Vintage Pioneer SX-750 Receiver Issue

[millman]

Hello everyone!

I've got a Pioneer SX-750 receiver that I bought in 1976 and have been using it in my garage for the past 25 years. It has always worked flawlessly and sounded fantastic. Just recently the left channel has developed a constant static and crackling noise that is present even when the volume is turned all the way down. It will quite often cause the protection circuit relay to trip. When you turn up the volume the static does not increase and gets hidden by the music source. I've blown out the unit with compressed air and gave it a good bath with contact cleaner but the static still exsists. Anybody have any idea's on what the cause could be? I definately want to repair this unit, it sounds fantastic and is very beautiful to look at! Thanks

[HayZee518]

pioneer

check the dual controls on the tone controls. these were all linear taper.
pioneer made use of thin wafer resistance material and berryllium copper wipers.
the volume control varied the input to the first amplifier array inside.
check your grounds at your speaker outputs. a faulty filter capacitor will cause a scratchy sound all the time.
these are the large cans soldered to the circuit board.

[valveradio]

i would look for possible dry joints on the pcb. the fault sounds like a noisy driver transistor or pre-driver to that channel, the only way to find out is to swap the transistors over one by one from the faulty channel to the good one and vice-versa, if the noisy channel changes to the other channel then you know its a transistor fault. output transistors have also been known to fail this way too.

good luck!