• Please note that Audiokarma will be offline briefly for updates early on Monday morning, September 28th.

SX-780 unsteady DC Balance

mykidsbutler

mykidsbutler

New Member
Working on a SX-780. Originally had a lot of distortion. Deox’t. Did a recap and replaced some transistors. Everything I have done has improved the distortion to the point were it is barely noticeable. Last thing I did today was to recheck the DC balance. The balance now swings anywhere from 30 to 40mv. Is this a problem? Any ideas why this is happening? Have not deox’t VR5 or 6. Ordered new pots to replace them just to try. For now have set balance so 0 is dead center of the “swing”. Any ideas what to look for would be appreciated!
 
Register to hide this ad
It will vary some as the amp warms up, but should settle down. That does not sound terrible though.
On the 780 I just worked on I unsoldered VR5 and VR6, soaked them in D5, exercised them, dried them out, soaked them with F5, exercised them, let them dry out, and put them back in.
The adjustments went from very touchy to smooth and stable. I could set them to less than 10mVdc and they would stay.
You might try re-pasteing the STK ICs.
If you have not seen the replacement/upgrade thread for the STK-0050s look here: http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
This will also allow you to set the bias on the output transistors. I have used it my self, works like a charm! wizard.gif
 
Last edited:
merlynski said:
It will vary some as the amp warms up, but should settle down. That does not sound terrible though.
On the 780 I just worked on I unsoldered VR5 and VR6, soaked them in D5, exercised them, dried them out, soaked them with F5, exercised them, let them dry out, and put them back in.
The adjustments went from very touchy to smooth and stable. I could set them to less than 10mVdc and they would stay.
You might try re-pasteing the STK ‘tICs.
If you have not seen the replacement/upgrade thread for the STK-0050s look here: http://audiokarma.org/forums/index.php?threads/stk-0050-replacement-for-sx-780-and-others.721181/
This will also allow you to set the bias on the output transistors. I have used it my self, works like a charm! View attachment 1272700
Click to expand...

Did my checks after about 45 minutes of Molly Hatchett so it should have been warmed up. I’ll try the remove and soak deal tonight. I did the discrete STK replacement. Great improvement!

slimecity said:
That sounds like a very small range of variance to me. If it swings in the order of volts (not mV) thats when you have issues.......
Click to expand...

Ok. I’m a 6 month rookie to audio repair. I get a little OCD when it comes to settings. I don’t like moving targets. Since it is fairly normal I feel better. Thanks!
 
Both channels swing 40mv's?
 
Last edited:
40 mV is very little, nothing to worry about, if it gets into the 100mV range then start to worry. You can test with your speakers to determine if you hear a pop when you turn the speaker switch on/off. select a source with no signal, start to turn the volume up, set balance for each channel to test. any pops?
 
If I can embellish.
The 40mv's is small and considering the there is no DC Balance adjustment provision, it's pretty good. But it should be stable, at least better than a 40mv swing.
Granted, its a 780 with minimal circuitry.
I maybe over blowing the reading but whenever I have this issue it bites me for not following through.
 
Last edited:
zebulon1 said:
If I can embellish.
The 40mv's is small and considering the there is no DC Balance adjustment provision, it's pretty good. But it should be stable, at least better than a 40mv swing.
Granted, its a 780 with minimal circuitry.
I maybe over blowing the reading but whenever I have this issue it bites me for not following through.
Click to expand...

o_OUm . . . the SX-780 has a DC Balance adjustment. VR5 and VR6. :naughty:
 
I replaced the original STK's with the upgrade discreet modules. Using the old ones for wheel chocks for my riding lawn mower. Should have some new trim pots coming today in the mail. Will replace VR5 or VR6, maybe both and give it a try to see what happens.
 
You must log in or register to reply here.
Back
Top Bottom