jeffsplat88
Member
Hello Folks,
I don't post very much. I read a lot. So in an effort to contribute to the folks I've learned so much from, I'll do my part with this little tid-bit. Take it for what it's worth.
AK is running slow for me tonight so I don't know if I'm simply repeating well-known information. But I'll do it anyway.
Pioneer SX-880. It appears that many people have discovered there are some problems with the protection circuit. The first, most obvious problem is that Pioneer didn't do their homework in calculation the power dissipation on the transistor-regulator that runs the protection circuit IC. This poor transistor has to drop 50 volts down to 13 volts at several hundred milliamps. This causes that poor transistor to cook and eventually die, usually taking the PCB and a few other components with it. This is a design flaw so if you have an 880, you are well served to modify it now instead of waiting for it to fail. The mod is simple if you are knowledgeable with electronics.
To fix the power dissipation on that transistor, simply DOUBLE that big power resistor that is mounted on the heat sink. It's a 220 ohm 5 watt. Take another and bend the bracket to fit them both in place, and wire it in series so that you get 440 ohm at 10 watts. This lowers the voltage on that regulator transistor which will give it a little more life. Those of you following the schematic will note that those resistors also feed the relay. My testing showed the relay still worked fine with the lower voltage present.
Now, on to the second problem. I run a pair of vintage polks that are rated about 6 ohms nominal but drop to a little above 4 ohms DCR. The Stock 880 hated these speakers, as well as my JBL Jubals. Not necessarily due to the low DCR (the Jubals are much higher), but likely due to excessive inductive kick back current from the speakers back into the amp circuit. I could test the system with 4 ohm resistors all day and have no problems. But the speakers would cause the protection to trip annoyingly early.
So I did a little digging. Those of you who've studied this circuit see that the service manual calls out a Wheatstone bridge circuit that sits across the emitter resistors on the power amp. It turns on a transistor when it sees a load smaller than a prescribed value (note that it's rectified so it's only monitoring half the cycle, and it's only on one emitter vs. both the pos and neg circuit). I tried futzing with the Wheatstone values in an attempt to lower the speaker impedance detection. I lowered this to 25% of the stock value and still could not keep the circuit from tripping with the polks. It improved, but not good enough. There is something else at play here. So returned the Wheatstone values to the stock values and I looked deeper down the circuit. In monitoring the trip signal that feeds the protection IC, I could clearly see it jumping when I turned it up. Jumping a lot. The circuit is clearly too sensitive.
What I also found was that the C418 (or C2 in the circuit description in the service manual) was mounted BACKWARDS relative to the DC voltage applied to it. So I replaced that cap, and also bumped up the value form 47 to 220uf to decrease the hair-trigger from all the pulses it gets from the bridge. This was the fix. Now my 880 can drive the polks and the Jubals into clipping with out tripping. Yet, when I put a pair of polks on one channel (well below the rated load of the amp), the circuit trips at high volume, as it should.
In summary:
1) Increase the power resistor mounted to the heatsink from 220 5w to 440 10w. This will save the regulator.
2) Pull C418 (C2) and reinstall in the correct polarity a slightly larger cap - I used 220uF. I seem to recall that even the PCB had the cap labeled backwards so this was a design error from day one. Check the voltage on that cap before you pull it, label the pos and neg voltage, and reinstall the new cap in accordance with the DC volts you see (you need to push the amp into tripping to see the volts on that cap). I wish I could be more descriptive but it's been a few weeks since I did the fix and can't remember whether it was miss-stuffed or miss-labeled. But it was definitely backwards.
Anyway, my 880 is happy now. Hopefully you others who may be experiencing problems can make your 880 happy too.
PS: I will say that the next thing I will want to do is buy a set of modern beefy transistors to replace the stock power amp transistors. Modern ones can take much more current than those old ones, with higher HFE. So that too will go a long way towards idiot-proofing the 880 for years to come. Cheap insurance.
I don't post very much. I read a lot. So in an effort to contribute to the folks I've learned so much from, I'll do my part with this little tid-bit. Take it for what it's worth.
AK is running slow for me tonight so I don't know if I'm simply repeating well-known information. But I'll do it anyway.
Pioneer SX-880. It appears that many people have discovered there are some problems with the protection circuit. The first, most obvious problem is that Pioneer didn't do their homework in calculation the power dissipation on the transistor-regulator that runs the protection circuit IC. This poor transistor has to drop 50 volts down to 13 volts at several hundred milliamps. This causes that poor transistor to cook and eventually die, usually taking the PCB and a few other components with it. This is a design flaw so if you have an 880, you are well served to modify it now instead of waiting for it to fail. The mod is simple if you are knowledgeable with electronics.
To fix the power dissipation on that transistor, simply DOUBLE that big power resistor that is mounted on the heat sink. It's a 220 ohm 5 watt. Take another and bend the bracket to fit them both in place, and wire it in series so that you get 440 ohm at 10 watts. This lowers the voltage on that regulator transistor which will give it a little more life. Those of you following the schematic will note that those resistors also feed the relay. My testing showed the relay still worked fine with the lower voltage present.
Now, on to the second problem. I run a pair of vintage polks that are rated about 6 ohms nominal but drop to a little above 4 ohms DCR. The Stock 880 hated these speakers, as well as my JBL Jubals. Not necessarily due to the low DCR (the Jubals are much higher), but likely due to excessive inductive kick back current from the speakers back into the amp circuit. I could test the system with 4 ohm resistors all day and have no problems. But the speakers would cause the protection to trip annoyingly early.
So I did a little digging. Those of you who've studied this circuit see that the service manual calls out a Wheatstone bridge circuit that sits across the emitter resistors on the power amp. It turns on a transistor when it sees a load smaller than a prescribed value (note that it's rectified so it's only monitoring half the cycle, and it's only on one emitter vs. both the pos and neg circuit). I tried futzing with the Wheatstone values in an attempt to lower the speaker impedance detection. I lowered this to 25% of the stock value and still could not keep the circuit from tripping with the polks. It improved, but not good enough. There is something else at play here. So returned the Wheatstone values to the stock values and I looked deeper down the circuit. In monitoring the trip signal that feeds the protection IC, I could clearly see it jumping when I turned it up. Jumping a lot. The circuit is clearly too sensitive.
What I also found was that the C418 (or C2 in the circuit description in the service manual) was mounted BACKWARDS relative to the DC voltage applied to it. So I replaced that cap, and also bumped up the value form 47 to 220uf to decrease the hair-trigger from all the pulses it gets from the bridge. This was the fix. Now my 880 can drive the polks and the Jubals into clipping with out tripping. Yet, when I put a pair of polks on one channel (well below the rated load of the amp), the circuit trips at high volume, as it should.
In summary:
1) Increase the power resistor mounted to the heatsink from 220 5w to 440 10w. This will save the regulator.
2) Pull C418 (C2) and reinstall in the correct polarity a slightly larger cap - I used 220uF. I seem to recall that even the PCB had the cap labeled backwards so this was a design error from day one. Check the voltage on that cap before you pull it, label the pos and neg voltage, and reinstall the new cap in accordance with the DC volts you see (you need to push the amp into tripping to see the volts on that cap). I wish I could be more descriptive but it's been a few weeks since I did the fix and can't remember whether it was miss-stuffed or miss-labeled. But it was definitely backwards.
Anyway, my 880 is happy now. Hopefully you others who may be experiencing problems can make your 880 happy too.
PS: I will say that the next thing I will want to do is buy a set of modern beefy transistors to replace the stock power amp transistors. Modern ones can take much more current than those old ones, with higher HFE. So that too will go a long way towards idiot-proofing the 880 for years to come. Cheap insurance.
