Vegas 400 Loud Hum Then Blows Fuse

[Mark Kuban]

Bought a basically nonworking Vegas 400 for cheap off of craigslist. What it does is make a loud hum for 30 seconds or so and then it blows the power fuse. Any ideas where to start troubleshooting?

[Lane Gray]

Anything that will pass enough current to blow a 6A fuse WILL be hot. I'd pull the chassis, give it a fresh fuse (what a shame they don't have circuit breakers in the AGC form), turn it on and wave your hand just above it after it blows again. The bit that feels hot is probably the culprit.

I'd also be inclined to look first for signs of heat damage, then at filter caps and maybe put an ohmmeter on the coils of the transformer.

Again, I'm not a tech, just a cat with some logic.
EDIT: That logic leads me to suspect something in the power supply rather than the preamp or power amp section. The amps run on lower voltages, so would need INSANE current draws to suck 6A of house current.

[Marvin Born]

Check for a shorted output transistor or driver transistor. Then check ESR of the filter caps.

[Mark Kuban]

Thanks! Will attempt to find those things mentioned. Not much experience working on solid state amps, just tube amps.

I if have too much trouble, I guess I'll take it to a tech.

[John Billings]

Maybe get some Component Cooler, if you can find a Radio Shack that's still open. Handy stuff! Better than waving your hand around. Remove the chassis, spray the cooler, and turn it on. Anything overheating will show right away. At least I think that's the right method! I haven';t used my can in 20 years, so maybe someone else can comment on it's use? And if it's applicable in this case? I think it will show parts that have heat issues, but maybe not shorts or opens?

[Stephen Cowell]

If it was an output shorted it should take the fuse out immediately no questions asked (and make less noise). You probably have a blown rectifier in your power supply, I'd start there... next would be filter caps.