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APC Alarm System from HL:A in The Black Mesa Facility.

It is really loud turn it down! Also when the resonance cascade happens activate the alarm. Cool right! I don't know if the addon's work but I have the Half-Life: Source Content packs which don't... exist anymore??? So if everything is an error and purple textures check the workshop for Half-Life: Source Content like textures or models and try that. If you want models to appear use the Half-Life: Co-Op game mode. One more thing: Also it will keep the models loaded after being mounted in Half-Life: Co-Op. So if you go to sandbox and load the save the models will appear I think. I remember that works for me.