Opening an encapsulated submodule for repair

This section shows an example of how to open an encapsulated submodule. The earliest ARP submodules, mainly those found in early model 2500 and 2600 synthesisers, were completely filled with hard potting compound which cannot be removed without the use of special solvents. The instructions given here are useful for work on submodules that were manufactured using the two-layer encapsulation scheme, see the introduction to submodules.

The submodule viewed from the pin side. Notice the manufacturing date code, 25-74 (apparently meaning week 25 of the year 1974), carved in the potting material.

Here, one side of the plastic shell has been sawed off using a metal saw. It is advisable to saw as close to the edge as possible, using a thin blade so as not to damage any components or the circuit board.

The plastic enclosure can now be lifted off. In case of difficulty, saw off another side of the shell.

Removing the thin layer of silicone rubber that lay against the large surface of the plastic shell reveals the solder side of the circuit board.

Shown here is the module with some of the silicone rubber removed along the edges. The hard epoxy layer can now be crackeled into several small sections using side cutters.

Care should be taken when crackling the epoxy so as not to damage the protruding mounting pins. Make several cuts close to each pin. Beware of epoxy pieces flying across the room, though.

In some cases areas of the epoxy layer may seem to be stuck to the silicone layer. This can be the result of tall components having made contact with the epoxy layer. Carefully peel off the epoxy without using excessive force.

The most laborious phase is removing the silicone rubber around the components. A sharp surgical knife may be used to cut and scrape off large pieces, and a needle to access tighter places.

The finished module. Notice that most of the semiconductor components in this module have gone through some kind of selection process (indicated by paint spots).

The mounting pins survived the process, even if they got a little bent. The module is now ready for fault-finding and repair.

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