![]() TRIAC can control both positive and negative half cycles of AC signal input. TRIAC can operate in four different modes. ![]() TRIAC function by either positive or negative gate control voltage. The idea is that you switch it on some way through each mains half-cycle, and at the zero crossing, it switches itself off. TRIAC can conduct in both directions it can thus be described as a bidirectional device. I've had a look at the circuit, briefly, there is is useful block diagram also, this part is 'AC control'it is a thyristor driven transformer, I don't know how familiar you are with thyristors, basically they are an arrangement of transistors exactly like Q402 Q403, and when turned on, they latch on until power is removed.Actually TR401 is a triac, which is just a pair of thyristors in antiparallel. You should read less than a volt.Is this thing blowing fuses? Triacs are notorious for asymmetric firing which gives small DC offset in their output which will overheat an inductive load and fry it.That effect gets worse with temperature so warm the triac with your heat gun and watch for DC. I havent studied that schematic in any detail.Seems you could unsolder the primary wires to transformer, lift R408 and rig a lamp test as in that sketch.If you do that, measure voltage across the lamp with DMM set for DC. In circuit testI'm not sure i'd force that triac full on, something might be turning it off because it detects a fault.
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