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

Johnny_Law

Johnny_Law

o.O
Subscriber
Here's a doozy. I received the unit with no lights working and the vu gauges pegged at 1 watt with the power on.

I found that R401 had been replaced with a 1/4 watt resistor and was blown, so in went a 1/2 watt 33ohm resistor. I also had a spare pair of STKs which went in. I also re-soldered the bases of the STK sockets.

Now it sounds great except for distortion in the left channel ... but only with a certain STK in it. With the volume all the way down, there's a hum that can be best described by the following videos I took.

With the STKs switched, there's no hum in either channel and it sounds great. But of course I want to know why this is happening in this configuration!!

Any ideas?



Thanks in advance,
//Ryan
 
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This sat in my garage for a while ... and today I hooked it up again to see if I can finish the repair. It isn't doing that loud hum any more.

But it is still doing the chirping from the videos, but will play music on top of it, and the music sounds fine (pretty much). For example when I first posted this update it was playing loudly, but the left channel is also producing a pretty dirty signal, sounding almost like when you're listening to an FM channel with imperfect reception, which I believe is the same hum / chirp sitting under the music.

I found that when I wiggle the STK packs around, the hum briefly disappears, but comes back as soon as I move the volume pot.

It has good STK packs in there - one is new, the other tested fine.

Time to resume chasing the hum and wacky dirty signal.
 
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It could be a ground problem if its effected by touching the volume control. Cheeck your grounding posts that the amp and other controls are tied to.
 
Listening to the video... it guess it could be some sort of oscillation... have you got a scope to check signals from one stage of the amp to another? Have you tried using it with all the covers back on? I had an amplifier one the bench once which had a bad hum on one channel but once the bottom cover was refitted it went away.

Though looking at the video again, it looks as though the STK power packs aren't clamped down to the heatsink?
 
Well ... found some funny business from a previous owner (the factory? hope not).

There was a diode on the underside of the circuit board connecting R505 to D501. But removing the diode did nothing to resolve the weird pulse / humming.

C401 is missing, as is C308. Edit: My good SX-680 doesn't have C401 or C308 either. Shoot.

HOWEVER, testing the voltages at Q301 and Q302, when I tested the second pin in, the humming disappeared until I removed the multimeter lead. Might this mean the transistors are bad?? The voltages were close to spec.

(Yes the STKs were sitting there before, they are tightened to the heat sink now.)
 
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If the humming dissappears when you start to probe it, you might be on the right track... the schematic I ahve for the SX-680 is a little poor but if I'm reading it correctly... Q301 and Q302 are the dual transistors responsible for signal input and feedback loop. I would suggest maybe double checking the power supply voltage that feeds this first stage of the amplifier and the feedback loop itself. If it's just one channel acting up then it woukd point to a component in that channels circuit being out of spec... How does the DC offset look on the bad channel?

Transistors can cause noise and it could be the transistor or another one within that circuit begining to break down but it could also be that the circuit is out of balance because of a faulty component. A diode on the opposite side of the board isn't unheard of but it might be worth trying to find it on a schematic... it looks like it could be bypassing the transistor Q502 which looks to be a voltage regulator. D501 is a zener diode - usually used here to provide a reference voltage for the transistor. This seems suspect to me... and I would check the voltages around Q502 as this looks to be feeding the initial stage of the amplifier... my thoughts is that there might be a short or too much voltage....
 
The hum / pulsing hum comes through both speakers ... not just one. :(

Good idea on the DC offset - both channels are at around 60mV. Something certainly isn't right!

The voltages at the transistors, from top to bottom as oriented on the board, are 0 / -32.2 / 0.6 / -28.4 / 0 for the one to the left on the board, and 0 / -32.2 / 0.6 / -30.6 / 0 for the one on the right.

  • Measuring the first pin removes the noise and replaces it with a faint FM signal with buzzing on both;
  • the second slows the pulse on both;
  • the middle slightly lessens the pulse on both;
  • the fourth increases the speed of the pulse on the right, but causes the pulse to disappear and then slowly reappear on the left, but only when the multimeter lead is removed;
  • the fifth makes the pulse faster and louder on both.

So the only difference between the two is the 4th from the top pin on the left transistor, which has a lower voltage, and causes the pulse to stop for a moment as soon as the multimeter lead is removed.

At this point, I'm beyond my previous experience and am hoping you can draw some conclusions ...

Forgot to mention: I did check the supply voltages at Q401/Q402 and they measured pretty close to reference.

And, Q502 measures at -24.7 / 5.3 / -21.0. I can't tell from the diagram if that's correct.
 
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These two transistors are a very important part of the amp which are one of the main parts that govern the DC offset. I've not had experiance with this amp but it seems to be similar to other Pioneer amps I have worked on. You should have (according to the schematic) 0.7V on the middle pin of those transistors. I'll need to look at workout what you need on the others but I would check the power supplies first - im paticulaly curious of Q502 as that diode you described doesn't sound like it should be there.... although that doesn't feed these two, it is feeding a stage behind the two 5 pin transistors which looks to be supplying them and so it's worth checking.

I'll need to look through some schematics but I would test the power supplies and take votlage measurements of Q502.

Do you have a good quality schematic that you can read easily?
 
Thanks - I just measured Q502 and updated my post. I am using the one off of hifiengine which is decent but not high-res.

I also measured Q501 to compare ... very different values. From top to bottom I measured 12.9 / -24.8 / 12.8.

So these aren't right ... will keep tracing backwards.
 
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Johnny_Law said:
Now it sounds great except for distortion in the left channel ... but only with a certain STK in it. With the volume all the way down, there's a hum that can be best described by the following videos I took.
With the STKs switched, there's no hum in either channel and it sounds great. But of course I want to know why this is happening in this configuration!!
Any ideas?
Click to expand...
Sure, instability/oscillation issues. Who knows what bjt die (devices) they used in those modules.
I see Pioneer put the tone control circuit in the feedback loop of the power amp = trouble = introduces possible instability and I suspect you hit on one.
I say stick with the setup that works and leave it be.
 
Johnny_Law said:
Thanks - I just measured Q502 and updated my post. I am using the one off of hifiengine which is decent but not high-res.

I also measured Q501 to compare ... very different values. From top to bottom I measured 12.9 / -24.8 / 12.8.

So these aren't right ... will keep tracing backwards.
Click to expand...

Thats true that they are different.. but looking from the schematic it looks like they are supposed to be different. The votlages on Q501 seem good for now and the readings for Q502 seem ok. You should have 4.9V on the collector and -25.5V on the base. I'm not sure on the polarity of the 4.9V it could be + or - I can't tell if its missing on the schematic or a misprint. Your reading of 5.3 seems ok but could be a little high. I'm not sure there...

Can you measure the voltage across D501? As in, put one probe either side of the diode and measure the voltage? Just to check that the zener diode is ok. If possible too, you could check it, with the power off for continuity to make sure its not shorted - I kinda doubt it is but I would check to be sure its ok as they can be failure points.

Another thing to check would be the voltages on the 5 pin transistors agian - where you got 0V re read it on a mV scale as there should be 100mV present on both of these as far as I can see...
 
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QSilver said:
Thats true that they are different.. but looking from the schematic it looks like they are supposed to be different. The votlages on Q501 seem good for now and the readings for Q502 seem ok. You should have 4.9V on the collector and -25.5V on the base. I'm not sure on the polarity of the 4.9V it could be + or - I can't tell if its missing on the schematic or a misprint. Your reading of 5.3 seems ok but could be a little high. I'm not sure there...

Can you measure the voltage across D501? As in, put one probe either side of the diode and measure the voltage? Just to check that the zener diode is ok. If possible too, you could check it, with the power off for continuity to make sure its not shorted - I kinda doubt it is but I would check to be sure its ok as they can be failure points.

Another thing to check would be the voltages on the 5 pin transistors agian - where you got 0V re read it on a mV scale as there should be 100mV present on both of these as far as I can see...
Click to expand...

Thanks so much for sticking with this with me.

Voltage across D501 is 21V. For continuity, I get a brief beep at first, then a display of ~930 (units?) on my Diode meter / continuity checker.

The top and bottom pins of the 5-pin transistors, top pins are both ~105mV, bottom pins are both ~85mV.
 
Johnny_Law said:
Thanks so much for sticking with this with me.
Click to expand...

No problem, just trying to help get your unit going again. :thumbsup:

I think that voltage is ok from D501 - I think it gives a reference voltage to the emitter of Q502 to supply the initial stage of the amp with a voltage of 21V... afaik this is a 22V power supply using D501 - a MZ-220 which I believe is a 20V zener diode unless anyone can correct me.

I've found this thread about a similar problem which would probably be worth reading through. MTF guides him through it and it has a lot of reference voltages in there so maybe have a read of that - I'm going to and see if I can see any similarities.

http://audiokarma.org/forums/index.php?threads/sx680-need-help.384959/
 
Thank you ... I followed that thread, Mark is so helpful, that's fantastic. Following that thread, all of my voltages seem fine. There has to be something I'm missing ...
 
Yes, its a great post and good to have MTF pointing out what some of the sections do. Can help clear the fog sometimes! The next thing I would think of is component failure or fatigue and I would probably get out the scope and see if I can trace back from the STKs where the pulsing or distortion is coming from. Do you have an oscilloscope?
 
The manual that hooligan has in that thread seems like it's really clear and detailed. I wonder where he got it?? It'd be very helpful to have a clearer circuit diagram to reference.

I don't have a scope, but I do have other receivers that I could use as recommended by MTF. I did manage to successfully traced the signal forward to Q304 (I think it was Q304, diagram not in front of me now), but I need to trace it backwards the other way.
 
Alright you'll love this. I went to follow the signal upstream - using a change in tone upon touch by my (+) multimeter lead (with (-) grounded). I found that it likes to randomly go away, and then it will come back either in a few moments or as soon as I turn up the volume and back down. So then I started to play with my bass and treble pots and it also goes away after I fiddle with the knobs for a moment.

Unpredictable results seem much more difficult to diagnose!

In the midst of the ticking/thumping going in and out, I did trace it up to - surprise - right where that diode was. EDIT: Nope, went farther up, looks like it starts at Q303. Edit again: swapped Q303 with my good 680 and didn't solve it. Humph.

So here I am typing this with no more tone thumping and no idea why it went away. (Edit - back now ... edit: gone again now, edit: back)
 
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