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still a niggling doubt (ok, hope) that it might be ok
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With the module disconnected, check between the DC input and motor output terminals for evidence that the power transistor is shorted, which would be a common failure mode and the likely reason for the retrofit zeners. If it is indeed shorted, the unit must be U/S but also can possibly be repaired by replacing the transistor, external to the module if more convenient.
Like Tony, I would try to unpot it. If the potting material is hard and cannot be prised out, I would put in in the milling machine and remove the case leaving only what's apparently attached to the transistor. Then it would be easier to chip away around the transistor leads and attach flyleads to a temporary external device, leaving much of the module potted and therefore mechanically intact. Of course, whatever drives the pass transistor may be damaged too, but unlike a full bridge, failure of one transistor does not automatically jeopardise everything around it.