A Couple Of SX 780 Questions.

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Ismellelephant

New Member
I did a search on the site and found a lot of useful info to try and get my SX780 working again. Everything lights up ok, source etc. but I get no click and no sound out or readings on the watt meters. I resoldered all the power packs leads and the transistors with the heat sinks. I checked the voltages on the Darlington Power packs on each side and found way different voltages on some of the pins, for example Pin1 is about -13v yet on the other darlington the same pin is about -38v. I have two questions, shouldn't the voltages be the same on both darlingtons? And if only one darlington was bad would you hear no sound at all? I am waiting for schematics to arrive but I couldn't help poking around. Thanks for any info.
 
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Can't help you a bit but just gotta ask where you are that you can smell elephants? Welcome to AK. My question about the power packs would be: which voltage reading is correct? Guess the schematic will tell that though. Good luck.

mac
 
I smell them everywhere, lol
I found pinout voltage readings elsewhere on the web as follows:
Pin 1: Input pos(+)
Pin 2: Vcc neg(-)
Pin 3: Output
Pin 4: NC
Pin 5: NC
Pin 6: NC
Pin 7: NC
Pin 8: Output
Pin 9: Vcc pos(+)
Pin 10: Input neg(-)
And on my sx780 the two different voltages on Pin's 1 are negative. Yes I need the schematic.
 
What do they smell like, maybe I smell them too? The closest help I can be is that I fried one of these units while replacing the power pack( not much help is it?). Had it working and saw a loose wire and thought I'd just solder it a bit better real quick. The magic smoke got out. Never made that mistake again. Remember boys and girls: always unplug your stereo gear before you solder on it.

mac
 
I think a good piece of advice is to keep your power outlet (for plugging in gear) at eye level on your workbench so that you can see if you have the unit plugged in or not.
 
I remember someone posting the pics of where to solder on the 780 cause the heat caused cold solder joints causing the problem you described. This is common on the 780s. Try this. These pics are the ones I used to get mine fixed. I know this is an AK members pictures from his thread but I cant remember who put them up. Hope he does not mind.
 

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I'm not sure how easy it is for you to remove the Darlington Power pack. If it isn't too big a deal, maybe you could do something such as remove them and switch the right for the left one and see if the same voltage reading moves to opposite side. You might also find someone in the on the pioneer board who has one of these. I worked on one recently, but I only had to give it a good cleaning with deoxit and adjust the bias and DC offset.

http://www.electronix.com/catalog/product_info.php/cPath/6_308/products_id/12677

Also read this:

http://audiokarma.org/forums/showthread.php?t=48283&highlight=darlington

http://audiokarma.org/forums/showthread.php?t=46557&highlight=darlington
 
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Thanks for the links. I removed both Darlingtons from the amp and the voltages supplied to the pads were identical and made sense. One darlington had a short between pins 8 and 9 so I resoldered the Darlington with out the short back in and now one channel works fine so I ordered a new Darlington. I guess to answer my own original question, yes one Darlington being bad can cause no sound from both channels. Thanks for the help.
 
You would think that anybody willing to go that far in would fit in fine here. Must have been another hit and run.
 
ISmellBullephant

tentoze said:
And he had one of the best usernames. Ever.
Click to expand...

:D

Rulerboyz, is that the actual Darlington pack in that link? That's way cheaper than it sounds, if that is indeed the peice, hehe. I've got a 780 that is giving me a little trouble in the lower volume ranges, it's not consistent tho. I thought it was the volume pot, but so far no luck there. I'll try it again before I start tearing in deeper. Thanks for the link RBZ.


Yeah Strawman, he seemed pretty determined to get that puppy back in the hunt. Too bad, cause we could learn a lot from different projects like Smellephant's. That is, at least I could, being a green horn and all. Learning all the time, learning all the time.


Rebel
 
On the other hand Pioneer SX-780 is one of the most common receivers I've noticed getting posted (do a search). Chances are that if it involves this receiver it's been done before somewhere in the memory banks of this board.
 
This worked for me on my SX-780 5 years ago too.
"I am writing to you in regard to a Q&A you answered on 3/25/00 on the Classic Audio website regarding a Pioneer SX-780 receiver that intermittently failed to turn on. The person submitting the question reported that the protection relay was failing to kick in and turn on the outputs. I also own that receiver and several months ago it quit working with exactly the same symptom, but it was a solid failure rather than intermittent. I just found time to look into it recently and managed to find my owners manual and a wiring diagram and started to trace things out. What I found was that I apparently had two cracked solder joints on the D712 transistor. I am not an electrical expert but it appeared that this transistor supplied power to the PA3004 module which in turn energized the relay in question. Re-heating the joints on this transistor seems to have solved the problem. "

Charlie
 
CharlieBee said:
This worked for me on my SX-780 5 years ago too.
"I am writing to you in regard to a Q&A you answered on 3/25/00 on the Classic Audio website regarding a Pioneer SX-780 receiver that intermittently failed to turn on. The person submitting the question reported that the protection relay was failing to kick in and turn on the outputs. I also own that receiver and several months ago it quit working with exactly the same symptom, but it was a solid failure rather than intermittent. I just found time to look into it recently and managed to find my owners manual and a wiring diagram and started to trace things out. What I found was that I apparently had two cracked solder joints on the D712 transistor. I am not an electrical expert but it appeared that this transistor supplied power to the PA3004 module which in turn energized the relay in question. Re-heating the joints on this transistor seems to have solved the problem. "

Charlie
Click to expand...

Nice. :thmbsp:
 
Thats the beauty of mass production where common failures occur thru the line. I repair computers, laser printers, etc. Every new model introduction has some kind of issue, once you identify and have the resolution you're allright. Usually, if you see a problem once, you'll see it a thousand times, especially working for a large machine base organization.
 
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