Pioneer SX-780 Distortion

[Chris Rush]

Getting Neil Young "Hey,Hey, My,My" type distortion from left channel with volume set anything beyond three.
But not getting any distortion through headphones at all.
I seek wisdom from the AK masters....

 

[merlynski]

In Headphones WHEN speakers are active? They (phones / speakers) get the same voltage signal over the SAME wire, but at less current IF the speakers are not being driven.

Dirty protection relay contacts can do that. It's almost as easy to pop a new relay in, than to get the relay out, the case off and the contacts burnished clean.

Have you replaced the protection relay? Cost is about $10 (plus shipping) from Mouser.
653-MY4-02-DC48
The symptoms you describe are consistant with damaged relay contacts. You can also temporarily bypass the relay by soldering a jumper between the relay pins on the bottom of the pcb . . .
Have you tried different speakers, swapped left and right with the speakers, deoxited the speaker switch (and other controls)?

 

[Chris Rush]

Have you replaced the protection relay? Cost is about $10 (plus shipping) from Mouser.
653-MY4-02-DC48
The symptoms you describe are consistant with damaged relay contacts. You can also temporarily bypass the relay by soldering a jumper between the relay pins on the bottom of the pcb . . .
Have you tried different speakers, swapped left and right with the speakers, deoxited the speaker switch (and other controls)?

All controls are completely deoxed. swapped speakers l\r and a second pair. Cannot find an (obvious) bad solder on the board any where. I will look into relay.

 

[merlynski]

All controls are completely deoxed. swapped speakers l\r and a second pair. Cannot find an (obvious) bad solder on the board any where. I will look into relay.

Seems like you are working through the necessary steps to get it fixed. May be the STK module . . .
That relay worked perfectly in the SX-780 I rebuilt. While you are in there change the driver transistor Q26 (2SC1384) to a 512-KSC2690A (Mouser #). The original gets flaky as it ages.

 

[Chris Rush]

Seems like you are working through the necessary steps to get it fixed. May be the STK module . . .
That relay worked perfectly in the SX-780 I rebuilt. While you are in there change the driver transistor Q26 (2SC1384) to a 512-KSC2690A (Mouser #). The original gets flaky as it ages.

I thought about the STK, but with perfect sound through headphones, I thought maybe not.

 

[merlynski]

I thought about the STK, but with perfect sound through headphones, I thought maybe not.

Headphones draw very little current so they are very easy for the STK to drive, and the same thing applies to the damaged relay contacts, less current to cause problems with the signal. Speakers draw way more current, so more distortion.
Have you seen the thread about the custom replacements for the SX-780 STK?: STK-0050 replacement for SX-780 and others

 

[merlynski]

You could try swapping the modules and listen for the distortion on the other side.

 

[Chris Rush]

You could try swapping the modules and listen for the distortion on the other side.

Replaced STK and cleaned contacts (very gently with fine emery cloth) on relay, still the distortion persists.

 

[merlynski]

Replaced STK and cleaned contacts (very gently with fine emery cloth) on relay, still the distortion persists.

Is it still on the same side?

 

[EastPoint]

OK, when you say "replaced STK," what does that mean? What I would do is swap the STKs right-to-left and see if the distortion follows. Is there any visible damage to resistors or anything on the main board? Any previous work done?

 

[Chris Rush]

Is it still on the same side?

Yes, still on the left.

 

[Chris Rush]

OK, when you say "replaced STK," what does that mean? What I would do is swap the STKs right-to-left and see if the distortion follows. Is there any visible damage to resistors or anything on the main board? Any previous work done?

I've been over the board several times and cannot see any obvious damage to anything, single owner before me replaced the very same STK that I in turn replaced with newer that I had on hand.

 

[Chris Rush]

Swap the STK's they said. It will work they said. Did I listen? NOOOOO. Swapped STK's distortion moved, replaced bad STK, unit make happy sounds!
Once again, Thank you all for your time,patience and expertise. I am going to celebrate with Jackson Brown and Bob Seger.

 

[EastPoint]

It’s always a bad STK. I fixed three SX-780s in the last two weeks. All three had different problems. Each had a bad STK.

 

[merlynski]

Swap the STK's they said. It will work they said. Did I listen? NOOOOO. Swapped STK's distortion moved, replaced bad STK, unit make happy sounds!
Once again, Thank you all for your time,patience and expertise. I am going to celebrate with Jackson Brown and Bob Seger.

Two of my favorite musicians. You are most welcome, ENJOY!