piclist-bounces@mit.edu wrote:
> Other possibilities: Your transformer could be shorted. What
> are you going to do then?

Put some sort of current-limit in series with the AC input, then you can check T1

As already mentioned by others, I'd be suspicious of Q6, which is shown as 4 NPN in parallel. All it would take is for one Q to short, then your AC fuse pops. If on the other hand one goes O/C that causes more stress @ full load on the remaining Qs, possibly leading to another S/C. I'm not certain how equally they share the current, nor how well they are heatsunk. I'm sure there's a significant power-derating based on temperature. The area around Q6 has a dashed line on the schematic labeled PCB-3, which has some timkering possibilities if its truly a separate pcb. Would be a good place to add the fuse, since the regulation now happens after the fuse instead of before. Does it already have a fan? If not adding one might help with heat dissipation (and therefore life of the transistors)

Someone mentioned the output diode. It's drawn as a zener, but the BoM calls out a 1N5401. guessing crude reverse polarity protection, but a 3A diode won't blow the 5A fuse on the AC side. It might short itself out, though :)

But i just glanced at it, so maybe i overlooked some critical details.

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