared s/pdif cable (using Belden 1855A Brilliance 75ohm video cable):

7. cable soldered and held in place to the battery chassis via zip ties:

8. reassembled SBM-1 (crimped Canare 75ohm s/pdif rca termination):

9. signal test:

10. signal test #2:


Digital Quality:
Does anyone happen to have these photos archived somewhere? Thanks.


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