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My New Pioneer SX-980, but afraid to use it... (DC offset high?!?)

JoeDotCom

JoeDotCom

Super Member
UG, so this receiver looks awesome! it is in great shape. only missing one switch cover. it definitely needs a cleaning (detoxit) which i intend to give it.

I have not used my meter to measure DC voltage before... but i am pretty sure this is telling me that this receiver has a DC offset on both channels around .7v (in other words 700mV)... am I right? (am i using the meter right?) The reading was takes using the "DC offset and you" instructions.

I have a small (virtually non existent) budget to get this thing fixed up. Is it unusable with this DC offset? I don't expect the detoxit cleaning will help the offset at all will it? Is this amp going to wreck my speakers? I have tried the receiver out with a pair of "beater" speakers, and it seems to be working OK to my novice ears.

I am OK with soldering and tinkering, etc. but I am not super electronics guy ether. I really want to be able to use this receiver. Is it usable... or at least close?




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some other photos just for fun....

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Your meter is set for the 20V scale? So, .7 volts.
You should find the manual and read the alignment instruction thoroughly, understand what they mean and try to adjust the DC offset (and probably the bias first; what ever the manual suggests).
.7 volts is not good on a DC coupled amp.
 
I wouldn't use it. The protect circuit should be disengaging the speaker terminals at that high offset. I'd be surprised if it can be adjusted within tolerance, but it's worth a try.

I'd advise rebuilding the power supply and protect boards, and new diff pairs in the amps at least.
 
Thanks ryuuoh and Jailtime. That is a bummer.. but what I figured was the right move. Any idea of a general price for the parts involved? Is it a $30 order or $300 for all the parts I will need to replace? I will probably need to sell off a pair of speakers to cover it, but it will be worth it!
 
Adjusting the offset is a 15 minute job, and something you might be able to do yourself.
 
Thanks Echowars. I have read many of your posts, and plan to consult them a lot more during this project. Idownloaded the service manual and have been looking for the offset adjustment info. I hope to dive in soon to try that adjustment first. :)
 
DVjorge said:
Why you don't set your meter for 200 mV scale ??? Correct me if I am wrong.
Click to expand...

I did try that, the reading is off the scale, so the meter just shows "1". my voltage is too high to be read by that small of a scale apparently. Gonna try tweaking the offset in a few. :)
 
EchoWars said:
Adjusting the offset is a 15 minute job, and something you might be able to do yourself.
Click to expand...

yes. Dc offset is now below 100 millivolts. the photo showing the meter in 200 millivolt mode... that's awesome I'm so impressed! I'm going to look and warm up a bit and I'll re tweak these and get them down closer 0.

that's what the manual calls the 'center voltage adjustment'. Next up is the 'idle current adjustment'. :-)

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sweet! This is so fun! I've got both channels down below 1 millivolt. I can't wait to hook this thing back up! thanks for your help Echowars, ryuuoh, Jailtime and everyone else!


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Congratz, that's a nice receiver. You did a smart thing by checking the amp's health before hooking up your expensive speakers to it. Enjoy the sound~
 
idle current is easier, it doesn't seem to bounce around like the center voltage adjustment did. ( manual said to set this to 30 millivolts)

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In your first pic, that cap on the right has a shrunken jacket. Probably has had a hard life. Consider recapping the power supply for reliability.
 
Wow, nice work! I'm surprised that you were able to adjust it down that far.

And that cap with the shrunken jacket, you should have seen the SX1280 I worked on. Every cap in the PS was like that.
 
I am glad you guys noticed that. Is the jacket actually shrunken? or did the capacitor expand? This is just a sign of wear and may degrade reliability / performance right? It is not dangerous is it? No fire / speaker damage hazard?

I am timid to dive into replacing the caps in this amp at this point. I think I am going to use something easier first.. I am thinking speakers crossovers are going to be easier than power supply circuitry. I just need to decide which speakers to start with out of my Ditton 110's or Avid 102's. I have a pair of Boston A60's but they are the 'newest' of my old school bunch. And i dont want to mess with my Polk RTA 12b's yet ether.

I ended up DetoxIT-ing the switches from the front of the receiver... i tried taking the bottom panel off but did not see an easy way to get to the backs of teh pots/switches without doing more disassembling than i had time for. So i shot the detox in from teh front of the receiver and while it helped alot, there is still a bit of crackle in some of the switches.

I think at this point (as long as you guys think i am OK to run with that shrunken jacket-ed cap) I am going to install this receiver in my listening room and enjoy it a bit while i work on my speaker crossovers to beef up my re-capping skillz. :)



Thank you all for your help!
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I am loving this thing! I play it nearly every night. I have not owned a receiver that sounds this powerful. I love it!

Only thing it is missing is one of the switch cap/tops. (you can probably see it in the photos, it is missing form the mute switch)

Is there a good place that will sell me a single switch cover?
 
Not trying to hijack this thread, but ideal current would be another term for BIAS, right? and current is in amps or milliamps, not voltage. Why does every manual say ideal current, and talk about adjusting it to a voltage point? Yamaha says this, and so does Pioneer. It confuses me.

Can someone explain?
 
Directly measuring current would require refitting the meter to measure current, and then placing it in series with the current path in some fashion. Difficult (as it would mean desoldering something to insert the meter in-line to the flow of current), and dangerous, as the meter configured to measure current is a direct short to whatever two points it is connected to and mistakes get costly. It is much safer to locate a resistor that the 'desired current to be measured' flows through, where you can measure the voltage generated by the current flowing through the resistor, and calculate the current.

This is why the bias (or idle current) is nearly always referred to as a voltage generated across a resistor (or pair of resistors) rather than some direct current measurement.
 
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