Pioneer SX-850 problem

[SvendS]

Hi there !!

Is there anyone out there who can help me ?
I am the happy owner of an old Pioneer SX-850 receiver/amplifier, which I use for PA on my home theater setup.
Unfortunately it has started to constantly switch the loudspeaker relay on and off. Can anyone tell me where to locate the problem.

Hoping for an answer

Svend S. - Denmark

[StarMover]

I'm told that the Pioneer relays themselves are rarely the problem. I'm not an expert here, but it sounds like a circut protection fault. How many speakers are you running? What about your speaker impeadence? 4-ohm, 6-ohm, 8-ohm?

If you're running more than just two speakers, try hooking up two 8-ohm speakers and see if you still have the problem.

[schwarcw]

I have the same problem in an SX-828 (which is now in the closet :tears: ). It plays fine for a while, then the sound goes off. Come back, then goes off again. Of course when a repair tech friend of mine was looking at it, the unit operated fine.

Carl

[dnewma04]

I think Jack has the same issue with a SX-850, as well. A tech looked it over without finding the issue. I believe it happens when using only as a preamp as well as when using the internal amp. This may or may not eliminate the amp as the problem. Sorry, that was less than helpful.

[SvendS]

Answer to StarMover:

The load on the PA is a single set of 4 ohm loudspeakers, so that should not be a part of the problem. But I think you are right about that it looks like a circuit protection fault. Could it be an unbalanced speaker output with too high DC voltage ?? I'll better test for that.

[Fred Longworth]

When the protection relay triggers after the set has been running a while, one of the following three things is likely the cause:

(1) A failure in the power amp or amps, most likely in the differential amplifier stage, and probably a malfunction that causes an intermittent and LARGE offset voltage at output test point(s). In this case the protection circuit is doing exactly what it's supposed to.

(2) A failure in the power supply circuit in the regulators that feed precise voltages to the differential amplifier circuits in the power amps. Again, this would likely cause huge DC offsets at the output test points -- and, as before, the protection circuit is working normally.

(3) A failure in the protection circuit itself, often either a failure of the power supply capacitor coming off the rectifier for the protector circuit, OR a malfunctioning transistor in the protection circuit.

(1) and (2) are common; (3) less common.

You may also have problems with intermittent shorts in your speaker wiring or speakers. In this case, the triggering of the protector will be volume dependent, and the relay will reset to "normal" a few seconds after triggering.

Hope this helps.

Fred Longworth
http://www.repairaudio.com
StereoTech Classic Audio Repair

[SvendS]

Answer to Fred L. :

Thank you for a good and educating answer. Actually, it often starts up being mute, and then turns on for a few seconds, and then off again - so I think I will start by testing the power supply for ripple. It could be the old electrolytic capacitors giving out. - Anyway, I think your answer will lead me on the track.

Thanks - SvendS