Modifying the MVP SAS Board for full duplex

Modifying the MVP SAS Board
for full duplex operation.
By Kevin Custer W3KKC

Concept: Modification of the System-Audio-Squelch or "SAS Board" is necessary to allow a GE MVP Mobile radio operate full duplex to become a repeater. Modification is accomplished by performing several steps below. a few high resolution pictures are included in these steps to guide the builder and help insure the modification is performed correctly. These steps are not for the inexperienced builder.

Modification Procedure:

Remove the SAS board from the radio set. These boards vary slightly from revision to revision but are fundamentally the same. Remove the two screws from the Audio IC that secure it to the heatsink. Be careful as there are small "shake proof" washers under the heads of the screws. Next, remove the screw that secures the TO-220 transistor. Be Careful of the insulators above and below the transistor, they must be reinstalled later. Now, make a note of the connections you are about to disconnect. The front panel assembly can be removed and set aside by unplugging the cabling.

It is necessary to activate the receiver circuits continuously by applying power to the receive oscillator. This is accomplished by shorting between J904 pin 1 and 2 on the solder side of the board. I used a resistor lead clipping to make the jump. This is pictured in the first image below.
Now we need to defeat the oscillator control circuitry. This step requires cutting a trace on the SAS board so be careful. This step is accomplished by finding and cutting the PC board trace that connects J904 pin 2 to the hybrid IC U902 (long narrow black chip) pin 7. For reference, the pin numbers are molded on the top side of this chip at each end beyond the stamped on part number. This trace is cut in the short area between the IC's pin and the plated through hole on the solder side of the PC board and is pictured in the first image below.

For obvious reasons, it is necessary to disable the receiver muting circuitry. This is done at by severing a trace that connects to U902 (long black chip) pin 6, but this time will be cut on the parts side of the board near pin 7 of J905. This step is accomplished by finding and cutting the circuit trace that is in the form of a large "S". Cut the trace beside H11 as shown below.

Proceed onto Step 5 of the main conversion page.