Speaker Hum - Pioneer sx1000TW

[Terry Howard]

Need help to isolate and eliminate speaker hum when connected to Pioneer SX1000TW receiver. Hum is present regardless of source - even when no external source is connected. Hum constant level, volume regardless of volume setting. No interference or ground loop issues I can find. No leaky caps. Thoughts?

 

[cademan]

Capacitors can get electrically leaky and not show any signs of anything being wrong. Usually, volume independant hum is caused by old worn out or dry power supply electrolytic capacitors.

 

[Terry Howard]

Capacitors can get electrically leaky and not show any signs of anything being wrong. Usually, volume independant hum is caused by old worn out or dry power supply electrolytic capacitors.

Thanks! Not a complicated unit, think I'll come up,with a replacement plan.

 

[RTally]

As cademan noted, the likely culprit for the hum you hear is the power supply filter caps.

That Pioneer SX-1000TW was originally sold in 1971. At 50+ years old, I am sure many of the other caps are way past they "best by" date. I think it is time for a major overhaul. Seems like this receiver would make for a nice restoration project reading the HiFi engine comments.

 

[Terry Howard]

Thanks for taking the time to share your expertise.

 

[Terry Howard]

I replaced filter and speaker coupling caps, all caps on the power supply unit, and and working my way across the other boards to no effect at this time. Still have a few more caps to go but beginning to think they are not the source of the speaker hum.

I measured 175 volts to main caps (C12,13) coming off power transformer. Only drawing from 120v outlet. That doesn't sound good. Transformer issue?

 

[cademan]

All a power transformer does is step up or step down wall voltage. It can't cause hum through the speakers and if you are proof positive that it is not a power filter capacitor problem, then next check the diodes in the power supply. If it is not a single unit bridge configuration, a single open diode can cause hum.

Unless you have the wrong setting on your meter, the power transformer is not going to read more or less than it is designed. What is the voltage rating of the power supply capacitors? If they are under 175 volts, any more voltage would damage them or make them pop. Make sure your meter is set on AC volts when measuring the secondary of the power transformer.

Probably not a good idea to use the shotgun approach unless you really know what you are doing.

Got any pics of your circuit board? I don't need pics of the 1000 itself as I already know wnat it looks like. Just pics of the board or what you are actually working on.

 

[RTally]

I replaced filter and speaker coupling caps, all caps on the power supply unit, and and working my way across the other boards to no effect at this time. Still have a few more caps to go but beginning to think they are not the source of the speaker hum.

I measured 175 volts to main caps (C12,13) coming off power transformer. Only drawing from 120v outlet. That doesn't sound good. Transformer issue?

The Service Manual is available at HiFi Engine. The schematics and wiring diagrams show voltage levels. I suggest starting at the power supply and verifying the dc voltages. They should match what is shown in the manual.

I would then set the DMM to ac voltage and measure the ac component of the power supply outputs. There should be only a few millivolts of ripple. It would be best to measure this with a scope if you have one.

If the hum is at 120 Hz, it is likely from the power supply, particularly since you say it is on both channels and not affected by the volume level. It is best to troubleshoot to find the source of the hum instead of willy-nilly replacing stuff. After the hum is located and the issue fixed, then you can recap the receiver as needed.