SX-680 weak channel
- Thread starter Divotdog
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Good advice, (clean the pots too) but don't go looking for a relay.... they didn't use one on that model, just speaker fuses.
Is the channel weak on all sources, and in mono?
Divotdog
"I can fix it Grampa"
Thanks Ron, I already hit oll the pots, thats SOP when it walks in the door, I just was wondering if this model had any thing that I should be watching for, I was thinking on trying to set the DC offset next and see if that gets me anywhere, but I need to get a new multimeter as I seem to have left mine at a job somewhere. :sigh:
Divotdog
"I can fix it Grampa"
It seems to be all sources and in mono.
BTW there is no DC offset OR bias adjust. And it has NFB tone controls (they're PART of the power amp's feedback loop).
There are 3 caps per channel after the volume controls/ mono etc stuff, feeding the power amp. Since all frequencies are affected about the same, we can discount the tone control's effect.
4.7uF feeds a bipolar transistor(2sc1919) buffer amp, then there is a jfet(2sk117) muting circuit that operates on power up, and during function switching. A 2.2uF cap feeds the muting jfet and a 0.47uF cap takes the output of the muting curcuit and feeds it to the power amp. Those caps could be shifting in value (reducing the volume) or leaking ( upsetting the biasing somewhere).
C313 and C314 (100uF/35v) do power supply filtering for those 2sc1919 transistors, and if they are leaky they could drop the voltages to those transistors, loosing gain. Check the voltages on both caps.
IF you had a constant amplitude signal you could feed in (test tone, interstation FM hiss etc) you could take ac voltage readings and see if you can spot a drop in the signal level at both sides of each coupling cap.
There are 3 caps per channel after the volume controls/ mono etc stuff, feeding the power amp. Since all frequencies are affected about the same, we can discount the tone control's effect.
4.7uF feeds a bipolar transistor(2sc1919) buffer amp, then there is a jfet(2sk117) muting circuit that operates on power up, and during function switching. A 2.2uF cap feeds the muting jfet and a 0.47uF cap takes the output of the muting curcuit and feeds it to the power amp. Those caps could be shifting in value (reducing the volume) or leaking ( upsetting the biasing somewhere).
C313 and C314 (100uF/35v) do power supply filtering for those 2sc1919 transistors, and if they are leaky they could drop the voltages to those transistors, loosing gain. Check the voltages on both caps.
IF you had a constant amplitude signal you could feed in (test tone, interstation FM hiss etc) you could take ac voltage readings and see if you can spot a drop in the signal level at both sides of each coupling cap.
I'm the wrong guy to ask on that.... it takes a LOT for me to declare something a lost cause.
Divotdog
"I can fix it Grampa"
Just an update here, member Jpg123 helped me out on this one, once he got inside of it we found that the stk-0039 for the right channel was a goner, I was able to order two new ones from electronix.com for a grand total of $6.00 each, once they were installed it works like a champ, got it hooked up to my Dynaco A-50s right now and it sounds great, this one will be finding a new home at my dads to replace the old Kenwood KR2600 that I gave him a couple of years ago! 
