Pioneer SX-2500 distortion in speaker out but not headphones

[KeithD]

I have a strange output issue. I have distortion in the right speaker output with a speaker connected. However, if I turn the selector to Headphones, I don't hear distortion through headphones. If I keep the headphones plugged in and unplug the right speaker, then I don't get distortion through the headphones when the selector is set to any of the speaker settings. However, if I plug a speaker in to the R channel and still listen through headphones, there is distortion in the R channel. So, some how the presence of a speaker load is causing distortion both in the speaker and in the headphones. I've tried other working speakers, same issue. What could be causing this?

 

[Watthour]

Did you mean to type "SX-2500" and slipped, or is there a Pioneer unit with which I am unfamiliar?

That is a relatively common occurrence if there is resistance at speaker protection relay contacts or if a channel has a failed output device. Without the load of a speaker, a failed output can drive headphones somewhat faithfully, but adding read load from a speaker will reveal its inability to pass current.

If it is indeed the SX-2500 it does not have a protection relay, but the quasi-complimentary outputs are socketed, so they are relatively easy to remove and test.

 

[KeithD]

Did you mean to type "SX-2500" and slipped, or is there a Pioneer unit with which I am unfamiliar?

That is a relatively common occurrence if there is resistance at speaker protection relay contacts or if a channel has a failed output device. Without the load of a speaker, a failed output can drive headphones somewhat faithfully, but adding read load from a speaker will reveal its inability to pass current.

If it is indeed the SX-2500 it does not have a protection relay, but the quasi-complimentary outputs are socketed, so they are relatively easy to remove and test.

Thanks for catching that title mistake, I have corrected it.

I was thinking this was an output transistor issue. I did swap them one by one with a new replacement, but saw no change to distortion issue. However, I checked by putting the new one it, and when it did not solve the issue, I put the old one back in and moved on to the next one with same procedure. Do I need to replace both output transistors on a channel to see if it is solved? I probably should have tried this. I do have four new replacements.

 

[Watthour]

If you have a meter at hand, it would be a good idea to measure resistances of the driver transistors as well - Q7/9 and Q8/10. It would be sad to waste some new outputs if there is other damage.

Also, a dim bulb tester is going to be very useful with this unit since it has no protection relay.

Incidentally, it may be some kind of epidemic:

http://audiokarma.org/forums/index.php?threads/output-transistors-keep-blowing-on-sx-2500.845565/

There is a lot of useful information in that thread.

 

[sregor]

Swapping them one at a time usually will tell if output bad, but I suppose it is possible that two were bad (not likely though)
You also might check for open emitter resistors, or open connections all around the outputs, including the TO-3 sockets which I presume they use. I've seen bad or loose connections where the output plugs into. It isn't too hard to troubleshoot with a scope if you have access to one. Also be on the look out for broken solder connections (especially around base of wire wraps if this has them).Good luck

 

[KeithD]

If you have a meter at hand, it would be a good idea to measure resistances of the driver transistors as well - Q7/9 and Q8/10. It would be sad to waste some new outputs if there is other damage.

Also, a dim bulb tester is going to be very useful with this unit since it has no protection relay.

Incidentally, it may be some kind of epidemic:

http://audiokarma.org/forums/index.php?threads/output-transistors-keep-blowing-on-sx-2500.845565/

There is a lot of useful information in that thread.

Q7-10 had already been changed for their replacements. So, I don't suspect those.

Always use a DBT.

I've been following that thread by PoDuck and have made a few posts there.

Swapping them one at a time usually will tell if output bad, but I suppose it is possible that two were bad (not likely though)
You also might check for open emitter resistors, or open connections all around the outputs, including the TO-3 sockets which I presume they use. I've seen bad or loose connections where the output plugs into. It isn't too hard to troubleshoot with a scope if you have access to one. Also be on the look out for broken solder connections (especially around base of wire wraps if this has them).Good luck

OK, will check the resistors. It was working fine until I was setting the center point voltage and bias. Center point voltage went fine, but I'm thinking I touched something errant with the DMM checking current. It was after that the distortion started. It's possible one of those cement emitter resistors got fried? Will check. Won't be able to work on this until tomorrow and will report back.

 

[merlynski]

The headphone connection is before the speaker switches and plugs. Have you done a thorough job using Deoxit on the Power/Speaker switch speaker section? Also Deoxit the speaker connection plugs and sockets on the back, as well as making sure the speaker wires are clean and tight under the screw heads on the plugs. You might try putting the plugs in the B sockets or swapping the left and right speaker connections to eliminate the speakers themselves.

 

[KeithD]

The headphone connection is before the speaker switches and plugs. Have you done a thorough job using Deoxit on the Power/Speaker switch speaker section? Also Deoxit the speaker connection plugs and sockets on the back, as well as making sure the speaker wires are clean and tight under the screw heads on the plugs. You might try putting the plugs in the B sockets or swapping the left and right speaker connections to eliminate the speakers themselves.

I checked, this is not a speaker issue. It occurs on the right channel of both A and B connections. Swapped speakers, still right channel. Tried a different speaker/connector on R channel, same issue. Also Deoxit and Fader lube on the power/speaker selector.

As I mentioned, I think something must have happened on the R channel when I was trying to measure the idle current.

 

[KeithD]

Checked all 4 emitter resistors, and all read approximately 0.4 ohm (0.3 is spec).

Before I had this distortion problem, I had recapped PS, main amp and protect boards. Q1-10 were replaced with recommended transistors. After the distortion appeared, I swapped Q11-14 individually with a new MJ21194. When I heard no change, I put the original back in and moved to the next transistor. I have also put the original Toshiba transistors in for Q7-10 just in case I fried one of those, but again no change. I can work my way back in the circuit replacing transistors, but that seems a bit random. However, I'm not sure what to do next?

 

[merlynski]

What version is your Protector Unit: AWM-004?

 

[KeithD]

Yes, but there is a stamp after the 4 that looks like it could be a zero, so the stamp may be AWM-0040.

 

[merlynski]

Yes, but there is a stamp after the 4 that looks like it could be a zero, so the stamp may be AWM-0040.

Then you probably have the same version @PoDuck has, with the audio path looped thru the Protector . I was going to refer you to that but you already caught on to it

 

[KeithD]

Decided to replace Q11-14, since I had four MJ21194G transistors on hand. Did not solve the distortion issue. So, I've replaced Q7-14 so far, and no change. Seems strange this would be further back in the circuit. I can replace Q1-6 as I have more replacements on hand, but this seems a bit random. Any chance this is something else? I supposed it could be an e-cap on the board that was damaged. They were all new. Not seeing any fried resistors.