I have a Pioneer SX20 stereo receiver on the bench for repair. It is shutting down due to DC Voltage on the Outputs (Both Channels have 4.58V on the speaker terminals)
I have attached a Screenshot of the schematic.
I have determined that the reason I have 4.58 V on the output is because the base voltage is too High (8.66V) on the Base Pin of the D2390 Transistors and I am getting reverse voltage drop across Zener diodes DZ303 and DZ306 from the base of each D2390 Transistor to the 0.27 Emitter resistors which are directly connected to the speaker terminals.
I need to establish why this Base voltage is this high. Im not sure what governs this voltage. I do know that Q305L/R are the temperature compensating transistors which mount to the heatsink but as they seem to test out ok as do the main output transistors, Im not sure which further tests I should be doing. Can anyone suggest where this Base voltage is coming from and possible reasons for it being too high (I assume this is too high, as the Zener is conducting and giving voltage on the output, which im sure is not supposed to happen under normal conditions. Not all that clued up with solid state so doing this as a learning exercise. Hope someone on here has more of an idea than me at this point to enable me to continue working through the issue.
Thanks!
(incase the Attached Screenshot is not visible)
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html
Channel 1: at the terminals with the D2390 removed (Positive Rail Transistor)
E 4.58v
B 8.66v
C 59.3v
Channel 1: at the terminals with the B1560 removed (Negative Rail Transistor)
E 4.58v
B 0.5v
C -59v
Channel 2: at the terminals with the D2390 removed (Positive Rail Transistor)
E 4.58v
B 8.74v
C 59.3v
Channel 2: at the terminals with the B1560 removed (Negative Rail Transistor)
E 4.58v
B 0.5v
C -59v
I have attached a Screenshot of the schematic.
I have determined that the reason I have 4.58 V on the output is because the base voltage is too High (8.66V) on the Base Pin of the D2390 Transistors and I am getting reverse voltage drop across Zener diodes DZ303 and DZ306 from the base of each D2390 Transistor to the 0.27 Emitter resistors which are directly connected to the speaker terminals.
I need to establish why this Base voltage is this high. Im not sure what governs this voltage. I do know that Q305L/R are the temperature compensating transistors which mount to the heatsink but as they seem to test out ok as do the main output transistors, Im not sure which further tests I should be doing. Can anyone suggest where this Base voltage is coming from and possible reasons for it being too high (I assume this is too high, as the Zener is conducting and giving voltage on the output, which im sure is not supposed to happen under normal conditions. Not all that clued up with solid state so doing this as a learning exercise. Hope someone on here has more of an idea than me at this point to enable me to continue working through the issue.
Thanks!
(incase the Attached Screenshot is not visible)
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html
Channel 1: at the terminals with the D2390 removed (Positive Rail Transistor)
E 4.58v
B 8.66v
C 59.3v
Channel 1: at the terminals with the B1560 removed (Negative Rail Transistor)
E 4.58v
B 0.5v
C -59v
Channel 2: at the terminals with the D2390 removed (Positive Rail Transistor)
E 4.58v
B 8.74v
C 59.3v
Channel 2: at the terminals with the B1560 removed (Negative Rail Transistor)
E 4.58v
B 0.5v
C -59v
Attachments
Problem Resolved.
If anyone ends up in this same position feel free to message me and I’ll explain what I found and did to rectify this issue.
If anyone ends up in this same position feel free to message me and I’ll explain what I found and did to rectify this issue.
Or post it here so someone in the future searching can find the answer without having to track you down?
O.K. So...
I got lucky picking up on a potential between Analogue Gnd and Digital Gnd. This Value happened to be exactly the DC offset on the ouput.
What a ridiculous design this amplifier is....
It relies on the removable rear panel and the sheet metal screws to tie the analogue and digital grounds together. Without that rear sheet metal cover fitted while I’ve been fault finding, the Digital ground has not been tied to the star ground point. It has been Capacitively coupled only.
I Made a jumper wire up to prove the theory and the DC offset is gone. and IC 605 is now functioning too…
It is passing audio on both channels through my workshop speakers.
I have not studied a schematic this hard before and I have certainly learned a few things from this one.
To think that the amp may have suffered only from an intermittent electrical connection through its back plate connecting to the main chassis and has then created this floating digital supply voltage causing the dc offset through the drive circuitry which has then locked the amp into DC protect. Of course having the amp open with the back off it, was never going to clear.
I intend to clean the protective coating from the areas of sheet metal where the critical electrical connections are or leave a permanent wired connection in place so that Fault finding can happen with the cover off.
Thanks to the few of your suggestions.
Much appreciation.
Harry
I got lucky picking up on a potential between Analogue Gnd and Digital Gnd. This Value happened to be exactly the DC offset on the ouput.
What a ridiculous design this amplifier is....
It relies on the removable rear panel and the sheet metal screws to tie the analogue and digital grounds together. Without that rear sheet metal cover fitted while I’ve been fault finding, the Digital ground has not been tied to the star ground point. It has been Capacitively coupled only.
I Made a jumper wire up to prove the theory and the DC offset is gone. and IC 605 is now functioning too…
It is passing audio on both channels through my workshop speakers.
I have not studied a schematic this hard before and I have certainly learned a few things from this one.
To think that the amp may have suffered only from an intermittent electrical connection through its back plate connecting to the main chassis and has then created this floating digital supply voltage causing the dc offset through the drive circuitry which has then locked the amp into DC protect. Of course having the amp open with the back off it, was never going to clear.
I intend to clean the protective coating from the areas of sheet metal where the critical electrical connections are or leave a permanent wired connection in place so that Fault finding can happen with the cover off.
Thanks to the few of your suggestions.
Much appreciation.
Harry
