Kenwood VR-407 protection problem

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

Reply to
sck0006
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Look at the output of each of the rear channels with a scope at the moment the protection circuit "should" unlock and let the unit play. Maybe a large DC spike at that moment. If so, you may well have a bad muting transistor. Used to be a fairly common problem on many brands, though I haven't seen one just lately...

Muting transistors are somewhat special and subs generally won't work. They are specially doped so that the large negative bias applied to turn them off won't damage the base-emitter junction. Sometimes digital transistors are used for the purpose.

It is possible to defeat the power relay on many models, and bring the unit up on a variac. This will usually allow you to check internal voltages, regulated supplies, offsets, etc without regard to protection status.

If the unit has 2 switched outlets, this is easy. Bring the unit's own power cord to one of it's own switched outlets, and use a double-ended POLARIZED cord to connect the other switched outlet to your variac (if you have one...) or a switched outlet strip. Make sure the polarity of the AC cord has not been compromised or defeated - that hot and neutral don't get reversed in the process, or there will be major sparks!

You're going to need a service manual if you don't already have one...

Mark Z.

Reply to
Mark D. Zacharias

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I think you're right, I need the service manual. I was trying to keep this as a budget repair, hopefully the manual will shed some insight.

I didn't see any spikes, dc voltage jumps to maybe -10mV, but that could just be from the amp shutting down, I'm not sure. Unfortunately one of my scopes is on the fritz again so I should probably tend to it before I do any further probing. Some of the best advice my dad gave me is "You have to have the right tools for the job & a clean workspace."

Stee

Reply to
sck0006

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