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Pioneer sx780 noisy right channel

guille go

guille go

New Member
Hi, this is my first post

I will try to be clear with what I am going to write, there are people who know a lot here.
I have a pioneer sx780 that I think was never repaired, the capacitors are original elna(all).
the right channel started to make random distortions,
when it starts to "heat up" they disappear.
I hear the noise with headphones too.
can it be a bad solder? q25 q19 and q20 (with heatsink) got hot and desoldered?
I don't have an oscilloscope to see the problem, but I hear it, where should I start testing?

Thank you very much for guiding me towards the solution, I appreciate it
 
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Welcome Aboard!

An oscilloscope should not be necessary to identify the problem(s) with the unit. There are several known common issues with the SX-780, and as long as the covers will be coming for access it would be sensible to address those. The very first thing to be done is to remove any dust and contaminaton from inside the unit. Following that, a thorough cleaning and re-lubrication of switches and controls should be performed. Many times that alone can solve these random issues.
 
tested in aux and FM

while cleaning the interior I found a rest of a wire between pin1 and pin7 of the fuses, could this cause the distortion?
 
guille go said:
tested in aux and FM

while cleaning the interior I found a rest of a wire between pin1 and pin7 of the fuses, could this cause the distortion?
Click to expand...

I am somewhat confused over that statement. Can you restate or define that further, please?
 
sorry but english is not my native language,

Stripping some wire in my workshop some rest fell inside my 780, and I found it deposited between pins 1 and 7 of the fuses.
can that cause any distortion?

express it better?
 
Welcome to Audiokarma!
If it was insulation from the stripped wire it would not cause a problem. If it was a piece of the wire itself, it might, but if no fuses got blown it probably did not. As Watthour said, there are many reasons for distortion in an old receiver. Cleaning the switches and pots is a good first step. De-Oxit D5 is good for switches, De-Oxit F5 Faderlube is good for pots. After that has been done you can look at replacing electrolytic caps and transistors. Here is a link to the list of known problem transistors: Top Ten Worst Transistors - noisy, failure-prone, whatever... and replacements.
 
the distortion is random, and it doesn't seem to be from switches or pots, I'll give you a read.
as I am also going to do a better cleaning of my equipment.
noise could be coming from the speaker jacks??
 
guille go said:
the distortion is random, and it doesn't seem to be from switches or pots, I'll give you a read.
as I am also going to do a better cleaning of my equipment.
noise could be coming from the speaker jacks??
Click to expand...
Unless it is a very bad connection, I doubt it would come from the Speaker jack, if it plays normally most of the time that makes me think not.
When you described that it disappears when the unit 'heats up' that tells me it is probably a transistor, or maybe a capacitor.
You should reflow the solder on Q25, Q20, and Q19 anyway, they are a known problem. They can also benefit from a bigger, better heatsink, especially Q25.
 
can generate that distortion even if they are power supply regulator transistors??
I saw resin bubbles next to those transistors (more on q25)
I'm going to try that solution
thanx!!!
 
If the power supply is not well regulated and is noisy it can cause noise and distortion in audio. Q25 unsoldering itself is one of the most common failures in the SX-780.
 
Good information about the right channel meter.
If Q25 has not been re-soldered, do so.
Do you have an oscilloscope or a signal tracer?
 
You can swap the STK-0050 left and right and see if your noise and distortion switches sides. If so your problem is the STK itself. You will have to unsolder them to refresh the heatsink compound on them anyway (a good idea).
 
I saw those replacements of the stk-0050 in some posts , but here in Argentina I don't get them, but I saw here a Chinese versions of the stk-0050, maybe I bought some for replacement.
I have been testing the sx780 for 3 days and the noise did not return, later I will review the darlington heatsink compound,
could you tell me the theory why only one channel comes out with noise?
Maybe that poor power supply affects the darlington of the right channel because it is more deteriorated

Thank you very much for your help, I appreciate it very much
 
guille go said:
I saw those replacements of the stk-0050 in some posts , but here in Argentina I don't get them, but I saw here a Chinese versions of the stk-0050, maybe I bought some for replacement.
I have been testing the sx780 for 3 days and the noise did not return, later I will review the darlington heatsink compound,
could you tell me the theory why only one channel comes out with noise?
Maybe that poor power supply affects the darlington of the right channel because it is more deteriorated

Thank you very much for your help, I appreciate it very much
Click to expand...

Each channel has a separate signal path through the amp with only power supplies and ground shared between them. If one power pack (STK) is having a noise problem it would not appear on the other channel, unless it is so severe as to affect the power supply.
 
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