Pioneer sx 650

SOURCE: Pioneer QX-949

From Charles (owning a QX 949, still working but with some investment into repairs by myself) :
QX 949 contains a "power protection cirquit". This cirquit reacts aswell on short-cirquit (either within the output-amplifier cirquit {transistors} or by faulty speakers) as on overload (by playing musik toooo loud).
Your QX 949 might therefore either be affected by short-cirquit within one of the four output amplifiers or there might be a problem within the pre-amplifier or power section, causing an AC or DC signal to appear at the inputs of the output-amplifier section and therefore overloading the output, as if music was played too loud. First basic test: does the discribed "tick sound" (which means that after power-up the protection cirquit releases the relais again) also appear when the volume is turned down completely ? If not, the fault must be a signal produced within the pre-amplifier section before the volume regulation. If yes there still could be a pre-amp problem after the volume regulation but it´s more likely to be found within the output-amp section. One won´t get round to measure this section (oscilloscope) at it's signal inputs to exclude a faulty signal to arrive there. If this is not the case I would recommend to disconnect and reconnect the four output-amps from the power supply one after the other, for finding which one causes a short-cirquit. But before doing this: try out what happens if you change QX 949 into 2-channel mode by turning the socket on the rear panel. Doing this the output amps for the rear channels are deactivated. If your QX 949 was working under these conditions it would limit your further search to the amps for the rear channels.
There is also a small chance that the power-protection cirquit as such is faulty or that the power supply contains problems. The power supply has got 6 fuses that you will find underneath the bottom plate. Are they all still ok ? (If one of them is broken then there can be a positive or negative supply to the output-amps as these are receiving + and - DC-currents from the power supply)
Basically: this amplifier is almost about 30 years old. As the lifetime of electrolytic capacitors ends after about 15-25 years there appears the need to replace them all (on all the prints). I did it with my QX 949... but you definitely must love yours to do that, it's quite an investment of patience and time.
And: servicemanuals.net display on their webpage availability of QX 949´s service manual. If you can't find it I might be able to scan some pages.
Charles (Switzerland)

SOURCE: SX-950 Stereo Pioneer Receiver

The relay is a protection device for the speakers. It will shut off the speakers when problems are detected internally. I would suspect that some of the capacitors in the front end are in need of replacing. This is a common item for units this old. I have restored a number of these for customers over the years with much success. Figure about $15-$20 in parts for this repair plus the local labor rate. This unit is definately worth a refurb. What happens is that there is dsome DC voltage being leaked into the final amplification stage. This triggers the protection circuit to shut off the speakers. DC voltage on a speaker will heat up the coil and eventually destroy the speaker.
Dan

SOURCE: I have a Pioneer VSX-D411

Circuit problems -- take to pro/tech for check put

SOURCE: Pioneer SX-750 Old School Need Help

Check the speaker wire. Sometimes it can just be loose in the terminals and cause distortion. I have had the problem before.

Cheers!
DellMan94

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