TV/VCR/Stereo Troubleshooting/VSX-D508 Pioneer

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QUESTION: Model VSX-D508 Pioneer receiver that flashes power off after being on about 5 seconds. Changed out all three audio ic's with factory origonals, still happens. Any Ideas ?

Nick

ANSWER: Nice old receiver.  Worth struggling with to get it rolling again.

Most common causes of protection mode are: 1) audio power transistors/FET have overheated and shorted down, 2) power supply section feeding the power amp section fails, 3) one or more of the loudspeakers or wires going to speaker is shorted.

Troubleshooting steps:

ONE:  Disconnect all speakers at the back of the receiver; make sure volume is down to zero. If receiver does not go into protection when speakers are disconnected you must check for a bad speaker or wire.  Check them carefully.  A strand of wire jumping over terminals can cause it.  One speaker with shorted voice coil can do it.  If amp goes into protection mode now with all speakers disconnected it means the receiver has internal failure.

TWO:  Since you have replaced the ICs it may be a defective IC (new ones sometimes fail or are improperly installed; did you do it or a service tech do it?).  I don't know if this receiver has individual transistors mounted on heat sinks or a power ic module for each channel, but if separate transistors/FETs at the output you may have one or more channels that need replaced.  The cause of failure can be dried out heat grease between the transistor and heat sink - which happens when rcver is old.  The heat eventually dries out the heat grease and allows the temperature of the transistors to go too high and short out.  

THREE: In the power supply the first components that go are the big electrolytic capacitors in the ripple filter section that feeds the audio power amplifiers section.  Often you can observe bloating or electrolyte oozing out of the cap.  If you have a voltmeter you should watch the voltage build up when you turn on the amplifier.  If it builds slowly and then it goes into protection mode then it may be the caps need replacing.  Not expensive, but lots of labor taking the old ones out and putting in new ones (and you'll probably need alternate replacement parts as finding exact mounting may be difficult causing it necessary to find some that have the right voltage rating and capacitance but wrong shape to mount which may require some creative way to mount them.)

Hope this helps you find the solution.  Let me know if I can help further.



---------- FOLLOW-UP ----------

QUESTION: Thanks so much for the help, however having done all of these things...we still have the same problem. Turn on looks OK for 5 seconds, then goes into "power off" again. Double checked soldering (many years experience), replaced three IC's with factory parts... as well as both FETs. STILL same problem. HELP ? NEVER even hear click in speakers ( Our known good speakers hooked correctly)

Thanks
Nick Swan

Answer
Step ONE was to determine if the cause was internal or external to the receiver.  Did you perform that check - with all speaker wires disconnected from the rear of the set?  If you still get receiver shut down then it is an internal matter - but could be both; if a speaker is shorted it could be the cause of something failing inside the set. (Even a 'good' speaker can be bad under certain drive conditions. If there is dc at the output terminal it could be a sign of amp trouble that is causing the cone/voice coil to deflect the cone to where there is a short with the pole piece of the speaker.)

And, in step THREE did you test or replace the electrolytic caps in the power supply?

Replacing the FETs and ICs will give no solution if they are not the problem.

Do you have a voltmeter where you can check internal voltages?  For example, at the power amp stage if you put the meter on the main B+ line feeding the power outputs you can see what happens when you switch the receiver ON.  You said it quits in about 5 seconds; so work backwards and try to determine where it is failing; if you clip the main lines feeding the power output section and it still goes down, then look at the power supply.  Clip the connection to the filter cap(s) to see if they are sucking down the B+ to the rest of the amplifier.

That is how I'd go about it.  This requires troubleshooting at the component level.

And, if you don't trust an IC you can put the scope on the inputs and outputs to see if signal is getting through.  

Keep struggling; I have an inclination you'll find it this time.  

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