• 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

My SX-780 just died!

sbparks

sbparks

Active Member
Hi Folks,

I let my teenage son use my beloved SX-780 that I've had *forever*. Unfortunately, he he a poor job of attaching the speaker wires to the speakers. He's using old speakers with 'way too close' wire terminals. He wasn't careful and had copper strands across the screws that resulted in a short at the speakers.

He claims the receiver powered up and then "all the lights went out".

I said, "What do you mean - It was working perfectly for me!". (I'm sure many us of have had these kind of conversations with teenagers.) To be honest he's a great kid, he just needs to be more thorough at times!.

So here's what I've found: A burnt 4amp fuse that when replaced, burns out again in about 1 second. Before it burns, the receiver's displays light up.

The simple fix didn't work. Poop!

What to check? I'll take my 780 to the repair shop if necessary, but I'd love to teach my son a little about repair if possible. I know that I've had the output devices (STK-0050's) replaced in the past. That may have nothing to do with the problem at hand.

I do appreciate the help!
 
Register to hide this ad
If it blows the fuse right away, chances are one or both of the packs are blown.

I have a 780 myself that I had to replace one of the packs in it due to a blown channel.

I don't remember, but is there more than one fuse inside?

Also do you know which channel had the short?

Scott
 
Hi,

If you want to teach your son repairing, teach him how to replace STK's :D

Anyway, if you look at the schematic (can be downloaded from the AK database) you'll notice that if you desolder pins 2 & 9 of the STK-0050's, they'll not be able to draw current, anymore. Also, a simple ohm-test between the pins 2 & 3 / 8 & 9 should yield shorts on the defective IC.
So, desolder both, test and you'll know which one to resolder and which one to replace. :yes:

Kind regards,

/tri-comp
 

Attachments

tri-comp, you went right to the heart of the topic! :D

That's where I was headed with my questions... you don't beat around the bush do ya?!? :D

Scott
 
Hi,

You're right, no beating...:thmbsp:

Let's just hope the 780 gets on the road again. The repair should be simple enough for someone handy with a soldering-iron, solderwick and a cheap multimeter.

Let's hear the results, please.

/tri-comp
 
Hey Guys, Thanks.

The speaker that was poorly wired was the left one. Does that mean he had that wire attached to the left channel? I wouldn't bet on it!

OK. I can perform the test to see if the power pack was toasted. No problem there.

I'm thinking that I should replace both parts. Here's why: The meter readings are slightly mis-matched when I use the unit AND one of the power packs doesn't match the other (a previous repair job).

Would it make sense to order two packs and just do them together?

Who do you recommend I order from?

Thanks again!
 
Follow up Question:

To perform the test, just desolder 2 and 9 and leave the device on the board (the rest of the pins are still attached)?

In other words, do I need to remove the device to test it? Or can I leave it installed? Would the rest of the circuits allow this test to be performed?

Thanks.
 
Hi,

Yes indeed, you can leave the rest of the pins attached. The dead short you should notice on the defective chip will not be affected by the rest of circuits.

/tri-comp


P.S. Take a look at the attached pdf's. They'll show you the inners of the chip and probably give you some idea of, how the output circuit is constructed.
 
Same thing happened to my SX828 except from ebay, turned on for a few seconds then died, blown fuse then would die and blow fuse instantly afterwards. It was diagnosed with a shorted output transistor. I'm going to attempt to repair it myself by replacing all outputs and their driver transistors.
 
sbparks said:
Hey Guys, Thanks.

The speaker that was poorly wired was the left one. Does that mean he had that wire attached to the left channel? I wouldn't bet on it!

OK. I can perform the test to see if the power pack was toasted. No problem there.

I'm thinking that I should replace both parts. Here's why: The meter readings are slightly mis-matched when I use the unit AND one of the power packs doesn't match the other (a previous repair job).

Would it make sense to order two packs and just do them together?

Who do you recommend I order from?

Thanks again!
Click to expand...


www.electronix.com and at $5.39 each, I would do both.
 
Thanks Mark! Saved that website's home page in my favorites... could come in handy! :yes: :thmbsp:

Scott
 
SILVERAUDIO said:
Whats the difference between the STK-0050 and the STK-0050II?
Click to expand...

A picture is worth 1000 words....

attachment.php
 

Attachments

  • stk0050_stk0050II.jpg
    stk0050_stk0050II.jpg
    28.3 KB · Views: 83
I received my shipment of the two replacement STK-0050 power chips.

I started work on the 780 to test the chips as suggested by SPL db and MarktheFixer. I was able to isolate the pins on the ICs and perform the tests on pins 2&3 and 8&9. One channel tested good (no short) and the other channel (Left channel, happened to be the one that my son wired sloppy) showed these pins as a short.

Just as you folks suggested!

Anyhow - I will replace BOTH chips just for good measure. Thanks so much for the suggestion (and I hope this is all that was wrong with the unit!).

Some follow up questions... This time about soldering: I used the solder wick to pull out the old solder. This was a tedious process, but it did work.

Should I apply flux to the solder wick to get it to 'draw' the solder better?

How can I clean up the board surface before applying the new ICs? I'd like this work to appear clean and professional. As the board is now, there is some reside. I'd like to do a nice neat job.

Thanks Guys!
 

Attachments

  • CIMG0829.JPG
    CIMG0829.JPG
    157.1 KB · Views: 30
  • CIMG0830.JPG
    CIMG0830.JPG
    167.3 KB · Views: 25
  • CIMG0831.JPG
    CIMG0831.JPG
    151.8 KB · Views: 33
  • CIMG0832.JPG
    CIMG0832.JPG
    147.5 KB · Views: 31
That residue is what's known as heatsink compound - it helps transfer heat from the pack
to the heat sink (the large piece of black metal).

You'll need to clean the old stuff off (I use Isopropyl alcohol and a rag) but be careful, that
stuff gets everywhere.

Next, you'll need to clean up the spots where the tabs of the pack solder to.

Then add new heatsink compound, mount the new pack (should see just a very small amount
of compound seep out around the pack after the screws are tightened).

Then resolder the pack connections - you might want to check the emitter resistors as well
(there should be somewhat large white rectangular looking resistors near the packs) - I had
one of these that was bad when I rebuilt my 780.

The heatsink compound can be had at Radio Shack if you have one in your neck of the woods,
otherwise, Parts Express is the way to go.

Here's a couple of links... first one is a great tool for removing old solder... it's heated and works
great (big key here is to get in and out quick, the less heat that stays on the circuit board traces
the better - less chance of lifting a trace).

The second one is the heatsink compound.

Desolder Tool

Heatsink Compound

Hope this helps... others will chime in as well!

Scott
 
SPL db said:
That residue is what's known as heatsink compound - it helps transfer heat from the pack
to the heat sink (the large piece of black metal).
Click to expand...

Scott,

I'm sorry, I wasn't very specific... I meant to ask about cleaning up the solder and the residue from the desoldering process. I was wondering if there is a good way to get the PCB clean to start the soldering process again.

I've got some heatsink compound from an earlier process so I'm OK there.

Thanks.
 
See the first link that I posted... the desoldering tool will work wonders! :yes: :thmbsp:

Scott
 
You must log in or register to reply here.
Back
Top Bottom