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SX-780 issues fixed now distorted sound

Z

Zeroedin

New Member
Title says it all. I had an issue with the Darlington packs one of them blew out because of my stupidity one of the wires touched for a speaker connection. Lesson learned... After reading several threads on this site about replacing them, testing for resistance, so on and so forth, I decided to give it a shot in replacement.

After testing several components mentioned in other threads, I deduced that the Left channel of the Pioneer was shot.

So I went ahead and tried to find a reputable seller that sold one of the Darlington packs the stk-0050. End up purchasing two of them.

Once they arrived from Amazon, costing 22 dollars each, I went ahead and desoldered the old dead Darlington, fired up the reciever and thus no more 4A fuse pop and protection relay clicked. I then did what anyone would do and listened to a song and turned the unit off

Reinstalled a new Darlington and this thing was made my Pioneer so distorted, on left channel... It was unbearable... Like a dying seel in the desert squelching....ow it's stuck in protection mode after installation.

Is there something I might have missed? Thanks for advice....
 
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check the 0.22 ohm 2 watt resistors, the packs can take them out.

gotta desolder one leg and air gap it from the circuit to accurately measure it's resistance.
 
markthefixer said:
check the 0.22 ohm 2 watt resistors, the packs can take them out.

gotta desolder one leg and air gap it from the circuit to accurately measure it's resistance.
Click to expand...
I'll take a look when I get home. I should add, that the channel was slightly distorted before the Darlington died. I remember having to tap the side of the reciever to get the signal to go through undistorted. Maybe connections need Deoxit?

Also... I figured it's possible the Darlington from Amazon was a counterfeit... Which seems to be quite common.

I did see a discreet stk-0050+ IC on eBay for 59.99. would the problem persist if this module was installed and 2w resistors are in check?
 
Sounds like the typical protection relay failing. Try tapping on the top of it see if distortion gets better/worse.
 
EastPoint said:
They are all counterfeit. These haven't been made in 30+ years.

Clean the controls with Deoxit D5. All the controls. Switches too.
Click to expand...
It was counterfeit and completely junk. Pulled it out and the relay clicked once more.

What other options are there to replace the Darlington? (I seen an STK0050+ on eBay for 60)
 
ebay, delta9electronics is our own rcs16
and the modules / kit STK0050
was our answer to the solid wall of dubious quality implementations and counterfeits of the module.

search for stk0050 kit to get right to rick's listings.
 
markthefixer said:
ebay, delta9electronics is our own rcs16
and the modules / kit STK0050
was our answer to the solid wall of dubious quality implementations and counterfeits of the module.

search for stk0050 kit to get right to rick's listings.
Click to expand...
Thank you, exactly what I was looking for. I'll pick up a set.

Are there any additional modifications to the pioneer required after the install of these chips?
 
So I got the stk-0050+s ready to go. I ran into another issue. My relay is becoming intermittent. I installed one of the STK-0050+ modules, and went to test... Now it's unit is stuck in protection mode. Go figure...

I got frustrated with it and left for a couple of hours, ate some dinner, came back and fired up the pioneer. Wouldn't you know it? CLICK goes the relay.

Turned it off. No click after.

Any ideas? I'm probably certain it needs a new relay. Should I check anything else as well?
 
is Q26, the relay driver transistor original? They do get tired and become heat sensitive. IMO they are too small.
That is, if the protect is unjustified, i.e.
the PA3004 is calling for the relay to be engaged and it isn't..

Replace with ksc2690.

Have you checked the three problematic power supply transistors?
(they're standing up and heat sunk) They get hot and bust their solder joints.
 
markthefixer said:
is Q26, the relay driver transistor original? They do get tired and become heat sensitive. IMO they are too small.
That is, if the protect is unjustified, i.e.
the PA3004 is calling for the relay to be engaged and it isn't..

Replace with ksc2690.

Have you checked the three problematic power supply transistors?
(they're standing up and heat sunk) They get hot and bust their solder joints.
Click to expand...
Are you talking about Q25, as described in this webpage perhaps? https://www.olegvintageaudio.com/2020/12/pioneer-sx-780-receiver-restoration.html?m=1
 
Is it possible to have two different revisions of the pioneer?

Here are pics of all the components in mine. I seen a few differences.
 

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