Pioneer SX-780 receiver problem

[CHale]

Hello--I've got a "vintage" Pioneer SX-780 stereo receiver that has
bitten the dust. My son said it just faded out over a couple of
seconds and quit; we found the 4A slo-blo fuse out when we opened it
up. (this unit has a bunch of fuses on a board, but I think this is
the biggest one?) I replaced it and turned it on, and the fuse blew
again and smoke appeared around, I'm pretty sure, one of the two
"Darlington Power Pack STK-0050" devices heat sunk to the large main
sink in the center of the unit. I disconnected the two large orange
and red wires leading from the fuse board to the Darlington area, and
saw 78 V DC there, with polarity nominally correct for the Darlington
per the silkscreen symbols on the PCB. With those two wires still
disconnected, the fuse no longer blows and things all power up (all
the lights and such; didn't check to see if I had a "single channel
stereo" situation now, but I could do that).

What thinkest thou, electronics guys? Think I just need a new one of
those Darlington blocks? (are those things pretty easily found?)
Or...? Thanks and I look forward to your advice.

C. Hale, Lafayette CO

 

[David]

If the STK-0050 is really bad, good luck finding one. They are very scarce
and as a result will cost you when you find one.

 

[Mark D. Zacharias]

They can still be had. I get them from East Coast Transistor Parts.

800-645-3516

If there was smoke, there may be another part in that area bad now -
probably a resistor.

Mark Z.

 

[Tim Schwartz]

Mr. Hale,

I'd agree with the other poster's that one of the STK IC's has probably
shorted. While (as suggested in another reply) the voltage regulator
almost certainly could use a resoldering, I've never seen these bad
connections cause the fuse to blow.

An easy way to see if one of the STK's failed is to put your voltmeter
into the lowest OHM range (x1 ideally) and measure adjacent pins like
1-2, 2-3, 8-9, etc. and compare the 2 IC's. If one of them has a dead
short where the other one does not, then you've likely figured out which
one is bad. Don't forget to order some heat sink compound (heat sink
grease) to put between the replacement IC and the heat sink, or the new
one will fail too. It might not be a bad idea to carefully add some
more compound under the remaining IC as well, as sometimes the factory
was too sparing with the stuff.

When installing the new IC torque the 2 mounting screws firmly and
evenly, but not too tight, as if you manage to flex the metal plate of
the IC, you are likely to damage it.

Regards,
Tim Schwartz
Bristol Electronics

 

[El Meda]

Mr. Hale,

I'd agree with the other poster's that one of the STK IC's has probably
shorted. While (as suggested in another reply) the voltage regulator
almost certainly could use a resoldering, I've never seen these bad
connections cause the fuse to blow.

An easy way to see if one of the STK's failed is to put your voltmeter
into the lowest OHM range (x1 ideally) and measure adjacent pins like
1-2, 2-3, 8-9, etc. and compare the 2 IC's. If one of them has a dead
short where the other one does not, then you've likely figured out which
one is bad. Don't forget to order some heat sink compound (heat sink
grease) to put between the replacement IC and the heat sink, or the new
one will fail too. It might not be a bad idea to carefully add some
more compound under the remaining IC as well, as sometimes the factory
was too sparing with the stuff.

When installing the new IC torque the 2 mounting screws firmly and
evenly, but not too tight, as if you manage to flex the metal plate of
the IC, you are likely to damage it.

Regards,
Tim Schwartz
Bristol Electronics

If you can't find the STK0050, you can use the STK0080 instead. Beware
of the fake STK's: if it's too cheap, it's probably fake.

 

[CHale]

If you can't find the STK0050, you can use the STK0080 instead. Beware
of the fake STK's: if it's too cheap, it's probably fake.
---
Ing. Remberto Gomez-Meda <[email protected]>
http://ingemeda.tripod.com/
INGE - Ingenieria Electronica.
Puerto Vallarta, Jalisco, Mexico.

Thanks for everyone's feedback so far; it's invaluable. I'll start
looking into repair accordingly. I've looked up the SX-780 on eBay
and entire, functional units are going for about $65-70...so, I guess
it would be pretty foolish to spend more than that, frankly. It's
such a pretty box, tho, I hate to abandon it. At any rate, I'll
proceed with looking for the Darlingtons, and let you all know how
this goes. Thanks again--Charley