Distracted by an equipment repair; a ZLA Controller missed an incorrect altitude assignment read back that resulted in an airspace entry infraction.

Narrative

Restricted Area 2501 was released FL270 and above. I had one Heavy Air Carrier on frequency at FL270 over the restricted area. Air Carrier X; also a heavy; checked on at FL270 over the restricted area. Both airplanes were landing LAX on the RIIVR arrival. At the same time; the URET slew ball was being replaced by tech-ops and there was a bit of distraction in the immediate area of my sector. When the first Heavy cleared the restricted area; I gave him a clearance to cross GRAMM at and maintain FL190. Apparently Air Carrier X read back the clearance and started descending. I did not hear this and thought it was the first Heavy that read back the clearance. Within a few seconds; I noticed that Air Carrier X was below FL270 and descending into the restricted area. I told Air Carrier X to maintain FL270; and then asked him when he got descent. He said he was climbing back up and that I had given him GRAMM at FL190. Recommendation; listen more carefully when equipment is being repaired or when the sector is closed or moved to another position.