This application note discusses a modification of the 40-BIT PORT (p/n INN-8484) which creates a "silicon fuse" - an easily replaceable IC which protects the rest of the circuitry in case of electrostatic "zap" to the SIMM socket. Recent experience indicated that the IC located in U19 is the one which seems to be more likely to be zapped in such occasions. Due to our long original heat sink, U19 has been soldered directly to the 40-BIT PORT board, and thus requires desoldering equipment for replacement.
This application note outlines a procedure that will provide a socket for U19 and therefore making U19 an easily replaceable "SILICON FUSE". To achieve that, the original long heat sink of the 40-BIT PORT regulator should be replaced by a shorter arrangement. While other ICs may potentially be damaged due to static, we believe that the new "SILICON FUSE" will be a fast remedy for most problems. Therefore, this modification is now a standard feature in all 40-BIT PORT units manufactured since May 1995.
The procedure requires desoldering equipment, as U19 should be removed from the board. We, therefore, recommend doing it only on units that have been returned by customers (after all, the overall failure rate of U19 is still acceptably low). INNOVENTIONS performs this modification on every unit sent for factory repair.
The kit of parts for creating the "silicon fuse" has been assigned p/n INN-8484- KIT2 and is available from INNOVENTIONS. It includes a 20-pin socket, 2 replacement U19 chips, and a set of the new heat sink arrangement.
1. Remove the 40-BIT PORT board from its case by removing the 2 socket screws and the eight mounting screws. Please make a note where the various nuts and washers are used for the final assembly.
2. Fig. 1 shows the area of the board around U19. Remove the hardware of the heat-sink and take it out (it will be replaced by the new heat-sink arrangement).
3. Locate and de-solder U19. Solder the 20-pin socket at U19 with the IC tab pointing to the left, like the other ICs on the board. Insert a fresh U19 IC in the socket.
4. Fig. 2 shows the arrangement of the new heat-sink. It comprises two heat sinks, one is placed under the regulator, the other on top. Please include the spacer and all the washers as shown in the drawing.
5. Assemble the 40-BIT PORT in the case by reversing step 1.
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