Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

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Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by CaptainClock on Mon Jul 24, 2017 9:44 pm

Hello everyone, I'm currently working on a Pioneer QX-9900 for a friend of mine and it seems to be going in and out of protection mode for some reason, it will do the initial powerup click of the protection relay when I first power it on, but then after a few seconds the relay kicks in and puts the reciever in protection mode, but then after another few seconds the stereo comes out of protection mode, but then in another few seconds it goes back into protection mode and then stays in protection mode. I was thinking that possibly the amplifier coupling capacitors might be bad (because that's what happened with my own personal Pioneer Receiver when I first got it). So could anyone on here tell me if I'm on the right track here or if I'm just way off in my guess?
Any help and tips with this would be appreciated.

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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by Tony V on Mon Jul 24, 2017 10:13 pm

Sounds like your on the right track to me and not an expensive investment at all to check. Make sure you replace all of them as all it takes is one to trip the protection relay.
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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by CaptainClock on Wed Jul 26, 2017 2:09 pm

OK, so I'm guessing that on this unit the Amplifier Coupling Caps are under the chassis? I'm asking because on my Pioneer SX-1000 TD the Amplifier Coupling Caps were on top of the chassis but with the way this QX-9900 is built there isn't any place on top for any amplifier coupling caps so it seems the only other logical place for them would be underneath the receiver chassis. I looked at the service manual but it didn't really say at all whether or not the Amplifier Coupling caps were on top or the bottom of the chassis.

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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by CaptainClock on Wed Jul 26, 2017 7:40 pm

OK so I removed the bottom cover on the Receiver and sure enough the Amplifier coupling caps were indeed on the underside of the receiver, the bad news is that it seems that the wires connecting to the coupling capacitors are wrapped onto the capacitor terminals instead of being soldered onto the terminals, and it seems there's been some work done on this unit before as there seems to be some fuses that were added to the coupling capacitors on one side and else where that don't appear to be original or from the factory and they're fairly large fuses at that, how do I go about figuring out if the coupling capacitors are actually bad or not?

pictures of the capacitors are posted below.





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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by Tony V on Thu Jul 27, 2017 12:32 am

Those fuses are there to protect the output transistors in case one of the cap's short or if someone touches the speaker wires together causing a short thus still protecting the output's. They can also protect the speakers from a voltage spike caused by a short. I would leave them in. Everything else looks pretty much straight forward. The only good way to check capacitors is to have an ESR meter or capacitor checker. No more than they cost i would replace them concidering the age of this receiver along with at least the main filter cap's.
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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by CaptainClock on Thu Jul 27, 2017 6:04 pm

That's what I thought, and interestingly enough they used the infamous Elna Caps in this thing so all the more reason to suspect that the caps are more than likely shot in this unit and why its not working right.
I got my ear out on some nice Nichicon Audio Grade Capacitors as replacements for the original Elnas. So hopefully This Receiver will work for another 40+ years without any issues with the Nichicons in place! Very Happy
As for the Power supply Caps I'm not sure where where they are in the grand scheme of things in this receiver, I know the Power Transformer (which is a huge brick of a transformer) is dead center in the chassis so possibly the power supply caps might be near by?
Also What's the easiest way to remove the old Amplifier Coupling Capacitors? I'm asking because when I tried to remove them while they were still wired in place they wouldn't budge. Also I'm assuming that the Red and Orange Wire Leads are Positive and the Black leads are negative?

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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

Post by CaptainClock on Thu Aug 03, 2017 10:33 pm

Well I got the replacement capacitors in but the only problem is that they are half the size of the originals and because of that they won't fit in the original capacitor's mounting rings so I'm not quite sure how to go about putting in the new capacitors into the stereo receiver without having to modify the chassis. Any ideas?

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Re: Need Help troubleshooting a Pioneer Quadraphonic Receiver Model QX-9900

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