NUE-PSK Digital Modem Power Supply Modification A digital modem for PSK31 field operation ... without using a PC!

The Problem

Shortly after going to production status and starting to ship the modems, we discovered a couple of errors in the "power supply" area of the pc board.  The unusual arrangement we use with two internal 9V batteries (yielding 18V) was conflicting with an external power supply when connected to the J1 power connector.  Current would flow from the batteries back to the nominally lower voltage external supply, causing the batteries to heat up and fail in pretty short order. 

One solution to this condition is to remove the batteries whenever an external power supply is being used.  But a better solution is to make a few simple modifications on the pc board, as described and illustrated below.

The Solution

In general, what we shall guide you through is a way to change the wiring around for the battery leads in order to utilize the normally-closed contact already present in J1.  When no external power is being applied to J1, the normally-closed switch in the jack completes the batteries' negative connection to ground.  But when an external supply is plugged into J1, that internal switch opens and the batteries are electrically  removed from the circuit.  [Click any photo below to see a full-res view.)

Photo A Backside of pc board with red wire added from J1 (on right) to the GND test point pad (on left).

Photo B Backside of pc board with red jumper going to the GND test point pad (at top). Also shows the TRACE CUT in the middle of the photo.

Photo C Close-up view of the red jumper soldered tot he switch pad for the J1 power connector.

Photo D Top side of pc board showing the upper black batt lead moved down to the adjacent red lead; and the lower black lead moved down to the GND test point pad.

1) Move lower black battery lead over to a nearby test point pad called "GND".  This pad is conveniently unused and floating.

2) Add a wire on the bottom side from that GND test point pad over to the middle/unused pad for the J1 power connector.

3) Move the upper black battery lead down to the lower red battery lead.

4) Cut the trace on the bottom side that connects the two red battery pads. 

Test

1) Connect two 9V batteries to the clips, turn on the modem and observe normal operation.

2) In the Config menu, dial up the Battery Voltage menu and observe about 16V or so.  (Actual voltage depends on how fresh your batteries are.)

3) Plug in an external power supply and your will see the modem reset and restart due to the brief power interruption when the plug was inserted to J1.

4) Once again dial up the Battery Voltage in Config and note the lower voltage of the external supply now being displayed. This tells us that the switch in J1 opened, removing the batteries from the circuit, and allowing the external supply to power the modem.

Button Things Up

1) Disconnect the power supply and batteries.

2) Screw the pc board back onto the standoffs in the lower enclosure shell using the four nickel-plated screws.

3) Place the top enclosure shell on and screw it in place using the four black screws.

4) Remove the battery cover, connect the two 9V batteries, and screw the battery cover back in place.

You can now keep the batteries in the modem all the time, even when the modem is being powered by an external supply.

If you don't wish to make this modification yourself, we'll be happy to do it for you .. just mail the modem to us and we'll turn it around for you right away.  Contact us for details.

If you have any questions, please contact me (my email address is shown below in the footer of this page), or post a message on the NUE-PSK email list in Yahoo Groups.

73, George N2APB

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Page last updated:  March 5, 2008