Pioneer Sx780 not working AC power problems.

[merlynski]

Is the transformer you are using to step down the AC voltage an isolation transformer, or an autotransformer? What is its power rating?

 

[Carlkits]

Is the transformer you are using to step down the AC voltage an isolation transformer, or an autotransformer? What is its power rating?

Autotransformer.
750vac in 60 Hertz.

 

[merlynski]

I ll try, but probably the old wiring of the building is the problem.

I have to agree with this.

Do you have any way to borrow an isolation transformer from somebody? An autotransformer does not isolate your amp from any RF or DC that may be present on your mains circuit. Are there any devices with switch-mode power supplies on the mains circuit? You may be having distortion of the AC waveform on your power line. Did the Electrician that inspected your wiring do a true-RMS measurement of the voltage at the time the problem is occurring? An isolation transformer, by itself, may not fix an AC supply distortion problem.
I am going to speculate that in the evening more people are home and using their computers and other devices that have switch-mode supplies and the AC mains cannot handle the load distortion. Does the problem go away late at night after most people go to bed?

I did some reading about AC power in Brazil and it appears to have gone through many changes over the years.

 

[Carlkits]

I have to agree with this.

Do you have any way to borrow an isolation transformer from somebody? An autotransformer does not isolate your amp from any RF or DC that may be present on your mains circuit. Are there any devices with switch-mode power supplies on the mains circuit? You may be having distortion of the AC waveform on your power line. Did the Electrician that inspected your wiring do a true-RMS measurement of the voltage at the time the problem is occurring? An isolation transformer, by itself, may not fix an AC supply distortion problem.
I am going to speculate that in the evening more people are home and using their computers and other devices that have switch-mode supplies and the AC mains cannot handle the load distortion. Does the problem go away late at night after most people go to bed?

I did some reading about AC power in Brazil and it appears to have gone through many changes over the years.

Unfortunately the only way is buying one, almost 100 dollars, and maybe is not the solution.
The electrician came during the day, but we inspected wiring and is out of regulation.
They should be 00gauge 10mm e they are 10 gauge 2.5mm.

The receiver now have one exclusively outlet connect in a exclusively circuit breaker, and I disarmed all the circuits and still clicks.

Here in Brazil, between 5 PM to 9 PM ac power is more expensive, we call it the Rush hour, it's a way to reduce power consumption by using a higher tax.

But it's rush hour, everybody is going to use electric devices.

I guess the only way is re wiring.

 

[petehall347]

some sort of power line conditioner / filter might help . we can only guess what is going on here but i have to agree with mark about the ac detect on the protect ic . shame we cant see what an oscilloscope can see . that would help a lot here .
if i remember rightly the ac mains voltage doesn't sag so something else is going on . maybe the frequency is shifting or its just noise from motors starting up .

 

[Carlkits]

some sort of power line conditioner / filter might help . we can only guess what is going on here but i have to agree with mark about the ac detect on the protect ic . shame we cant see what an oscilloscope can see . that would help a lot here .
if i remember rightly the ac mains voltage doesn't sag so something else is going on . maybe the frequency is shifting or its just noise from motors starting up .

I use one filter line with dps, now is 10.15 pm and is clicking .

Probably gonna have to rewired from the main box on the floor of the building.

I really don't want to give up on this Sx780, mine other Sx626 is working fine on the same outlet.

 

[markthefixer]

I have been researching this most of the afternoon.
Scoping the power would be my fondest wish, but I realize that is nearly impossible.
I have encountered this type of fault here in the forums at least twice, and they involved the PA3004 protection chip..
Did not necessarily understand WHY the fixes worked, but they WORKED.

One was with AK'er Brivan and his SX-980. He had a 21 VAC point being monitored by the protect.
The protect circuit had a series 1500 ohm resistor, and a grounded 10k resistor - making a voltage divider of 0.87 % . 21v x 87% is 18.2 VAC.
THAT point was connected to a 470,000 ohm resistor which was connected to pin 7 of the PA3004 chip.

What was done was the 1500 ohm resistor was bypassed/shorted and the 10,000 ohm resistor removed.
Thus feeding 21v AC to the 470,000 ohm series resistor and pin 7.

I propose disconnecting R320 (1 megohm : 1,000,000 ohms) from ground,
leaving R319 as a 1 megohm series resistor from pin 7 to approx 49 volts AC.



My previous investigations suggested strongly that pin 7 of the PA3004 has an impedance of approximately 10,000 ohms, so the unit may NOT even turn on. (0.485v AC)
In that case take the opened end of R320 and connect it to pin 20, paralleling the 1 meg resistor with another 1 meg resistor. Thus turning the pair into a 500,000 ohm resistor.(0.978v AC)

 

[Carlkits]

I have been researching this most of the afternoon.
Scoping the power would be my fondest wish, but I realize that is nearly impossible.
I have encountered this type of fault here in the forums at least twice, and they involved the PA3004 protection chip..
Did not necessarily understand WHY the fixes worked, but they WORKED.

One was with AK'er Brivan and his SX-980. He had a 21 VAC point being monitored by the protect.
The protect circuit had a series 1500 ohm resistor, and a grounded 10k resistor - making a voltage divider of 0.87 % . 21v x 87% is 18.2 VAC.
THAT point was connected to a 470,000 ohm resistor which was connected to pin 7 of the PA3004 chip.

What was done was the 1500 ohm resistor was bypassed/shorted and the 10,000 ohm resistor removed.
Thus feeding 21v AC to the 470,000 ohm series resistor and pin 7.

I propose disconnecting R320 (1 megohm : 1,000,000 ohms) from ground,
leaving R319 as a 1 megohm series resistor from pin 7 to approx 49 volts AC.

View attachment 2311002

I only disconnect R320? Gonna ask help from a cousin who is good with electronics repair.

 

[markthefixer]

yes. perhaps you should make the 500,000 ohm series resistor at the same time?

 

[Carlkits]

Well guys

Is 4.38 AM, the whole neighborhood is sleeping and I turned it on and its clicking, so is low voltage from the provider of power.

I bought one multimeter to make those readings, Amazon will deliver tomorrow, and I'm searching oscilloscope for rental.

I'm gonna try myself follow the instructions and I comeback with the results.

 

[Carlkits]

Good night guys!
Thursday I ll have one oscilloscope plugged in the receiver.
My cousin is gonna bring the oscilloscope, first gonna record the results and post in here.

Something else I should look?

Should I make the R320 suggestion before the oscilloscope?

I see some light in the end of this thread!

 

[markthefixer]

I believe you should try the R320 suggestion, perhaps even putting R319 across it to make 500,000 ohms instead of 1,000,000 ohms

 

[Carlkits]

Good night to all

Gonna start the fix in the unit.

Wish me luck.

I took pictures of the soldering near the Stk0050

 

[Carlkits]

Trouble shoot and fix the problem causing the relay to drop before doing major parts replacement. It is probably DC Offset drift OR, one of the most common problems with the SX-780: Q25 runs very hot and the solder on the legs gets heat stressed and becomes intermittent. Reflow the solder joints on Q25, Q20 and Q19.

View attachment 2304442

View attachment 2304449

Adding more heatsinking, to Q25 especially, will help with preventing this in the future.

Just for checking
Are those ok?

 

[merlynski]

Hard to tell from the picture, nothing obvious on the transistors. Would not hurt to reflow them.

 

[Carlkits]

Good night guys

Unfortunately I have to deal with the lost of a dear member of my family and did not make the R320 fix.
I bought a soldering iron and I'm practicing on a older equipment.

Wednesday gonna try find the r320 and trying to succeed.

I I'm exchange the wiring of my apartment, first I exchanged all the circuits breakers, and did not make any difference, now I'm gonna replace the wiring.

Thanks in advance for all the help !

 

[merlynski]

Take care of your family matters first, we will be here for you . . .

 

[Carlkits]

Hello

I took the unity to the repair shop and they did exactly as suggested, and reflow everything.
When I got home I plug in power and was ok, after one hour I hooked up all the cables and speakers and start to clicking again.
Unfortunately I'm back to the Ak lab.

 

[markthefixer]

The R320 & R319 suggestions?

 

[Carlkits]

The R320 & R319 suggestions?

Yes