Hi All. I'm looking for some expert tech troubleshooting advice on a Pioneer SX20 Amplifier. The initial issue was that I just had a standby light and would not turn on.
The first thing that I found while probing around was that a small transistor was not getting base voltage to switch on the main 240v relay for the main power transformer. After doing more research I have found that the base voltage is low due to it being in protection mode - which makes sense. I have located the service manual and found that there is a way to clear the fault from the memory and reset the amplifier. This will allow it to turn on long enough to bring up a menu to see what condition the fault is (ASO detect, DC detect, Over Temp Protect, or power protect) The read out is DC Detect every time!
I have removed a zero Ohm Link which couples the DC detection circuit to the speaker terminals and sure enough the amplifier will stay on, enabling me to continue troubleshooting. I have found that there is around +5V DC on all of the speaker terminals (Speakers A- L&R, Speakers B- L&R)
The Output Transistors all test out O.K. I have measured these out of circuit, as do the ceramic 0.22ohm Emitter Resistors test o.k.
I have taken some voltages with all four of the Output transistors removed to help eliminate where the issue is coming from and this is what I get:
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
What stands out to me is the 8ish volts on the base connection on both channels of the D2390. I assume that this is not normal. I am just not experienced with solid state amplifiers to know what to look for here. I picked up this amplifier as non working yesterday as a fault finding project and hoping to learn something from this along the way. I just need a little guidance at this point if anyone is willing to give me any suggestions. I cant upload the PDF to this as its too large, so instead here is the link where it can be downloaded or viewed. https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html
Thanks in advance! Cheers.
The first thing that I found while probing around was that a small transistor was not getting base voltage to switch on the main 240v relay for the main power transformer. After doing more research I have found that the base voltage is low due to it being in protection mode - which makes sense. I have located the service manual and found that there is a way to clear the fault from the memory and reset the amplifier. This will allow it to turn on long enough to bring up a menu to see what condition the fault is (ASO detect, DC detect, Over Temp Protect, or power protect) The read out is DC Detect every time!
I have removed a zero Ohm Link which couples the DC detection circuit to the speaker terminals and sure enough the amplifier will stay on, enabling me to continue troubleshooting. I have found that there is around +5V DC on all of the speaker terminals (Speakers A- L&R, Speakers B- L&R)
The Output Transistors all test out O.K. I have measured these out of circuit, as do the ceramic 0.22ohm Emitter Resistors test o.k.
I have taken some voltages with all four of the Output transistors removed to help eliminate where the issue is coming from and this is what I get:
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
What stands out to me is the 8ish volts on the base connection on both channels of the D2390. I assume that this is not normal. I am just not experienced with solid state amplifiers to know what to look for here. I picked up this amplifier as non working yesterday as a fault finding project and hoping to learn something from this along the way. I just need a little guidance at this point if anyone is willing to give me any suggestions. I cant upload the PDF to this as its too large, so instead here is the link where it can be downloaded or viewed. https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html
Thanks in advance! Cheers.
Your link triggers a malware warning, but I when I tried again:
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html?page=51#manual
The first suspects are C302(LR) and C306(LR) which are DC blocking caps for the power amp, however caps usually dry up and go open, not short. Open decoupling caps could cause oscillations which may cause a DC offset. Such caps include C340(LR), C342(LR) and C344(LR). It would be useful to know if it passes audio signal, and a scope would reveal any oscillations.
This amp also has 6+ linear voltage regulators, which are also prone to failure:
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html?page=13#manual
Fortunately, these regulators can be quickly checked by measuring their output voltage. Just be careful not to short them with your meter probe. Given that both channels have the same problem, the power supply (common) is a good bet. Note that failed regulators generally short, so the output is too high. Failed decoupling caps on these regulators can make them misbehave even though the regulator itself is OK.
This amp has a mute circuit in front of the power amp, which could be the source of DC. Turn the mute function off and on and see if it effects the output offset.
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html?page=51#manual
The first suspects are C302(LR) and C306(LR) which are DC blocking caps for the power amp, however caps usually dry up and go open, not short. Open decoupling caps could cause oscillations which may cause a DC offset. Such caps include C340(LR), C342(LR) and C344(LR). It would be useful to know if it passes audio signal, and a scope would reveal any oscillations.
This amp also has 6+ linear voltage regulators, which are also prone to failure:
https://www.manualslib.com/manual/1911908/Pioneer-Sx-20-K.html?page=13#manual
Fortunately, these regulators can be quickly checked by measuring their output voltage. Just be careful not to short them with your meter probe. Given that both channels have the same problem, the power supply (common) is a good bet. Note that failed regulators generally short, so the output is too high. Failed decoupling caps on these regulators can make them misbehave even though the regulator itself is OK.
This amp has a mute circuit in front of the power amp, which could be the source of DC. Turn the mute function off and on and see if it effects the output offset.
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I really appreciate your reply Steve. I have tested all 6 linear voltage regulators and all test very well (unfortunately)
I tried your suggestion and enabled/disabled the mute function on the remote control. The base voltage was sitting at 8.66, it dropped to 8.56 and quickly climbs back up to 8.66v when the mute is enabled and no change when it is disabled.
I dont appear to have an audio output. but have been able to probe with my scope and verify my test signal that is present at the input is present at the Recorder Output.
I decided to follow the troubleshooting included in the service manual for troubleshooting "no sound on the Speaker A terminals" They seem to conclude that IC605 will be defective (VOL IC) as I dont have a signal at C122L/R . I do have relevant +/- 15v etc at this IC. This conclusion doesnt make alot of sense to me as it seems unrelated to the DC protection circuit issue I have been troubleshooting unless this could be inter related somehow. I have been ESR testing random small aluminium electrolytic near the output stage and they have tested out ok, but might look at this again as per your suggestion.
I have been doing some more probing of the audio signal to find out where I loose it and it is quite early on. The 1khz 1v test signal can been probed on pins 1 + 32 on IC605. On the output Pins 21 + 12 I have no ac signal but 6.5v DC is present on the output pins of this chip. (Surely this cant be right) I removed one leg of C122(L) to check further and yes it is DC Voltage leaving the IC. I have confirmed the 9V supply to the chip is normal.
Could this be upsetting the applecart and causing the DC Protection issue I have had also?
Could this be upsetting the applecart and causing the DC Protection issue I have had also?
I have actually been able to replace this IC and still have the same issue. the replacement came from another faulty Pioneer SX20 so I suppose there is the chance that the same fault exists with the replacement IC but I feel it is unlikely. The data sheet for the IC in question can be found at this link:
https://datasheetspdf.com/pdf/1100576/NewJapanRadio/NJU7391A/1One thing I will try is setting the amplifier to “pure direct” mode to see if it passes a signal on to the proceeding op amp after the audio processor IC when in this state. Anyway it looks like I may have more than one issue with this amplifier. If anyone has anything to suggest I would be keen to hear it. Thanks!
https://datasheetspdf.com/pdf/1100576/NewJapanRadio/NJU7391A/1One thing I will try is setting the amplifier to “pure direct” mode to see if it passes a signal on to the proceeding op amp after the audio processor IC when in this state. Anyway it looks like I may have more than one issue with this amplifier. If anyone has anything to suggest I would be keen to hear it. Thanks!
Another update on this:
I have bypassed IC605, injecting a test signal into IC601 by lifting the positive leg of C122L/R and verified signal on it's output and all the way through to the speaker terminals and is a beautiful looking sinewave at a healthy output level of 30V at a 0.5v 1khz test signal on the IC601. This was checked without a dummy load on the speaker terminals. As soon as I introduce the dummy load WITH test signal connected the protection relay clicks on. This I assume that this is the ASO Detect triggering. The Amplifiers fault log does not have any info on it's inbuilt protection detection log when it clicks off. It doesn't seem to latch under this condition as it will constantly try to to turn off the protection relay, and turns it back on every few seconds.
The DC on the speaker terminals is still present even though I have been able to get the ac signal now on the amplifiers output (even though i still have the issue with the audio/VOL processing IC to come back to later)
If I connect the dummy load to the amplifier WITHOUT the test signal, the Output transistors for this particular channel heat up quite hot which indicates to me some sort of inbalance or biasing problem but a little stuck with this right now. Hopefully somebody has a suggestion.
It is definitely a common issue across both channels which is interesting.
Thanks!
I have bypassed IC605, injecting a test signal into IC601 by lifting the positive leg of C122L/R and verified signal on it's output and all the way through to the speaker terminals and is a beautiful looking sinewave at a healthy output level of 30V at a 0.5v 1khz test signal on the IC601. This was checked without a dummy load on the speaker terminals. As soon as I introduce the dummy load WITH test signal connected the protection relay clicks on. This I assume that this is the ASO Detect triggering. The Amplifiers fault log does not have any info on it's inbuilt protection detection log when it clicks off. It doesn't seem to latch under this condition as it will constantly try to to turn off the protection relay, and turns it back on every few seconds.
The DC on the speaker terminals is still present even though I have been able to get the ac signal now on the amplifiers output (even though i still have the issue with the audio/VOL processing IC to come back to later)
If I connect the dummy load to the amplifier WITHOUT the test signal, the Output transistors for this particular channel heat up quite hot which indicates to me some sort of inbalance or biasing problem but a little stuck with this right now. Hopefully somebody has a suggestion.
It is definitely a common issue across both channels which is interesting.
Thanks!
Problem resolved. If anyone ends up in the same position, feel free to message me and I’ll explain what I found and did to rectify this issue.
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