I have a Kenwood VR-407 that keeps going into protection (turns off & power light blinks). The owner stated that it was a sudden occurrence, no lightning strikes, shorted wires, etc... This unit uses two amplifier modules, one STK413-020A for front & center channels, and a STK412-020A for the surrounds. I removed both modules, the unit powered up no protection. I put in the front module, the unit powered up no protection. I removed the front & put in the rear, the unit went into protection. I assumed the rear module was blown, purchased the new one for ~$30.00, put it in, & the unit still goes into protection. I don't have the service manual on this unit, but from what I can tell, each module uses the same four power supplies. It looks like there's a =B165V & a =B135V. Both supplies come up w/ no ripple before the unit goes into protection. I wasn't able to get any sort of quantitative measurements from the output terminals (at the amplifier outputs, of course the rear panel terminals are cut off because of the speaker relays), but I can safely say that none of the output terminals are shorted to any supply or ground. I checked all thru hole semi's for a nominal pn drop, nothing shorted out there, which brings up another point.
How does everyone check transistors/diodes in-circuit? I've heard many times that just checking the pn junction voltage isn't enough, and I'll admit I've had a few that had a proper voltage but were actually bad, but for the most part shouldn't it suffice?
I'm sure this will probably open up a battle between greats, but I'd love to hear from everyone's experiences on this matter.
Thanks, Steve