[HELP] SX-939 - No output audio

joaokmo

New Member
Hello folks! I bought a Pioneer SX-939 from a seller who initially claimed that the unit was working in good condition. Then the unit worked well in the first few days, but soon thereafter presented a crackling sound on the loudspeakers (see video attachment). So I thought that one of the amplification phases of the sine wave could be compromised, as I do not have an oscilloscope, I was not able to verify it. Then I went straight to the replacement of the drivers of the section of power amplifier, being the pairs of transistors: B536 to A940 and D381 to C2073.
After the replacement, the output sound was at a very low volume! After turning off the unit and on hours later, it did not even have any output at all.
PS: I've noticed that there is no relay click after all... (i suppose that there is no power going on through the power amp board?)
Hope someone with their expertise could help me to fix it, i would be very grateful. =]

Attachtment:
 
Report1: I managed to adjust the BIAS to 50mv and DC Off-set to 0v on the left channel, but i couldn't adjust the DC Off-set of the right channel, i just turn the trimpot in both ways but the multimeter still displays -38.14v. The BIAS in the right channel is fine, adjustable... (I followed the service manual instructions). Maybe the DC trimpot is faulty? Any ideas?

Note: I'm not applying any 8 ohms dummy resistor at speakers outputs, does it influence?
 

Attachments

  • service manual section.JPG
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make a list of amp board and protect board voltages .
also best make a list of power supply voltages too .
p.s welcome to AK
 
make a list of amp board and protect board voltages .
also best make a list of power supply voltages too .
p.s welcome to AK
Hi, Pete! Thanks for your responsiveness. I'm little lost here... Do you mind tell me where exactly i should put the multimeter pointers? (sounds newbie, i know) lol. I mean, any specific terminal? I just want to get the things right.
 
look in the manual . find the schematics . around the edge of the boards are pins with wires attached . all have number identification .
set meter to DC volts .black probe goes to shiny chassis .red to each pin . record and post the voltages .
saying that there are actually some AC voltages on the power supply . next to the fuses .
what prompted you to change the transistors ?
 
look in the manual . find the schematics . around the edge of the boards are pins with wires attached . all have number identification .
set meter to DC volts .black probe goes to shiny chassis .red to each pin . record and post the voltages .
saying that there are actually some AC voltages on the power supply . next to the fuses .
what prompted you to change the transistors ?
Ok. I figured it out, then i will post the DC voltages of each terminal here. I replaced only the driver transistors pairs to their equivalents, i done that mainly because the symptom that i related at the first message (see the YT video). And there is no relay click at all...
 
ok the original problem is most likely within the power supply . concentrate on those voltages first ..
then we need to find what went wrong with the now bad channel .
important bit .....
do you have what is known as a dim bulb tester DBT ? look it up .it is easy to build and will save things burning up in less than a second .
 
I've done the measurement, and here is the list:

PINS:
1/16 = 40v
2 = 40v
3 = 0v (GND)
4 = 31.88v
5/19= 31.88v
6 = 0v (GND)
7/22= 41v
8= 41
9= 0v (GND)
10= 0v (yellow wire that comes from the relay and this is the Pin that should be monitored with the multimeter to adjust the DC Offset as shown in the service manual)
11= 0v (yellow wire that comes from the protection board) (don't know if this should be 0v)
12= 0.014v (white wire that comes from the protection board
13= 0.014v (purple wire that comes from the protection board)
14/29= 50v (red wire that comes from the secondary of the transformer passing through filter caps positive side¹)
15/30= 50v (orange wire that comes from the secondary of the transformer passing through filter caps positive side²)
18= 0v
24= (GND)
25= -38.20V (green wire that comes from the relay and this is also a pin to set the DC offset to 0v)
26= (same trail as 25 pin)
27= -38.30v (black and white wire that comes from the protection board)
28= -38.18v (silver wire that that comes from the protection board)
 

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check the ohms of the big white emitter resistors . should be 0.5 ohms .. if ok check other resistors comparing channel to channel . any that appear much different un-solder one side and check again .
 
check the ohms of the big white emitter resistors . should be 0.5 ohms .. if ok check other resistors comparing channel to channel . any that appear much different un-solder one side and check again .
Ok, Pete. Checked it now, they're all working good... measuring 0.5 ohms on both channels. Thanks!
 
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