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Pioneer SX-750 Has Constant Hum - Any Ideas?

I bought a pioneer sx-750 off ebay. It was supposed to be free of any issues but has a constant hum. I have ruled out any obvious grounding issues. Even took it to another house with one speaker and got the hum. It stays at the same level when you increase the volume. The caps look good throughout. Any suggestions are greatly appreciated. I hope to get this back to a listenable unit. For now, it's listenable with enough volume to help mask the hum but it's still noticeable. Thanks!



Re: Pioneer SX-750 Has Constant Hum - Any Ideas?

As with other post on this, bad caps are likely. If it were mine, I would replace all caps in all of the supplies.

Something else to confirm would be the supplies that feed the pre-amp\tone ctrl section. (think your unit is fed from a 21v bus, then may be reduced further on different modules with zeners). If the supply regulator shorts, the supply will be high with lots of ripple on the line. On Zener supplies if the zener opens, the line will be high and can cause hum.

I think your unit also has a split 48v line. These high voltage regulators feed the power amp stage. On the larger 1080 and up series, (not sure on yours), these regulators were not heat sink'd sufficiently and fail, and\or almost always cause the caps in the area to dry out but will look okay physically.

I mod these units by moving these pass xistors to the main plate or add additional heat sinks. These are usually located on the power supply\protection pcb.

If the audio is mostly clear, not distorted, and just has a hum on top of it, power supplies would be the place to look. If audio is very distorted, might have a bad output stage.

Good luck
Chaz

Re: Pioneer PL-12D

Does anyone know how my son can connect the above (my old record deck) to a modern amplifier, which doesn't have the input jacks of my old amp (Sansui AU2200).

Many thanks

karodimitrov's picture

Re: Pioneer PL-12D

What kind is your amp? Inputs should be standard-RCA.Same as the your Pioneer and Sansui.

Re: Pioneer PL-12D

Thanks for this - when I advised to son, he said the issue was not the output connection but the power connection - something I had misunderstood.

I assume therefore that it should simply be a case of cutting off the two pin power connection on the back of the turntable and attaching a 3 pin plug (with a fuse of no more than one amp).

Would this be OK?

karodimitrov's picture

Re: Pioneer PL-12D

Far as I understand you,you want when you turn on the amplifier to be powered and turntable.Or vice versa, when you turn on the turntable - be powered amplifier.Or just powered them separately?
Sorry for my bad English! Google Translator helps hard too.:)

Re: Pioneer PL-12D

Originally the turntable was powered from the old Sansui amp, but having the turntable separately powered will not be a problem; the only question is the best way to do this.

I have now confirmed that the old two jack connectors can simply be cut off and replaced with a standard mains plug with a small fuse, so problem solved.

Many thanks for replying.

Re: Pioneer PL-12D

Originally the turntable was powered from the old Sansui amp, but having the turntable separately powered will not be a problem; the only question is the best way to do this.

karodimitrov's picture

Re: Pioneer PL-12D

Simple electrical power strip with 2 or 3 sockets.Without rebuild the turntable.In one socket switch amplifier,in another-turntable.

karodimitrov's picture

Re: Pioneer PL-12D

PS. Is not it easier to use a simple electrical power strip with 2 or 3 sockets?So will not need to rebuild the turntable.

Re: Pioneer SX-750 Has Constant Hum - Any Ideas?

Put a scope on the power supply filter caps to see if you have high AC ripple riding on either
the B+ or B- lines.

karodimitrov's picture

Re: Pioneer SX-750 Has Constant Hum - Any Ideas?

If you switch to a free AUX (no signal) and again there hum- lternate power capacitors.They are over 35 years old.