Pioneer sx 680 Right VU meter Pinned at Max

keithpgdrb

keithpgdrb

Super Member
Hey all,

friend of mine asked if I could look at his receiver that didnt work. I told him I was no expert, but I could give it a try.

got the unit, an old sx-680 . when powered up with nothing connected, the right VU meter just pins to max. Haven't hooked anything up to it to test if it makes sound. and if there is a huge power issue, don't think I will.

where should I start in the diagnosis here?
Thank you all in advance for your wisdom.
 
there is either dc to the meter or its oscillating in the amp .
more likely dc which will also likely be on the output . so don't connect speakers yet .
check dc volts at speaker terminals ..or maybe better than stressing the meter pull the 5 amp speaker fuses and check dc volts at the empty fuse clips . fuse 4 is right channel
 
Didn't get a chance to do anything but open up the case tonight. But I could see straight away that one of the fuses inside was blown. Looks like by number 49? Have to verify which one tomorrow.
 
Looks like one of your stk-0039 Darlington power packs bit the dust, need ground referenced voltage measurements of both packs, pins 1,2,3, 8,9,0.

Insulate all but the tiniest portion of the tip of the probe, so when the probe slips (we ALL slip) none of the pins will be shorted together. That short will be instant death for a good pack.

BUT with that 5 amp output fuse blown, the meter shouldn't be indicating at all.

You have to make your voltage measurement for the channel at the hot / feed / amplifier side of the fuse, so read both sides of a particular fuse, and see if the readings are different.

The fuses feed in opposite directions, so the side with pin 49 and pin 48 for the other channel, are the COLD side, towards the speakers, NOT the amplifier outputs.
 
Thanks Mark. What is the easiest way to access those pins? can they be accessed from underneath?

The hot side of the blown fuse is 32V.. Same for both sides of the good fuse. about 32V
 
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it seems both channels are blown then . or something else is amiss:scratch2:
voltage readings for each power pack should clinch it ..be very careful not to short anything .
 
I don't understand why those readings tell us that both channels are blown. I believe the left channel works. At least that's what I was told.
 
32v at both fuses tells me they both have problems ..they both should be very close to zero volts .
on another note when measuring voltages black probe to chassis red to whatever you are getting voltage for ..report back whether its negative - or positive+

with the power off unit unplugged measure ohms of the emitter resistors .. the 0.22 ohms 2w ones ..
 
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I get what you're saying. I should also make sure I'm testing the right fuses. So we're all on the same page with what numbers mean what
 
ok. popped off the bottom which made testing those sensitive places a breeze. Here is the result. I will admit that I am embarrassed, because I have no idea which one of these is for which channel. both essentially read the same.

If you are looking at the unit from the front...

readings on the Left side.
1 33.6V
2 -33.2V
3 32.7V
8 32.7V
9 32.9V
0 33.6V

On the right
1 33.6V
2 -33.3V
3 32.4V
8 32.4V
9 32.9V
0 33.7V
 
just to be clear, here is the fuse I'm referring to.
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Where is the resistor you want to to check?
attachment.php
 

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the resistors aren't there ..they are replaced with solid wire .they go to pins 8 and 3 .
the readings are suggesting both packs to be shorted out ..
 
I suppose taking out the packs and checking them outside of the unit is the next step correct? Are there replacements available for these?
 
That unit has been molested by a moron. WHAT was he thinking?

"I can't find 0.22 ohm resistors, and that's so close to zero, a wire should do..."

Now there are two dead packs.

Get the packs, and the resistors. Also get some thermal transfer compound / grease (the white stuff between the pack and the heat sink plate).

Then, for safety's / $$$'s sake, (don't want to hurt the new packs) test out of circuit Q303, Q304.

edit - yes, probably both the two packs AND four resistors from eBay and may end up coming over from the pacific rim...
can do it for under 20 bucks if you are willing to wait for them to arrive.
 
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I'm in agreement Mark. when looking at the underside, I think some other things have been done as well. I see evidence of some altering. I'll snap a pic, maybe you guys can see what was done. in the meantime, I'll let my friend know what has been found so far. see if he wants to spend the bread to fix it.
 
I thought it was odd as well, but it looks like a factory solder job. After replacing the bad stk, and shotgun replacement of caps and transistors it's running like a champ.
 
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