Question about Kenwood VR-407 Receiver
A lightning strike took out a nearby transformer and now my kenwood amp will blow one of the internal 7 amp fuses if the power is turn on to it. The local shop took a look and said that this amp has very strange circuitry and they couldn't help me. Another shop said that the output transistors were all blown but did not know where to get parts. I found a source but they are about $50 apiece and there are 8 of them. It still sounds odd to me that all 8 would be damaged as I believe that this amp has two rails of 4 transistors each, with 4 of them coming on line only at very high settings. The amp was off at the time of the lightning strike. There is no physical damage apparent to the transistors or the heat sinks that they are mounted to. Any ideas? Thanks ratmoul
You can check the output transistors for short circuits with the low ohms range or diode checker . If they have failed, then it is often a good idea to replace all as much stress can be placed on an output array in this type of failure. to repair reliable, all outputs on an offending channel should be replaced.
Make sure the fuse you replaced was the same type of fuse. check markings.... T stands for sloblo... a fast blo duse may let go at turn on. The power supply may have a shorted rectifier diode in it also.
be aware however, every time a fuse of that rating blows... more damage may be occurring to the amp.
Posted on Dec 15, 2007
It might be easier for a novice to find the emitter resistors (white rectangular ceramic with three pins exiting) there are four of them
each is a pair of resistors @0.22 ohm each side.
Connect a multimeter to the center pin, and each outer pin in turn,
if you find an open circuit in any of these then the respective output transistor has failed (shorted to the power rail + or - depending on whether its a NPN (positive rail) or PNP (negative rail) device that`s failed. As explained above, you will probably need to have the low power pre-drivers replaced as well as this amp is DC coupled,
I have found current limiting resistors in the signal path tend to protect the first and second stages from damage, including the input differential pair.
Posted on Jan 30, 2014
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