Hiya!

Found a Pioneer SX-680 today for ten bucks at local thrift store. Unable to test there (no speaker wire) but took a chance anyway. Tried it at home and seems to have some problems. WHen I set it to mono, both channels work, but in stereo one side drops to very low volume and is very distorted. Also, after playing for a while you start to hear rapid popping coming from the speakers. The popping is not too loud, but noticeable. I just ordered some deoxit so I have not been able to clean the pots yet. Any suggestions? Thanks! :D

Clean first, then evaluate. :thmbsp:

If anyone is gonna set you on the right direction its Glen.
I was gonna say the same thing. I have saved many receivers with low channels and dropouts by giving it a good cleaning.
Good luck and let us know how it turns out.

As you have been twice advised, treat switches and controls with Caig DeoxIT D5. This will likely bring back the missing or intermittent channel.

* * * * *

Pioneer power supplies run hard, get hot. You may find that power resistors and regulator transistors need resoldering

* * * * *

Popping is often caused by a failure in the differential amplifier stage in the audio power amps.

* * * * *

Hope this is useful.

Fred Longworth
http://www.repairaudio.com
StereoTech Classic Audio Repair

Well, I sprayed down the pots today, and although the scratchyness is gone now, I still have problems with the left channel. Seems like when I use the tuner in stereo I can barely hear the left channel. buthe they are both fine in mono. When I use the CD player through it, it sounds lower on the left channel, but fine on the right when in stero. Both are the same leval in mono. What gives? :worried:

I noticed this has a lot of wire-wrapped connections. Should these be soldered to improve them? Should I just solder them, or remove them, clean, and then solder? Why was wire-wrapping used in the first place? :scratch2:

Spray down the controls again, and exercise them a bit. I have run across this quite a few times.

I noticed this has a lot of wire-wrapped connections. Should these be soldered to improve them? Should I just solder them, or remove them, clean, and then solder? Why was wire-wrapping used in the first place? :scratch2:
Think I remember a thread about wire-wrapping here somewhere but I couldn't find it. Not a tech myself but my inclination would be to leave the wire wraps as is and clean externally w/a little deoxit and a toothbrush. Don't see how soldering over would be helpful, if it ain't broke - don't fix, it school of thought.
Regards

edit: Ah - found it, search this for info "wire wraps - should I solder them all up?" not sure how to paste exact location, sorry Avocado is green.

When you treat the switches and controls, it's real important to get the DeoxIT inside the control or switch through whatever apertures are available. Then, you need to work the device a number of times. As a general rule, I rotate a control fifty times forward and back through its entire rotation and I exercise a switch a hundred times.

If you don't exercise the device enough, the treatment won't "take."

Sometimes I see people working a switch or control super-fast. Don't do this. You can be brisk, but super-fast operation can actually damage the device.

Good luck with your machine.

Fred Longworth
StereoTech Classic Audio Repair
http://www.repairaudio.com

I noticed this has a lot of wire-wrapped connections. Should these be soldered to improve them? Should I just solder them, or remove them, clean, and then solder? Why was wire-wrapping used in the first place? :scratch2:

I asked the exact same question about this, for the same reciever, about a year ago - the answer was the wire wrapping is more reliable than we think it is. The advice was to leave them alone. I know that on another audio site out there there is a guy who recommends soldering wire wraps up to anyone who asks about them - but I don't believe he actually knows what he's talking about!

Try taking the bottom off and find the screws that hold the main circuit board onto the chassis. Loosen them and then gently tighten them again. I think the circuit ground traces contact the chassis at one or more of these screws and if they are loose or corroded the ground will not make contact. I had an sx-680 that would work when cold then go dead when warm, then back to working again if left on for several hours! I retightend these screws and that all stopped. If they look corroded clean them up and wipe with Deoxit or contact cleaner.

Also, keep cleaning the controls (especially the mono/stereo switch) and working them with the unit off. Don't turn it on for several hours after spraying cleaner into any of the controls.

good luck!

Thank you all for the advice! Thats what I love about this place! :D

I cleaned the contols several times, so all the scratchiness is gone from those, however the main problem is till there; left channel volume very low in stereo but both are strong in mono. I am really a novice at this, but I am trying to learn. I did find a scematic for this receiver, but I really do not know what to try next. Is it possible the Darlington Modules (STK 00039 I beleive they are marked) is dective? Would it cause a problem like this in stereo? It seems to me that if it were bad it would not work at all, however when set to mono is it possible only one module ls actually providing power to both? Would they be working combined when in mono, then seperate in stereo? If so, that might explain why I hear both sides in equal strength. any suggestions? Thanks!

The mono / stereo switch is well before the power amp, usually even before the tone controls. Your power amp sections, when working fine on both speakers with the mono switch set on mono, are fine. Your signal source to the amplifier needs attention.

One thing that that is overlooked is that people try to spray the deoxit in from the front, and when it "dosen't work" get frustrated.

De-oxiting involves physically getting to each and every switch in the signal path, including things like am/fm/phono/aux switches and tape monitor switches and visually seeing the spray go in nooks and crannies to get to the actual contacts.

I haven't looked at your model to see what the switches are, but that am/fm/phono/aux etc switch, if rotary in action, must be carefully and accurately spritzed to hit the actual contact areas. The other switches in that SIGNAL area are the tape monitors. Both can do exactly what you described.

I think you are on the right track, and just need to be aware of just how many switches those signals travel through.

Thank you mark for that information. I did spray all the switches down once, but sprayed several problem ones, like the volume and balance controls, several times due to scratchiness when turned. They appear to be working fine now, so perhaps I need to focus on the other switches you mentioned more throughly. I will try that this weekend and post the results. :yes:

Yes, look to all the switches and spray them from the rear trough holes you find there! Filter, Muting, loudness, monitor ,..

Front panel layout of sx-680:

power
headphones
speaker a select
speaker b select
bass tone
treble tone
mono/stereo select
FUNCTION suspect
MONITOR 1 STRONG suspect
MONITOR 2 STRONG suspect
balance Done & ok
volume Done & ok
loudness

The three I have capitalized are the most likely problem spots.
From your description it is probably the tape monitor switches as both the FM and CD player are affected.
Did you connect the cd player to AUX?
If so connect the cd player to the tape 2 play (l & r) jacks and push in the tape monitor 2 switch, this will eliminate 3 of the 4 contact sets that the signal has to travel through.
You could also take the two interconnect cables you used to hook up the cd player and connect the tape 1 rec( l & r) to the tape 1 play( l & r) depress the tape monitor 1 switch and see if that affects the fm sound in stereo.
Next you could do the same with tape 2 rec & play.
You could even connect the cables from tape 1 rec to tape 2 play, depress BOTH tape monitor switches and see what happens.
This could help localize which monitor switch is giving problems.

Well, I followed Mark's advice, after spraying down the controls and switches with contact cleaner again. When I plugged the CD Player into Tape 2 play, the volume was at full stength on both channels! Cool! I tried tape 1 and phono and there was full volume on both as well. I even tried the AUX input again and that too was now working at full volume. I did try connecting Play on Tape 2 to record on Tape 1 and the tuner was still low on the left channel. So I see that the driver stage is working fine, and the problem is either: the path of the signal from the tuner has a problem between it and the driver, or the stereo section of the tuner has a defect. Since the left channel does work (abeit reduced volume) I am going on the assuption it is the signal path that is the problem.Where might I check next? :scratch2:

What type of test equipment do you have? a digital volt meter ? ( right now I would find myself reaching for an oscilloscope, but that's me..).

The function switch contacts for the tuner ( as opposed to the wiper on the function switch, which IS contacting the aux input switch contacts on that switch) are still the strongest suspects.

Since all the other positions on the function switch work on both channels,
(AM L & R, Phono L & R and (i know...) Aux L & R) ??

We need a service manual, if repeatedly actuating the switch THROUGH the FM position (am,fm, phono and back) doesn't eventually clear it up.

There are a few tricks to try with a dmm before we need a scope (or tech), but that manual is needed to find the correct points to probe.

Then again this is assuming your comfort level for doing stuff like that is high, and we won't break more than we fix.

I tried to make some comparasons to an sx-780 but the sx-680 does NOT have an fm mute switch, so I don't know exactly what's in there.

The 780 has a stereo decoder ic, a de-emphasis switch (on the back) a pair of "normal" transistors that amplify L & R, then another pair of JFET transistors that do the mute for L & R , after which the signals are fed to the function switch. so the 780 has a decoder ic, 4 transistors, a switch and assorted whatnot that can go bad... the 680 ??? read the manual.

Anybody have a sx-680 .pdf to download???

YES, you can download the schematic from my website here (http://www.loeskenweb.de/Manuals/Pioneer/SX-690/)
Its from the 690, but thats the same in black!

Good luck :thmbsp:

I do have a DMM, but no scope. I do have a copy of the schematic that Agger hsd losded ina previous post. I am no expert, but I can follow instructions and use the meter.

I have the schematics and the layout drawings. THANKS agger!!

That is an interesting function selector switch, it is a linear arrangement.....

Are you sure that you got de-oxit into the left side end sections, there are 6 contact sets in 3 groups of 2. (just checking).(outlined in red and green)

There is a pair of transistors buffering the pa1001-a stereo decoder chip. They are q7 and q8.

You need to read the AC voltages on pins 4 and 5 (outlined in purple) of the pa1001-a stereo decoder chip. These are the left and right channel audio outputs. It has to be operating and tuned to a good station. I would guess that you should see about 100 millivolts ac, but the important part is whether or not the voltages are similar. These voltages will be referenced to a good ground. Use a clip lead if at all possible to the ground, so that you don't have to split your attention away from where you are probing.

BE CAREFUL to not short any pins together.... you need a very fine tip on the probe, and it should be sharp to minimize slipping, (use a file) then get a wrap of insulation (tape perhaps) around all of the exposed probe EXCEPT the very end of that sharp tip.

I have also outlined in purple the outputs of those transistors, q7 and q8, on both sides of c151 and c150, the output coupling capacitors. You need to take AC and DC readings on both sides of both capacitors, with the reciever set to fm, again referenced to ground.

I outlined the left and right signal paths in red and green on the layout.

Well, the voltage on pin 4 and 5 is about 19-20 milliamps, so it appears to be the same on both.

For voltage, I get .07 to .1 (fluctuates) volts on both pin 4 and 5.

Well, the voltage on pin 4 and 5 is about 19-20 milliamps, so it appears to be the same on both.

For voltage, I get .07 to .1 (fluctuates) volts on both pin 4 and 5.



Uh... no milliAMPS or amps readings PLEASE, a dvm on volts mode only "sips a little current off of the top" whereas a milliamp or amp current reading is a bottomless pit, where the meter is essentially a short circuit (to ground in this case), and that can destroy things very very fast.

Since the readings on pins 4 and 5 are similar, the decoder ic is doing it's job.
Pin 4 is the left channel which is the low channel, pin 5 is the right channel which is ok.

The readings on both sides of c150 and c151 will tell the tale....
Remember, two readings at every point, AC volts and DC volts....
c151 is for the left channel and is suspect, c150 is the right channel and is the "good reference"

Either the transistor (q7 for the suspect left channel) isn't doing it's job, or the capacitor c151 is either open, thus not passing the appropriate amount of signal, or the capacitor is electronically leaky or shorted, thus preventing the transistor q7 from operating correctly.

Okay, I finally got some time to do the readings, and here they are: for C151 the Positive end read 100-130 millivolts (fluctuates) for Ac, and zero for DC. The negative end of C151 read 40-48 millivolts AC, and 4-5.5 millivolts DC. For capacitor C150, Both AC and DC were zero on the positive, on the negative the AC was zero, and the DC .33-3.5 millivolts. :headscrat

p.s. I just figured out I was looking at the receiver from the front, so it was the RIGHT channel that is causing problems, not the left. Sorry about the mix-up :no:

Looks like q8 is bad, or the circuit it is in isn't functioning due to a fault with a passive component.

This is supported by finding similar ac voltages at the left and right channel outputs (pins 4 and 5) of the stereo decoder integrated circuit, and then the ac signal disappearing at c151. The only things inbetween is r140 (8.2k), r142 (10k) and q8.

Several alternatives:

pull q8 and test to see if it is dead, your meter needs a diode test function
if it is dead then we need to find a replacement. Where are you on this planet??
or
swap q7 and q8 and see if the fault moves to the other channel.
if the fault moves to the other channel, then the transistor q8 is confirmed defective, if the fault stays on the same channel, then there is a deeper fault.

p.s. I don't know how your post sunday evening got past me, I was watching for it.

edit: There are quite a few replacements available. I have some 2sa725's that are similar enough to work, especially since the circuit is non-critical and running at 13.6 volts.( I think the 2sa726's used had a slightly higher voltage rating, the difference between 25 and 35 volts I believe). Or you could order some ZTX795A transistors online from mouser.com or digikey.com (about 1.13 - 1.25 $US), but I'm not sure about minimum orders/shipping/etc policies.

Shoot me another PM with address etc, and I can mail you a pair of 2sa725's that I have laying around... (one to use, one spare).

Thanks for the info mark. Well, I live not too far from you, as I am in Hammond, Indiana just across the border from Illinois. Well, I did some checking on those transistors (2SA726S) and I found that NTE makes a substitute marked NTE234 which parts express has in stock. I will look to see if the resistors are bad, but I think it is the transistor, because the signal is constant until it reaches the first pin of the transitor, then disappears after it. Should I replace both transistors? I would think if one is bad, the other may follow suit?

No, just because one went, doesn't mean the other will.

NTE transistors are not held in high regard here, but this isn't a power output transistor where a counterfiet could blow up due to inadequate die sizing or sustained power disapation characteristics.

my lettercarrier hasn't been here yet so I could hurry and get them out today.
You're close so it could get there fast...

Edit: on the wayyyyyyyy......

I got the transistors today you sent me. thanks! I tried replacing the suspect transistor but the problem is still there! :tears:
This sure is frustrating. I wonder if I could bypass the selector switch and see if a solid signal comes through. that do you think?

I got the transistors today you sent me. thanks! I tried replacing the suspect transistor but the problem is still there! :tears:
This sure is frustrating. I wonder if I could bypass the selector switch and see if a solid signal comes through. that do you think?

There are a lot of passive components in that circuit, we just proved that the active component (transistor) was working ok. Of course we are assuming that the transistor is lined up ok, with good solder conections.. ( to my sad experience, that may not always be the case)

You need to compare (with the power OFF) the resistances of the components from the good side to the corresponding component on the bad side. These are all the components arranged around q7 and q8 in the schematic diagram...

Following the diagram, start with the 8.2k resistors, going down, and don't stop till you hit the 100p capacitors....

We're pretty sure the signal going in is ok, but it doesn't come out, and the transistors are good.

There is a type of signal probe you could build, and it uses the amp section,
it is a 0.047 uf capacitor with one lead connected to the center lead of an interconnect cable, (the ground side of the connector is insulated) and is probably best plugged in at the aux inputs.The probe end is the other side of the 0.047 uf cap, insulate it well, leaving only a small bit of wire exposed. But the results in this case will most likely not be worth the effort to build the probe.

Well, I have been busy lately and finally got a little time to work on the Pioneer again. Tested the resistors and they were all within 10% of their values. Damn! Well, I had ordered some parts from Parts Express so I ordered some .47uf 50V electrolytics just to give them a try. I mean, they were only .9 cents. Pulled out the capacitor from the suspected bad side and replaced it with the new one and...IT WORKED! :nutz: The volume is now at full strength on both channels! Amazing how one little component could cause such a headache. I must admit I learned a lot from this. I am in your debt Mark. Thank you for advice and patience! :ntwrthy:

Read the HomerJSimpson thread I was troubleshooting, THAT was ONE dead (open circuit) resistor causing the problem... and it was a &!#*@ to find....

I was wondering what happened, and decided patient waiting was best...

So I waited... That capacitor passes the audio through while blocking the dc voltage, it is called a coupling capacitor, it probably leaked enough to dry out and shift down in value by at least a factor of 10...

I am indeed happy that another Pioneer is back singing again, congratulations on your accomplishment!!

:music: :D :smoke: