• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Assistance with troubleshooting an SX-780 requested!

beatcomber

beatcomber

AK Member
Hello all -

I have an SX-780 that has been recapped (not by me!). It was working flawlessly since the spring when I bought it, but a few days ago I moved it by car from one location to another, and when I set it up I discovered a newly fuzzy/distorted left channel. :(

Here's what I can confirm:
  • Distortion only with speakers - headphones sound fine.
  • Distortion happens with either A or B terminals
  • I have eliminated the cables and the speakers as the source of the distortion - it's definitely the SX-780
  • All input sources produce the same results
  • The solder joints on the wires to the speaker terminals appear secure
In my uneducated opinion, something was probably knocked loose during transit, and it's probably a simple fix.

Unfortunately my electronics repair ability is extremely rudimentary - I can't read a schematic and can barely use a soldering iron - so I'm hoping there are some simple things I can check before bringing it to a pro.

Thank you in advance.
 
Last edited:
Register to hide this ad
beatcomber said:
Hello all -

I have an SX-780 that has been recapped (not by me!). It was working flawlessly since the spring when I bought it, but a few days ago I moved it by car from one location to another, and when I set it up I discovered a newly fuzzy/distorted left channel. :(

Here's what I can confirm:
  • Distortion only with speakers - headphones sound fine.
  • Distortion happens with either A or B terminals
  • I have eliminated the cables and the speakers as the source of the distortion - it's definitely the SX-780
  • All input sources produce the same results
  • The solder joints on the wires to the speaker terminals appear secure
In my uneducated opinion, something was probably knocked loose during transit, and it's probably a simple fix.

Unfortunately my electronics repair ability is extremely rudimentary - I can't read a schematic and can barely use a soldering iron - so I'm hoping there are some simple things I can check before bringing it to a pro.

Thank you in advance.
Click to expand...
Further proof that a complete recap does not ensure a properly restored piece of electronics. You might have a faulty Darlington Power Pack.
 
beatcomber said:
Thanks!
Click to expand...
I see you're in Massachusetts. No guarantees, but can put you in touch with a great YouTube technician who did some fantastic work for me. He's also in Mass. No clue as to his workload, but he will work on vintage receivers and post the work onto YouTube. A
 
It's coming out protection.
With headphones it sounds good
Which means the relay is working.
The only component between the relay and the speaker terminals are the speaker select switches.
It could be those switches.
The headphones are routed directly from the relay so they are confirming the amps are operating correctly.
Must be the switch(s). Deoxit
 
Arthur Smith said:
I see you're in Massachusetts. No guarantees, but can put you in touch with a great YouTube technician who did some fantastic work for me. He's also in Mass. No clue as to his workload, but he will work on vintage receivers and post the work onto YouTube. A
Click to expand...

Thank you for the offer! I'd be interested.

zebulon1 said:
It's coming out protection.
With headphones it sounds good
Which means the relay is working.
The only component between the relay and the speaker terminals are the speaker select switches.
It could be those switches.
The headphones are routed directly from the relay so they are confirming the amps are operating correctly.
Must be the switch(s). Deoxit
Click to expand...

If it were really the switches that were the problem, wouldn't the issue be confined to one set of speaker terminals, and not both? Nevertheless, I'll give it a try!
 
:dunno:
Headphones sound good.
The headphones would cause less of a load on the amps.
Back to basics Offset and Bias readings?
Maybe its something more sinister.
 
Headphones don't draw as much current as speakers do and can easily run on the drivers of normal amps. Not sure if the Powerpacks will be the same.

Craig
 
llwhtt said:
Headphones don't draw as much current as speakers do and can easily run on the drivers of normal amps. Not sure if the Powerpacks will be the same.

Craig
Click to expand...

For precisely the same reason, marginal contacts in the speaker switches (and relay) can drive headphones acceptably but might not handle the load of speakers. Back to basics - Clean the switches first. Occam is waiting for his shave...
 
I met with the tech that @Arthur Smith recommended and handed over my SX-780, and he just got back to me.

Sure enough, one of the Darlingtons is going south. I just gave him the green light to order a matched pair so that he can preventatively replace them both.
 
HAve him save the GOOD old one as a spare and make sure you get it back.

EDIT 0020 15JAN17



Audio Lab of Ga. and B+D Enterprises usually have the best packs. As Sanyo stopped production of the packs over 20 years ago, there are no SANYO packs out there. Everything is now Chinese fakes. Some are better than others. EBAY IS ABSOLUTE LAST RESORT!.

http://www.audiolabga.com/data_html/STK0050.html One in Stock.
http//www.bdent.com Out of Stock that I can tell. You might call and see.

Also two of the guys collaborated on a replacement circuit for the 780. It replaces the power packs but requires the fabrication of a heatsink/attachment point for the board. See this thread for details.http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/ It would cost more, but the benefits outweigh the cost greatly,especially if you plan on keeping the unit. The BOM and instructions for the board and heatsink are included in the zip file on the 1st post. Read the whole thread.
 
Last edited:
beatcomber said:
I met with the tech that @Arthur Smith recommended and handed over my SX-780, and he just got back to me.

Sure enough, one of the Darlingtons is going south. I just gave him the green light to order a matched pair so that he can preventatively replace them both.
Click to expand...
Glad that you connected with him- he's the absolute best. Is he going to video the repair?
 
larryderouin said:
HAve him save the GOOD old one as a spare and make sure you get it back.

EDIT 0020 15JAN17



Audio Lab of Ga. and B+D Enterprises usually have the best packs. As Sanyo stopped production of the packs over 20 years ago, there are no SANYO packs out there. Everything is now Chinese fakes. Some are better than others. EBAY IS ABSOLUTE LAST RESORT!.

http://www.audiolabga.com/data_html/STK0050.html One in Stock.
http//www.bdent.com Out of Stock that I can tell. You might call and see.

Also two of the guys collaborated on a replacement circuit for the 780. It replaces the power packs but requires the fabrication of a heatsink/attachment point for the board. See this thread for details.http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/ It would cost more, but the benefits outweigh the cost greatly,especially if you plan on keeping the unit. The BOM and instructions for the board and heatsink are included in the zip file on the 1st post. Read the whole thread.
Click to expand...

Thanks, Larry! I will forward this info!
 
Your tech is clearly scamming you. A matched pair? That sounds so glamorous but actually means he is intending to pocket your original part and put in a pair of fakes.

Get your receiver back and part it out. There's no saving this one. Original, functioning STKs do not exist. Do not listen to these people on AK claiming otherwise, they are delusional. Your receiver is done for.
 
ZOOM said:
Your tech is clearly scamming you. A matched pair? That sounds so glamorous but actually means he is intending to pocket your original part and put in a pair of fakes.

Get your receiver back and part it out. There's no saving this one. Original, functioning STKs do not exist. Do not listen to these people on AK claiming otherwise, they are delusional. Your receiver is done for.
Click to expand...

I think you don't know what your talking about. No one is scamming nobody. Im sure he will get his original parts back and was made well aware that the replacement parts being used are reproductions as they don't make the originals anymore. I'm sure it will work just as good as the originals if not better. No need to scrap the unit or part it out. That's just ludicrous.
 
Last edited:
I doubt its the power pack. Those either work or don't for the most part.

My guess is dirty contacts in the relay or one of the switches is really being a pain.
I had to take the function switch apart on one of those once and polish the metal parts to get it to work right.

Recapping is not a guarantee of life long reliability. Transistors and diodes and resisters can still fail too. It may be one of those. I have fixed units that tech said were shot but turned out to be a simple part ahead of the power pack.

Then, there is the bad solder joint problem. Since it was recapped, we know someone has been in there and those are known to develop bad joints on some spots. I have seen work by people that can't solder properly.

Wishing you and your tech luck.
 
Last edited:
I was going to edit my post #13 again and ask you to mark the packs somehow that can't be wiped off and get BOTH of them back. This way if he does replace the packs, you have a way of checking up on him. Not saying he's dishonest but as touchy as these packs are, you don't want him saying "I replaced the packs" and you find out later that they weren't. Ask him for all replaced parts.

Disregard anything that Zoom has to contribute. I've never seen a semi-positive or positive post from him. Combine the Grinch, Satan, Lionel Barrymore's character in "it's a wonderful life", and any number of negative based personalities, and you have Zoom. He contributes nothing to the forums except for negative comments, and spurious ideas at best.
 
You must log in or register to reply here.
Back
Top Bottom