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SX-680 humming changes with STK pack / treble and bass adjustment

Yup, one of these kind of problems... I would think about checking solder joints although I do remember you talking about a diode that wasn't on the schematic? Could you photograph it? Does your good SX-680 have the same diode?
 
Thanks, will do so tonight. Seems like that signal path may in fact have a bad ground or joint somewhere if grounding it with the multimeter fixes it briefly. That or maybe a bad cap that I'm discharging when I ground the circuit?

No, the underside diode isn't on my good 680, but yes I did snap a picture of it:

IMG_20160312_170421209.jpg


And shined my flashlight on it in this pic:

IMG_20160312_170449743.jpg
 
I would question why that diode is there to be honest... its not original and its not in the schematic... and it might be bypassing part of the circuit. are you able to draw on a schematic what it is connecting to? I would be tempted to remove it and see what happens.. maybe on a DBT too...
 
Oh, yeah I did remove it (but I saved it just in case). All of this testing is post-removal. (Removing it didn't seem to change anything.) It was connecting R505 to D501.
 
Ah right ok. Well that's interesting... in which direction was it connecting them? It might sound strange but have you checked the grounding wires on the speaker terminals?
 
Ok so after all this, I narrowed it down to the aftermarket STK Pack. I had re-flowed all the solder and tested various things getting nothing. So I swapped STK packs with my good 680 ... GONE. Quiet as a mouse. 100% functional.

So that's the problem, after all. The dang aftermarket STK pack is junk.

I suppose the diode mystery will never be solved ... it doesn't seem to need that diode installed to work.

Now I'll put up a WTB thread for a genuine or tested STK 039.
 
Well done for finding the fault. I've found that problem with STK packs and that's one of the reasons I don't like to buy those pioneer amplifiers. They're so hard to find working... if only you could replace them with a small circuit... but never mind. It's working now, I hope you find a work STK for it!

I don't understand why the diode was there either but if its working without just see what happens. It will likely be fine but if you get any more trouble with it, check around that area again. It might have been put there to pacify something breaking down in circuit.
 
Yeah thanks a bunch for the help. You don't happen to know of a good reliable source for STK modules do you? I've been looking on eBay and other sites and trying to figure out what the best option is.
 
I had seen that the internals of the STK pack are in the schematic, so provided someone can work out the values, you could have your own PCB made and make your own aftermarket version. I know someone on the forum has done this with one (an STK-0039 IIRC) so it is possible. It's just a case of finding the right values and I think that would be the best way to go. Plus, if something went wrong with it in the future, you could just open it back up and replace the faulty part.
 
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