SX-780 powers on, no sound, no relay click

T

Truckasaurus

New Member
Can someone direct me to a guide on how to test the Darlington power packs while still installed? I saw in another thread that MTK posted how to on 12-06-2007, but I can't find that post. Any other beginner advice on what else to test would be also helpful.

Edit: I found a quick guide saying it should be -1.4v/1.4v on pins 1 and 10. This is what I got:

Right (away from power switch):
Pin 1: -1.348v
Pin 10: 1.188v

Left:
Pin 1: -36.52v
Pin 10: -33.94v

I believe there are also some transistors that I should check near the left pack, does anyone know what I should be looking for with those?
 
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Truckasaurus said:
Can someone direct me to a guide on how to test the Darlington power packs while still installed? I saw in another thread that MTK posted how to on 12-06-2007, but I can't find that post. Any other beginner advice on what else to test would be also helpful.

Edit: I found a quick guide saying it should be -1.4v/1.4v on pins 1 and 10. This is what I got:

Right (away from power switch):
Pin 1: -1.348v
Pin 10: 1.188v

Left:
Pin 1: -36.52v
Pin 10: -33.94v

I believe there are also some transistors that I should check near the left pack, does anyone know what I should be looking for with those?
Click to expand...
Problem in the left channel. There is s thread here on checking the "driver" section by removing the output IC and adding a couple of resistors.
It looks like the output module is probably bad.

Here is one of the posts about checking the 780 without the module.

merlynski

Post in thread 'Pioneer sx780 troubleshooting, high dc offset'

z-adamson said:
Double checking....

You mean the 0.22ohms 3 watt resistors, right? The ceramic bodied wirewound resistors.
Click to expand...
No, I mean unsolder and remove the STK pack, then put a 1kΩ resistor between pcb pins 0 and 8, and another 1kΩ resistor between pcb pins 1 and 3.
Pioneer SX-780 red dc-test.jpg
Like this^^

You can't drive a speaker with this configuration. It is meant to allow you to confirm/or deny the proper voltages at pins 1 and 10. If the driver section is good then the relay should close as well.
 
mrk229 said:
Problem in the left channel. There is s thread here on checking the "driver" section by removing the output IC and adding a couple of resistors.
It looks like the output module is probably bad.

Here is one of the posts about checking the 780 without the module.

merlynski

Post in thread 'Pioneer sx780 troubleshooting, high dc offset'

z-adamson said:
Double checking....

You mean the 0.22ohms 3 watt resistors, right? The ceramic bodied wirewound resistors.
Click to expand...
No, I mean unsolder and remove the STK pack, then put a 1kΩ resistor between pcb pins 0 and 8, and another 1kΩ resistor between pcb pins 1 and 3.
Pioneer SX-780 red dc-test.jpg
Like this^^

You can't drive a speaker with this configuration. It is meant to allow you to confirm/or deny the proper voltages at pins 1 and 10. If the driver section is good then the relay should close as well.
Click to expand...
Thanks. I ordered some resistors and will work on it when they come tomorrow and let you know how it goes.
 
mcdoug3721 said:
Missed the Left: Pin 1: -36.52v Pin 10: -33.94v which is indicative of shorted Q13/Q15
Click to expand...
Those are two of these three, yes? How do I check those?

Edit: Also, no shorts between 2 & 3 or 8 & 9.
 

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A quick check for continuity between emtitter and collector Q13 & Q15 left channel can be done with the power off. You have rail/collector voltage on STK pins 1 and 10.

1770762577903.png
 
mcdoug3721 said:
A quick check for continuity between emtitter and collector Q13 & Q15 left channel can be done with the power off. You have rail/collector voltage on STK pins 1 and 10.

View attachment 3696748
Click to expand...
Sorry, I'm quite new to this. I assumed the "bottom" (closest to the front of the unit) lead was the base, but the board shows a C, so I guess that's the collector? Is the emitter then the one on the top right (I can't see the other markings)? If that's the case, then there's no E-C continuity on, I believe, Q15 (the transistor closest to the power pack). I had to turn the power on for this reading, though, otherwise everything showed a voltage.
 
I get that's how they're diagrammed, but I was having trouble figuring out how they're positioned in the circuit. Again, I'm pretty new with all this. That said, I did figure it out, assuming the PCB labels are correct. Reads OL in diode mode between E and C with power applied on both transistors. Again, assuming that Q15 and Q13 are the two in this photo.

Edit: Sorry, I guess these are actually the good ones, and the center one, with continuity from black on E, red on C is the bad one.
 

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Truckasaurus said:
I get that's how they're diagrammed, but I was having trouble figuring out how they're positioned in the circuit. Again, I'm pretty new with all this. That said, I did figure it out, assuming the PCB labels are correct. Reads OL in diode mode between E and C with power applied on both transistors. Again, assuming that Q15 and Q13 are the two in this photo.

Edit: Sorry, I guess these are actually the good ones, and the center one, with continuity from black on E, red on C is the bad one.
Click to expand...
Those three transistors are the voltage regulators, not the driver transistors. I can see the heatsink through one of the holes in the PC board, and the heatsink stability tabs poking through the PC board. They supply power for both channels.
 
Power off, Q15 and Q13 show fine. Power on, Q13 shorts black on C, red on E, and Q15 shorts both ways, and also somehow causes my dmm to think it has a low battery and shuts off.
 
Truckasaurus said:
Power off, Q15 and Q13 show fine. Power on, Q13 shorts black on C, red on E, and Q15 shorts both ways, and also somehow causes my dmm to think it has a low battery and shuts off.
Click to expand...
You can't do that type of testing with the power on.

Are you looking at the right components now. The picture you posted has you in the yellow square with Q19, Q20 and Q25.

Q11,13 and 15 are in the blue box.


780pc.jpg
 
Yes, I'm in the right components now, Q15 (A912), and Q13 (C1885). With power off, they don't show shorts between E and C.
 
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I'd like to add that I found this thread talking about someone's pcb being different from the schematic, with an extra resistor and diode, and my board is the same.
 
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mrk229 said:
Problem in the left channel. There is s thread here on checking the "driver" section by removing the output IC and adding a couple of resistors.
It looks like the output module is probably bad.

Here is one of the posts about checking the 780 without the module.

merlynski

Post in thread 'Pioneer sx780 troubleshooting, high dc offset'

z-adamson said:
Double checking....

You mean the 0.22ohms 3 watt resistors, right? The ceramic bodied wirewound resistors.
Click to expand...
No, I mean unsolder and remove the STK pack, then put a 1kΩ resistor between pcb pins 0 and 8, and another 1kΩ resistor between pcb pins 1 and 3.
Pioneer SX-780 red dc-test.jpg
Like this^^

You can't drive a speaker with this configuration. It is meant to allow you to confirm/or deny the proper voltages at pins 1 and 10. If the driver section is good then the relay should close as well.
Click to expand...
I took the pack out and put the 1kΩ resistors in. The relay clicked, but the voltages are:

1: -6.86v
2: -33.56v
3: -73.0v
8: -71.3v
9: 33.56v
10: 6.67v

The guy in the thread you linked me to had 2/9 about 6.5v more, and 3/8 zeroish.

Edit: I also checked the power pack and there's a short between 8 and 9, which didn't show when I tested it in circuit. So I guess a new power pack is in order, but pretty sure there's something else wrong with those voltages, right?
 
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Truckasaurus said:
I took the pack out and put the 1kΩ resistors in. The relay clicked, but the voltages are:

1: -6.86v
2: -33.56v
3: -73.0v
8: -71.3v
9: 33.56v
10: 6.67v

The guy in the thread you linked me to had 2/9 about 6.5v more, and 3/8 zeroish.

Edit: I also checked the power pack and there's a short between 8 and 9, which didn't show when I tested it in circuit. So I guess a new power pack is in order, but pretty sure there's something else wrong with those voltages, right?
Click to expand...
Yes 3 and 8 zeroish is what we are looking for. Are you sure about the measurements? I don't think you can even find -71 volts in the 780.

Where is the black lead of your meter connected? Are you sure 3 and 8 are not millivolts?

Pictures are always helpful. Meter face with the setting dial visible, where the black meter lead is connected.
 
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