Pioneer QX-8000

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m1keC

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I have just started to restore a Pioneer QX-8000 quadraphonic receiver and started out by trying the different modes. It quickly became apparent that when in stereo mode that there was hardly any sound from the left channel. After trying a few other settings I then switches the receiver off and rather than the sound dying away it came on properly on the left channel as well as the right for a few seconds!
I'm suspecting that there must be a faulty capacitor somewhere - am I right in this diagnosis> I have checked the 4 2200uF caps on the output of the 4 amplifier sections and they all test a little higher than spec with low esr.
Any suggestions on where to start looking next?
 

par4ken

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Or maybe a transistor?
Most likely a capacitor. Ideally they all should be replaced. Often by visually inspecting the boards you can see a capacitor that looks swollen or has gunk oozing out of it, that will be the culprit. Check "HiFi Engine" to see if they have a service manual that you can download.
 

m1keC

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I have downloaded the service manual. On a visual examination all the caps look ok, no bulging or leaks. I intend to recap but would like to resolve this problem first. I guess when I power off the voltage goes down - that made me think it was possibly a faulty transistor that breaks down at working voltage. Next job is to scope the outputs of the 4 amplifiers to see if the problem is on just one of them. Do you still think its a capacitor causing the problem? It was my first thought but I can't see how the amp would work properly with decreased voltage then.
 

par4ken

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Pure speculation but perhaps the bad capacitor sort of works as the voltage drops. One advantage of working on a stereo unit is that you can compare the working channel to the non-working one to help narrow down the problem. Feed a signal in and trace it through comparing the good side to the bad one, at some point you will get to the trouble area. Yes I would try to find the problem before recapping.
 

m1keC

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After a bit more checking..... there are 2 amp boards each with 2 channels. One has front & rear left, the other front & rear right. With the selector in 4 channel mode both left hand channels are down so it seems that both amplifiers on the same board are faulty. And they both burst into life when powering up or down when the voltage is being reduced. I'll look on the circuit diagram to see if there is anything common that could cause it. Obviously the switch is a possibility but that has been de-ox'ed - maybe I will do it again in case....
 

DuncanS

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After a bit more checking..... there are 2 amp boards each with 2 channels. One has front & rear left, the other front & rear right. With the selector in 4 channel mode both left hand channels are down so it seems that both amplifiers on the same board are faulty. And they both burst into life when powering up or down when the voltage is being reduced. I'll look on the circuit diagram to see if there is anything common that could cause it. Obviously the switch is a possibility but that has been de-ox'ed - maybe I will do it again in case....
It feels like a capacitor problem, most probably an electrolytic that is somewhere in the signal chain common to both amps. I've downloaded the schematic from hifiengine and had a look so maybe in the Q Unit (AWM-001) or the Control Amp Unit (AWG-003). There are no common parts on the amplifier board shared between the 2 amps, so I don't think it is that.
 
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