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digitalsasquatch
08-07-2004, 12:00 PM
Howdy folks,
I've got this old Pioneer QX-747 receiver. Quadraphonic, wood, whole 9 yards. Looks good, worked good too last time I turned it off a couple years ago. Put it on a shelf and didn't touch it 'til today. Turned it on and tuned in a station. After a couple seconds the audio cut out. The signal meter seemed to indicate I was still getting a signal, but no sound could be heard. Before I could think to mess with any of the controls, the sound came back on. For a second or two. Then it cut out again. Then on again, then off. Then it stayed off. I tried adjusting all the little pots for the quadraphonic, volume, speaker selector. I tried switching over to quadraphonic mode and using the rear speaker outputs. Still no sound. I popped the case off it and looked around in there. I didn't see anything glaringly obvious like melted semiconductors or whatnot... Any ideas?

nielsvanm
10-05-2004, 08:28 PM
I've gotten a few vintage recievers, and in the beginning (after a few years of no use), they have all had similar symptoms to yours. What I found worked was to take off the cover and blow all the dust out with compressed air, and then hook it up to some speakers and crank it for a bit (half volume or so). Over time all of mine have sort of just worked themselves back into a good working machine, they just needed to warm up a bit.... Quirky little guys though...




Howdy folks,
I've got this old Pioneer QX-747 receiver. Quadraphonic, wood, whole 9 yards. Looks good, worked good too last time I turned it off a couple years ago. Put it on a shelf and didn't touch it 'til today. Turned it on and tuned in a station. After a couple seconds the audio cut out. The signal meter seemed to indicate I was still getting a signal, but no sound could be heard. Before I could think to mess with any of the controls, the sound came back on. For a second or two. Then it cut out again. Then on again, then off. Then it stayed off. I tried adjusting all the little pots for the quadraphonic, volume, speaker selector. I tried switching over to quadraphonic mode and using the rear speaker outputs. Still no sound. I popped the case off it and looked around in there. I didn't see anything glaringly obvious like melted semiconductors or whatnot... Any ideas?

dave_bullet
10-09-2004, 09:59 AM
No idea... but the forum linked below is far more active with lost of solid state specialists - I'm sure someone here will have an idea.

http://www.diyaudio.com/forums/forumdisplay.php?s=&forumid=4

Cheers,
Dave.

Kursun
10-09-2004, 12:07 PM
Howdy folks,
I've got this old Pioneer QX-747 receiver. Quadraphonic, wood, whole 9 yards. Looks good, worked good too last time I turned it off a couple years ago. Put it on a shelf and didn't touch it 'til today. Turned it on and tuned in a station. After a couple seconds the audio cut out. The signal meter seemed to indicate I was still getting a signal, but no sound could be heard. Before I could think to mess with any of the controls, the sound came back on. For a second or two. Then it cut out again. Then on again, then off. Then it stayed off. I tried adjusting all the little pots for the quadraphonic, volume, speaker selector. I tried switching over to quadraphonic mode and using the rear speaker outputs. Still no sound. I popped the case off it and looked around in there. I didn't see anything glaringly obvious like melted semiconductors or whatnot... Any ideas?I think you have a problem with the time delay circuit.
On every amplifier and receiver there is a time delay circuit. Everytime you switch on the component it mutes the loudspeakers for about 5 seconds until the voltages inside various stages stabilise. You'll recognise the action of this relay by a clicking sound just at the time of speakers going into action.

This circuit is usually a very simple one, probably a resistor, a capacitor and a transistor. The capacitor is charged through the resistor, and when it is fully charged it lets the transistor to kick the relay which in turn connects the outputs to the speakers. You may have a faulty relay. This is a electro-mechanical device and may go out of order in time.

CharlieBee
01-15-2005, 03:12 PM
I found the below post and looked at my SX-780 board, found a joint with a dark ring and resoldered several in that area and Olay, it works fine. Maybe you can find the appropriate area on your unit and check out the solder joints.

"Mr. Whyte,

I am writing to you in regard to a Q&A you answered on 3/25/00 on the Classic Audio website regarding a Pioneer SX-780 receiver that intermittently failed to turn on. The person submitting the question reported that the protection relay was failing to kick in and turn on the outputs. I also own that receiver and several months ago it quit working with exactly the same symptom, but it was a solid failure rather than intermittent. I just found time to look into it recently and managed to find my owners manual and a wiring diagram and started to trace things out. What I found was that I apparently had two cracked solder joints on the D712 transistor. I am not an electrical expert but it appeared that this transistor supplied power to the PA3004 module which in turn energized the relay in question. Re-heating the joints on this transistor seems to have solved the problem.

As the Q&A does not have an e-mail for the individual who originally submitted the question, I was hoping you could pass my experience on to him/her in the hope that it might be of assistance. I would encourage them to inspect the solder joints with a magnifying glass.

Please feel free to pass my e-mail onto this person as well. If they wanted a copy of the wiring diagram or manual I would be happy to make one for them.

Sincerely,

Mark Myers"

CharlieBee