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Pioneer SX-880 goes into protection when turned up - has anyone ever solved this issue?

EastPoint

EastPoint

Recycler
Subscriber
Hi folks. I have a Pioneer SX-880 that goes into protection when turned up to 10 o'clock or so. Sometimes it does it sooner, sometimes it does it later. It plays fine until you turn it up a bit, and then it starts cutting out. If you leave it at that volume, it'll play again for a split-second, then cut out for a few seconds, then come back in for a split-second, etc.

It won't do it with no speakers playing--if I turn off the speaker pushbutton it will be fine. It isn't hooked to a dim-bulb. It does it with balance set to either side or in the middle. It's not an issue with my speakers--they are AR11s and I've had no issues driving them with other gear. DC offset and what-not are fine--this isn't an issue with that.

This is a recurring issue with the 880, and there have been a half-dozen threads about it over the years. Here are a few:

http://audiokarma.org/forums/index.php?threads/sx-880-protection-at-high-volume.620033/
http://audiokarma.org/forums/index....880-enters-protection-when-overdriven.470111/
http://audiokarma.org/forums/index.php?threads/pioneer-sx-880-cutting-out.81118/
http://audiokarma.org/forums/index.php?threads/sx-880-shut-down-for-a-second.152177/
http://audiokarma.org/forums/index.php?threads/sx-880-protection-keeps-tripping.41614/
http://audiokarma.org/forums/index.php?threads/infamous-pioneer-sx-880-protection-on.415600/

So clearly this is a common issue with the 880. Unfortunately none of these threads appear to actually resolve the issue.

A few years ago, an AK member @jeffsplat88 claimed to have solved the issue, here:
http://audiokarma.org/forums/index.php?threads/pioneer-sx-880-protection-circuit-fixes.679172/

However, I tried upping the resistor to 440 ohms as he suggested, and my 880 refused to come out of protection. I tried bumping the 47 uf 16v capacitor up to 220 uf 25v and it didn't really seem to change things. I am pretty sure the polarity was correct as installed (and as marked on the board), but I tried reversing polarity and it didn't seem to change things.

I have recapped the majority of the SX-880. It had this issue before the recap and it has the issue now.

Any ideas?
 
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i have owned and rebuilt an 880 and an 890. i am by NO means an electronics expert. have you checked the bias and offset? i did do the jeffsplat88 mods to both sets and that resistor mod sure did cool down the power supply regulators. and both sets still performed well. as i recall its not JUST the capacitor value that is changed, its polarity is as well. i cant for the life of me remember exactly,but it seems the board silk screen was ass about from the schematic.i gave both sets away one is still here in an un aircon house and it plays great the other is in california. both owners seem quite pleased. at the time i was hoping that "mark the fixer" would chime in as to the validity of the mods.sadly he didnt, but as he wont let anyone steer the forum wrong i figured it was a mod that couldnt possibly lead to ruination and was quite easy to reverse if need be. my curiosity was piqued as i wondered if my sx 1080 might suffer with its super hot regulators in the same way. based on my experience. i recommend the changes. if it's "BUM DOPE" im sure someone will comment. i highly recommend bornes multi turns be used to replace the bias and offset pots.
 
I will check bias and offset tonight and then try the mods again. I checked the schematic and I do think my cap is in backwards like he said.
 
I would try changing c418 (47uF16 V). The function is to filter out audio so only DC will trigger protection circuit. I would also change it to an NP-Non Polar (or BP - Bipolar same only different name) cap. The schematic and parts list show regular polar type cap, but IMHO it should be a non polar type. Good luck with it.
 
Well, this is interesting.

I checked the DC offset and it was very close to zero. Bias was pretty low--it's supposed to be around 30ma and it was around 5-10ma on one side and 15ma on the other. I adjusted the bias to 30ma and then tried installing a 220 uf 16v cap backwards in place of the 47 uf cap, and tried the extra 220-ohm resistor again. The receiver is now working fine and has no protection issue, so apparently we're all good.
 
EastPoint said:
Well, this is interesting.

I checked the DC offset and it was very close to zero. Bias was pretty low--it's supposed to be around 30ma and it was around 5-10ma on one side and 15ma on the other. I adjusted the bias to 30ma and then tried installing a 220 uf 16v cap backwards in place of the 47 uf cap, and tried the extra 220-ohm resistor again. The receiver is now working fine and has no protection issue, so apparently we're all good.
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I'm no electrical engineer but from what I can tell is in the event of a failure that leads to DC offset on the outputs, some of that voltage is tapped to pin 4 of the protection IC which stops pin 3 of the IC sending voltage to the base of Q37 (or Q27 my schematic is blurry) which inturn stops any current flow thru the protection relay coil thus disengaging the relay contacts. (Q37 is basically a switch once it turns on it allows current to pass thru the relay coil to ground which then engages the relay contacts.) I think that 47µF cap you mentioned is part of the startup delay so you don't get a lovely THUMP thru your speakers at startup, have you noticed that it may take the relay longer to engage at startup? Might be worth upgrading Q37 to a KSC2690AYS as I have a sneeking feeling that was the original issue.
 
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Oh and if you do have this issue in the future again, it could also be something to do with the overcurrent protection circuit. Q33 and Q34. If they detect an over current condition or one of them shorts due to age they'll send negative rail voltage to the protection IC causing it to disenguge the relay and save the amp from damage or be a nuisence depending on why they engaged Q36.
 
Did you check all the emitter resistors on that channel? I have seen this with Pioneer's a few times (SX 50 and 80 series) where perhaps in the past the amp has had a catastrophic output failure, the output transistors are replaced, but the Emitter resistors were never checked, and on or two might have drifted high, so then when you bias it up, it all good running at low power, but when it starts drawing any significant current, the resistor is heating up and going even more hiR than before, so more voltage drop, and sends more voltage to the current sensing transistor and trips the protection.
Had this on the SX1250 I just rebuilt...
 
Kevzep, Once I even check my emitter resistors (SX-1250) on a power supply running some current through them to make sure their values didn't change etc. Ran them until they got quite warm to the touch, not burning hot but fairly warm. In my case the resistance value did not change and the current draw during the test also did not change as the temperature increased. Some thought I was wasting my time but since they play such an important role I wanted to be sure. Figured it was better than a plain ole resistance check.

Bob
 
Hi, I see this is an old thread. But I've just received a SX-880 with the same problem. It plays well, but after it's warmed up, and turned up loud, it will go into/out of protection mode. What I've noticed is that the dc offset on the left channel, will start to go high. After 5 minutes, I will set it to 0. But after a lengthy time, it will drift up over 300mv. The bias stays constant at 30mv for both channels, and the right channel dc offset also stays at 0v. So I believe this is what is causing the issue. I first though it was the differential pairs transistors. So to debug, I swapped them between channels. No difference, left still drifted high. So then I put new transistors in for q19/q20. Still no difference. After reading these older threads, they discuss the transistors in the protection circuit, q22/q34. I can see that they have already been changed as well as the caps c415/c416.
Anyone have any suggestions here. It plays well, but not if you want to really crank it up.
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C415,416 should be film types with 5mm lead-spacing if replacing them. Just because once they are film, never have to worry about them again. Usually folks change the q33,34 transistors as well. In this case use ksa992. This protection circuit is a slight deviation from the usual. Interesting that they did not use a pa3004 as in other models in that time frame.
It is quiet possible that there is nothing really wrong, in that it’s going into protection when playing it really loud especially if it’s driving low ohm speakers. We could get our sx- 950 doing the same when we blasted it, so we just turned it down a bit.
The dc offset of 300mV usually is not enough to cause the protection to trip. But Why it’s drifting to 300mV in only one channel ? something is drifting but it’s a dc feedback amp so any offset gets amplified by the gain in the circuit. It could be the pot but there are other possibilities like q8 too. Another possibility is some leakage in the input coupling cap. can’t see what or where it is with what you show.
You can determine if it’s offset or over current by scoping q36. Missing the rest of the protect circuit schematic for me to comment further.
 
This has been worked on before as those caps, c415,416 have already been changed to wima film caps. The transistors had already been changed. I just again put new in. From what I can see in this thread, this appears to be a not uncommon issue with this receiver. But no real solutions from what I can see. I damaged the trim pots when taking them out. I'm now waiting for new to arrive. When they do, I'm going to scope the path to see if there's anything showing up in the signal path.
 
Getting back to this 880. So still looking at the dc offset issue. I was able to bring it by putting the bypass wire back. Someone had removed it. I also swapped D3 and D4. Those diodes that are mounted to the heat sink for bias stabilization. And then I got the offset issue to move to the other channel. And it happens after it's been running for a while. Which is the same thing that happened in the original channel. The offset would drift up to about 300mv. So, what I don't understand is why is the diode affecting offset and not affecting bias. The bias is staying very stable once set to 30mv. And where can I find a new version of this diode?
 
I am resurrecting this thread because I came across the same protection issue with a friend's SX-880 (going into protection mode at around 10:00 on the volume knob) and was at my wits end after having replaced all of the components in the protection circuit. I was studying the schematic and noticed that on the schematic the emitters for Q33, 34 and 36 all point toward the rear panel of the amp. Q36 in this particular 880 was a 2SC1914A (the original spec) and its lead configuration was opposite that of Q33 and Q34 (BCE vs ECB) yet all three transistors were oriented the same way in terms of the flats facing in the same direction. So either Q33 and 34 or Q36 was installed backwards because its emitter is on the opposite side.

Simply orienting all three (Q33, 34 and Q36) so the emitter was on the side the schematic showed (pointing towards the back) solved my protection circuit issue! Since this amp has an unknown service history (it had a sticker from a service facility on it) I can't say when this error occurred but I would urge you to check the orientation of Q36 if you run into an SX-880 that goes into protection mode at around 10:00 on the volume control and you have exhausted all other paths to solving it.
 
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NHDoc said:
I am resurrecting this thread because I came across the same protection issue with a friend's SX-880 (going into protection mode at around 10:00 on the volume knob) and was at my wits end after having replaced all of the components in the protection circuit. I was studying the schematic and noticed that on the schematic the emitters for Q33, 34 and 36 all point toward the rear panel of the amp. Q36 in this particular 880 was a 2SC1914A (the original spec) and its lead configuration was opposite that of Q33 and Q34 (BCE vs ECB) yet all three transistors were oriented the same way in terms of the flats facing in the same direction. So Q36 was apparently installed backwards because its emitter is on the opposite side.

The PC board markings are inconsistent, or at least unclear with respect to the emitter location but Q36 was definitely installed with its emitter pointing towards the front panel. I replaced Q36 with a KSC1845FT but its pinout is opposite the 2SC1914A but I installed it consistent with where the emitter was on the transistor I removed. Simply turning Q36 around so the emitter was on the side the schematic showed (pointing towards the back) solved my protection circuit issue! Now the three transistors are oriented the same way but since the KSC1845FT's emitter is on the opposite side it is backwards when compared to the original 2SC1914A. Since this amp has an unknown service history (it had a sticker from a service facility on it) I can't say when this error occurred but I would urge you to check the orientation of Q36 if you run into an SX-880 that goes into protection mode at around 10:00 on the volume control and you have exhausted all other paths to solving it.
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Thanks, this is very good information.
 
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