Pioneer SX-980 Protection Relay Kicking Out

That is wicked looking ripple for a power supply rail :yikes:. Is it the same on pin 19 of the power supply board? Check PS FU3 and FU4, rectifiers D1-4 and C11 (reversed?).
Scope all the other outputs of the PS also, as well as DC voltages.
Signal is the same at pin 19 of the power supply board. I rechecked all the fuses on the PSB - all good (I pulled them out to be sure). I thought C11 and the bridge rectifier diodes might be suspect, as well. Will hopefully get that board pulled this afternoon.
 
Signal is the same at pin 19 of the power supply board. I rechecked all the fuses on the PSB - all good (I pulled them out to be sure). I thought C11 and the bridge rectifier diodes might be suspect, as well. Will hopefully get that board pulled this afternoon.
I checked the bridge rectifier diodes in-circuit with an ohmeter - they all tested good - and all tested with equal readings. I replaced C11 and C12, but am still getting the same waveform on the protection board pin 10 / power supply board pin 19. Pins 4, 5, 6, 7 and 8 of the protection board still disconnected. I hate to say it, but this is starting to look like a parts unit. Giving up for the weekend. I'll decide it's fate on Tuesday, I guess. Thank you so much for all the time you've put into helping me on this!
 
I don't have a solution to your problem but I thought I'd comment on your ripple. In other designs, they tended to use the fully filtered output of the regulated power supply to drive the protection relay. In the SX-980, they are using the higher voltage that appears at the first stage of filtering (Pin 19 +65VDC) which will contain significant ripple. The second stage of PS filtering takes place via C15 and Q1 that act as a capacitance multiplier which removes the remaining ripple. So, the ripple you are seeing on Pin 19 isn't totally unexpected. You will likely see the same ripple on the negative supply which can be measured on the negative lead of C16. It's possible that this ripple is sufficient to cause the protection relay to drop intermittently.
 
I checked the bridge rectifier diodes in-circuit with an ohmeter - they all tested good - and all tested with equal readings. I replaced C11 and C12, but am still getting the same waveform on the protection board pin 10 / power supply board pin 19. Pins 4, 5, 6, 7 and 8 of the protection board still disconnected. I hate to say it, but this is starting to look like a parts unit. Giving up for the weekend. I'll decide it's fate on Tuesday, I guess. Thank you so much for all the time you've put into helping me on this!

You are most welcome, :thumbsup: I learned a lot during the process! I am not wanting to give up. If you do decide to part with it PM me, please.
 
I don't have a solution to your problem but I thought I'd comment on your ripple. In other designs, they tended to use the fully filtered output of the regulated power supply to drive the protection relay. In the SX-980, they are using the higher voltage that appears at the first stage of filtering (Pin 19 +65VDC) which will contain significant ripple. The second stage of PS filtering takes place via C15 and Q1 that act as a capacitance multiplier which removes the remaining ripple. So, the ripple you are seeing on Pin 19 isn't totally unexpected. You will likely see the same ripple on the negative supply which can be measured on the negative lead of C16. It's possible that this ripple is sufficient to cause the protection relay to drop intermittently.

I was concerned by the 'oscillation' on the positive peaks and the sharp point at the bottom dips, to me that does not look like normal 120Hz rectifier and cap filtered ripple, even heavily loaded. I am now suspecting an oscillation amp circuit.
 
I was concerned by the 'oscillation' on the positive peaks and the sharp point at the bottom dips, to me that does not look like normal 120Hz rectifier and cap filtered ripple, even heavily loaded. I am now suspecting an oscillation amp circuit.
We would need to see an expanded view of that part of the waveform to know if it's some sort of ringing or oscillation. It's most likely being filtered out before appearing at the PS output.

The first filter cap isn't very large (220 uf) so it will tend to discharge somewhat between cycles under a normal load resulting in sharp dip at the bottom. I modified an existing PS simulation by changing the cap value to 220 uf and the ripple looked similar to the scope display minus the ringing/oscillation.
 
If anyone wants a different view of the waveform, I'd be glad to provide it. I guess my next plan is to separate the switch/pot boards from the rest of the receiver like I did before this all started happening. Maybe I'll see something. I just don't know what I could have done that would cause this - especially with the pre in/out jumpers removed and the wires removed from pins 4 through 8 of the protection board.
 
With the scope look at all the Power Supply pins for similar signals or oscillations. Look at the power amps' pin 11 (outputs). Output transistors' E, B & C. With ohmmeter and power off check continuity to ground (chassis) for every ground pin on the protection board, power supply board and the power amp boards. Those old wirewrap wires are brittle and may have broken off.
 
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With the scope look at all the Power Supply pins for similar signals or oscillations. Look at the power amps' pin 11 (outputs). Output transistors' E, B & C. With ohmmeter and power off check continuity to ground (chassis) for every ground pin on the protection board, power supply board and the power amp boards. Those old wirewrap wires are brittle and may have broken off.
Okay, I'll take a look at those things when I get back to the shop on Tuesday. Regarding pin 4 of the protection board - what kind of voltage should I see on the wire the connects up to it? I believe that wire is tied to both power amps, if I recall the schematic correctly. I mistakenly grounded that pin with the wire disconnected (I was thinking of pin 4 on the IC, not the board - oops), but when I did, the relay stopped cycling - stayed off, I believe. So, grounding pin 4 of the protection board causes the relay to stop cycling (staying off), so is there a particular voltage that should be applied to that pin to cause normal operation of the relay? Just wondering since disconnecting audio inputs/outputs from pins 5,6,7 and 8 really shouldn't matter. All I need for normal operation of the protection relay are the normal voltages provided on pins 1,2,9 & 10 - and a ground on pin 3. Leaving pin 4 as my wildcard, right now.
 
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Well, I just read through the protection circuit description (again) in the service manual. Turns out pin 4 of the protection board should see 0vdc from the power amps, which I believe I measured previously when the original board was connected up. So there goes that theory. Also, with the ripple in my 65vdc supply, the relay still pulls in solidly, although briefly. Starting to wonder if I have two bad PA3004 IC's. I may try to purchase one through Ebay to see if it makes a difference. I'll do some power supply voltage checks first, as recommended.
 
Another thought.....could I connect up an external +65vdc supply to pin 10 of the protection board (with the existing wire disconnected, of course) without wreaking havoc with anything in the receiver? Curious as to whether that ripple is giving me fits.
 
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Okay, I'll take a look at those things when I get back to the shop on Tuesday. Regarding pin 4 of the protection board - what kind of voltage should I see on the wire the connects up to it? I believe that wire is tied to both power amps, if I recall the schematic correctly. I mistakenly grounded that pin with the wire disconnected (I was thinking of pin 4 on the IC, not the board - oops), but when I did, the relay stopped cycling - stayed off, I believe. So, grounding pin 4 of the protection board causes the relay to stop cycling (staying off), so is there a particular voltage that should be applied to that pin to cause normal operation of the relay? Just wondering since disconnecting audio inputs/outputs from pins 5,6,7 and 8 really shouldn't matter. All I need for normal operation of the protection relay are the normal voltages provided on pins 1,2,9 & 10 - and a ground on pin 3. Leaving pin 4 as my wildcard, right now.

Are you saying grounding pin 4 of the protection board stops the relay cycling? The way the overload detection works is that there is a Wheatstone bridge formed with the output transistors 0.5 ohm emitter resistors (R18 and 19 on power amp board with Q8 at the center.) When an overcurrent condition happens the W bridge becomes unbalanced and turns on p-amp Q8. This pulls current through D2 on the protection board turning on Q1, which then pulls pin 4 of the PA3004 positive, which turns OFF the relay. This is a CURRENT operated circuit. Pin 4 of the P board is in a fairly high impedance state with a positive bias from Q1 BE. R2 (13k) and D2 (Q8 off) until the overload detection pulls it negative. Grounding P board pin 4 won't hurt anything. I started a thread about the PA3004 pin 4 a few days ago: ( http://audiokarma.org/forums/index.php?threads/pa3004-pin-4-question.835170/ ). The +13 vdc power supply comes off the same transformer winding as the +65vdc, different tap though. Look for noise on +13vdc, it powers the PA3004 and the Q1 circuit on P board.
 
I'll verify the operation of the relay when grounding pin 4 of the protection board when I get back to the shop on Tuesday. I'll check the +13vdc supply for ripple, as well as the others. Thanks!
 
I'll verify the operation of the relay when grounding pin 4 of the protection board when I get back to the shop on Tuesday. I'll check the +13vdc supply for ripple, as well as the others. Thanks!
The relay should drop out when P board pin 4 is grounded, that is normal functioning. When in this condition look at IC pin 8 with scope to see if it is still charging and discharging like before when relay was cycling, I expect it will not be..
 
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Yes, I believe IC pin 8 was still going through the charge/discharge cycle. I'll verify on Tuesday. What I'd like to know (and what the manual doesn't tell you) is what voltages at what pins, either board, IC or both, you will observe when the protection circuit functions normally and the relay pulls in after the initial delay. Concerning the protection circuit, the manual is very good at telling you what happens when things go wrong, but not so much when things go right.
 
power supply voltages should be steady regardless of fault conditions . concentrate on amp output voltages first if supply is stable . voltages are shown as working condition if any are way out they need looking at .
 
Yes, I believe IC pin 8 was still going through the charge/discharge cycle. I'll verify on Tuesday. What I'd like to know (and what the manual doesn't tell you) is what voltages at what pins, either board, IC or both, you will observe when the protection circuit functions normally and the relay pulls in after the initial delay. Concerning the protection circuit, the manual is very good at telling you what happens when things go wrong, but not so much when things go right.

I believe the voltages posted on the schematic next to the PA3004 pins are the steady state voltages with no audio output from the power amps, i. e. 0.0vac; no dc offset at the power amps, i.e. 0.0vdc; no overload condition i.e. Q1 in cutoff; and Q2 on (saturated) with relay pulled in; steady AC present on pin 7; and negative voltage present on pin 6. SM PA3004 Logic diagram shows 18vac w current limiting resistor to pin 7 and -13vdc with current limit resistor to pin 6. If those conditions are true the logic diagram S1 should be open, pin 8 should be a steady +7.2 vdc.
 
The relay should drop out when P board pin 4 is grounded, that is normal functioning. When in this condition look at IC pin 8 with scope to see if it is still charging and discharging like before when relay was cycling, I expect it will not be..
Okay, I grounded PB pin 4. IC pin 8 voltage does remain stable at about 0.5v, so you're prediction was correct. I was remembering a different situation, apparently. I'll check some voltages with the o-scope.
 
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