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Pioneer SX-780 No Sound, No Relay Click. Feedback wanted before repair.

sploofer

sploofer

Member
Hey AKers,

I know there are multiple threads on this topic and I have a pretty good idea what the problem is, but I wanted to get a second opinion before I start working on the solution.

I found this SX-780 earlier this weekend at a flea market. I had no way of testing it at the time, but it was in nice condition and the owner said it worked so I decided to take the $25 gamble. I got home and found that it powered up, all the lights work, the FM and AM VU meter bounces, the source light works, but there was no sound and no relay click.

I opened up the bottom to see if I could find anything problematic (leaks, damage, etc.). The first thing I noticed was that all the fuses were wrapped in aluminum foil and that one of the fuses appeared burnt (see picture below). I read about "the known cold solder joint issue" (http://www.audiokarma.org/forums/index.php?threads/pioneer-sx-780-issue.55876/) so I looked for that and it seems like I have this issue as well (see picture below).

I'm a complete beginner with soldering and repairing, but I think with a little bit of practice I could re-solder the joints. I have a soldering iron, solder and a solder sucker, but I don't have anything else (multi-meter, soldering paste, etc).

I was seriously considering taking this to a shop, but if this is an easy fix, I'd rather spend the money on tools I can reuse and get the experience repairing this unit.

Questions:
  • Why did the previous owner wrap the fuses in foil?
    • Should I replace the questionable fuse?
  • Should I buy anything else to assist me with this repair or should I re-solder the joints and see if that fixes the problem?
Bottom of the SX-780:
https://www.dropbox.com/s/fw38mqc5kowlh0l/20170917_092538.jpg

Close-up of the joints:
https://www.dropbox.com/s/y7v5knyiuscuj2l/20170917_092857.jpg

Close up of the Fuses:
https://www.dropbox.com/s/c2uxwjn5qh0q5d1/20170917_092910.jpg
 
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pins 4 5 6 7 are unused so not a problem ..
yes of course they all need the correct fuses installed before powering on again .
you want to be checking for dc voltage at the amplifier outputs . this can be done at L1 and L2 reference chassis ground .. it should be close to 0v .
 
sploofer said:
Hey AKers,

I know there are multiple threads on this topic and I have a pretty good idea what the problem is, but I wanted to get a second opinion before I start working on the solution.

I found this SX-780 earlier this weekend at a flea market. I had no way of testing it at the time, but it was in nice condition and the owner said it worked so I decided to take the $25 gamble. I got home and found that it powered up, all the lights work, the FM and AM VU meter bounces, the source light works, but there was no sound and no relay click.

I opened up the bottom to see if I could find anything problematic (leaks, damage, etc.). The first thing I noticed was that all the fuses were wrapped in aluminum foil and that one of the fuses appeared burnt (see picture below). I read about "the known cold solder joint issue" (http://www.audiokarma.org/forums/index.php?threads/pioneer-sx-780-issue.55876/) so I looked for that and it seems like I have this issue as well (see picture below).

I'm a complete beginner with soldering and repairing, but I think with a little bit of practice I could re-solder the joints. I have a soldering iron, solder and a solder sucker, but I don't have anything else (multi-meter, soldering paste, etc).

I was seriously considering taking this to a shop, but if this is an easy fix, I'd rather spend the money on tools I can reuse and get the experience repairing this unit.

Questions:
  • Why did the previous owner wrap the fuses in foil?
    • Should I replace the questionable fuse?
  • Should I buy anything else to assist me with this repair or should I re-solder the joints and see if that fixes the problem?
Bottom of the SX-780:
https://www.dropbox.com/s/fw38mqc5kowlh0l/20170917_092538.jpg

Close-up of the joints:
https://www.dropbox.com/s/y7v5knyiuscuj2l/20170917_092857.jpg

Close up of the Fuses:
https://www.dropbox.com/s/c2uxwjn5qh0q5d1/20170917_092910.jpg
Click to expand...

Blown fuses is a bad sign, fuses wrapped in tinfoil is even worse, something bad happened inside the amp causing those fuses to blow, and the previous owner just wrapped them in tinfoil probably making said problem worse. Buy a cheap $5 multimeter from harbor freight. Like Pete said, check the output voltage, test voltage at pins 8 and 3 on both the output IC's black probe to chassis red probe to either pin, should be 0v, hate to be a debbie downer but I think you may be looking at blown output IC's. Not sure what solder you have right now but I'd suggest you get some high quality flux core 60 / 40 leaded solder, flows and sticks much nicer than tin solder also lasts longer.
 
markthefixer said:
check the 3 supply voltages coming out of the 3 heat sinked power transistors in a row over near the corner.

Then if they are ok
remove the module, tie pins 1,3,8 & 0 together to establish bias circuit conduction, as well as providing a feedback path. 1 and 0 HAVE to be connecvted together by something or you will see the -38 and +38.

STAY THE %^^$^% AWAY FROM PINS 9 AND 2 = B L O O O O I E E E E E

Then see if both channels can be dc offset set to 0.000v

if one channel cannot be set to 0;000, try interchanging q11/q12 and q13/q14 IF they show good on the 6 way transistor diode check.

sitting at +38 AT PINS 1 AND 0 WITH THE MODULE in, means that there is no balancing pull down from the -38, and that is q11,q13 and R259
plus there is also their (q11 base for q11 and q13) being tuned on by the base voltage being supplied by the q7 & q9 differential input amplifier.

Q15 (along with R251, d1, r253 biasing and r261 sensing) is a 3mA current source circuit fed from the +39.5v rail. The collector can't be hanging in midair and work. That's what happens when the module is removed and nothing is done with pin 0. You COULD set your DMM for a current reading, and touch one probe to that pin 0 hanging in midair and the other to ground, so when it is working it should read about 3mA or 0.003A no matter if the other probe is grounded or connected to the -39.5v rail. IF q15 is a 2sa912 as specified it is a 150 volt transistor and will not be harmed by the 80v differential to the -39.5v rail.
Click to expand...

This quote is for another SX-780 thread. Best to read it before you get to far.
You'll need a DBT.
http://audiokarma.org/forums/index.php?threads/another-sx-780-issue.608200/

DBT build instructions:
http://audiokarma.org/forums/index.php?threads/dim-bulb-tester-build.758607/
 
Last edited:
Thanks to everyone for the suggestions. Next week I'll see if I can make a Dim Bulb Tester, but in the meantime, could someone suggest somewhere to buy a set of replacement fuses for the SX-780?
 
Ace Hardware has a great assortment in the "Hillman" bulk bins.
 
Hey All, I have a SX-950 that has same issues but I was listening to FM this morning for a few hours. Now...nothing but lights. Shall I follow the same advice or is the SX-950 different?
Thanks, Tony in Austin
 
Two completely different receivers, best to start your own thread.
Welcome.
 
test the fuses that are there and note any blown ones . i have a hunch someone was playing around that thought they knew something .
 
Eastham said:
Blown fuses is a bad sign, fuses wrapped in tinfoil is even worse, something bad happened inside the amp causing those fuses to blow, and the previous owner just wrapped them in tinfoil probably making said problem worse. Buy a cheap $5 multimeter from harbor freight. Like Pete said, check the output voltage, test voltage at pins 8 and 3 on both the output IC's black probe to chassis red probe to either pin, should be 0v, hate to be a debbie downer but I think you may be looking at blown output IC's. Not sure what solder you have right now but I'd suggest you get some high quality flux core 60 / 40 leaded solder, flows and sticks much nicer than tin solder also lasts longer.
Click to expand...

Your hunch was correct! It was a blown IC.

I ended up taking this into a professional and thats what he had found. He replaced it, cleaned it up and now everything works great. I originally planned on doing this myself, but I have zero experience soldering or working with electronics. I wanted to play it safe, especially since they have a warranty and accountability if any accidents happen, which might have happened with me.

Total cost of the repair was $160 and including the $25 I paid for this unit, I can live with paying $185 for a serviced and clean SX-780 that I bought locally, instead of buying off the bay.
 
sploofer said:
Your hunch was correct! It was a blown IC.

I ended up taking this into a professional and thats what he had found. He replaced it, cleaned it up and now everything works great. I originally planned on doing this myself, but I have zero experience soldering or working with electronics. I wanted to play it safe, especially since they have a warranty and accountability if any accidents happen, which might have happened with me.

Total cost of the repair was $160 and including the $25 I paid for this unit, I can live with paying $185 for a serviced and clean SX-780 that I bought locally, instead of buying off the bay.
Click to expand...

Typical of these receivers, Those output IC's are very, very sensitive to having the speaker terminals shorted. Glad it's working and that you're enjoying it!
 
sploofer said:
Your hunch was correct! It was a blown IC.

I ended up taking this into a professional and thats what he had found. He replaced it, cleaned it up and now everything works great. I originally planned on doing this myself, but I have zero experience soldering or working with electronics. I wanted to play it safe, especially since they have a warranty and accountability if any accidents happen, which might have happened with me.

Total cost of the repair was $160 and including the $25 I paid for this unit, I can live with paying $185 for a serviced and clean SX-780 that I bought locally, instead of buying off the bay.
Click to expand...


What did he use for a replacement?
 
I'm delighted to hear you got this unit working again. Tonight I re-created a log-on after not being on here for many years. My SX-780, bought new in the late '70s, suffered from the cold solder joint crack and for YEARS I lugged it around after college through several moves until with the advent of the internet and this site where I joined to see if the wonderful world-wide web had an answer. On this site I found detailed info and photos of the common bad solder joint. With nothing to lose, one weekend my son and I removed the bottom cover (scoffing at the "DO NOT REMOVE! No User Serviceable Parts Inside" warning on the panel), located the bad solder joint (we couldn't see anything but a potential hairline crack) and then applied a hot solder gun tip to remelt it. We buttoned the unit back up, flipped it over, connected speakers and powered it up. I described to my son what the "moment of truth" would be: the click of the relay and broadcasted sound. As expected, the unit was fixed and has been working reliably for the seven or so years since. Enjoy your 780!
And my thanks to all members here who helped me get my receiver working again.
 
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