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SX-580 120hz hum

J A

J A

Active Member
I've been digging around the forums and I think this is power supply related based on what I'm seeing. I have a 120hz hum (spectrum analyzer confirmed) from both speakers. It is not influenced in any way by volume, input changes, or switching channels. The only time it stops is disengaging the speakers with the speaker channel push- buttons. I don't hear a buzz from the receiver, just from the speakers. Also of note, it occurs whether anything is plugged into an input or not, and when plugged into different outlets and it is a steady volume.

Is this a power supply issue?
 
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I don't have a scope. Is SA schematic? If so I just downloaded the KC and KU version. what's the difference in them?

I'm picking up a multimeter today. Probably something used/inexpensive (I'm on a very tight budget) but I read a couple of multimeter threads and know to look for auto-range and trms. I may need a little guidance once I have those in hand to know what I'm doing. I did see a thread on what the power supply solder points should measure I think.

If a scope is mandatory there's one posted here locally (2 MHz input bandwidth, 10MHz repetitive, single channel, handheld). Would that be sufficient to get by for now?
 
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SA means Spectrum Analyzer in this case. Is that a Software SA on a computer you were using?
Personally, I intensely dislike Auto-range Multimeters. TRMS is good if the frequency response is (well) above 1kHz. Lots of cheap multimeters cannot respond to AC above 60Hz or so. Look for one that measures DC and AC volts down to millivolts, and above 200V; and also measures ohms below 1Ω and above 1MΩ
Scope is not mandatory. Spend the money on a better multimeter, instead, that will be your primary measurement tool.
I will be happy to help/guide the trouble-shooting process.
You will need a copy of the Service Manual and schematic. hifiengine.com is a free source.
KC is the Canadian version, KU is the US version. Often very little difference between them. The 220/240 versions primary difference is on the power supply transformer primary winding side, and then in the tuner sections.
 
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Cool. I would honestly like to buy a quality DMM. I would prefer not to elaborate but it's just not in the budget at the moment. I found a Tekpower TP9605BT for super cheap and am expecting to pick it up this afternoon if that will work (even as a single use item) I do intend to buy a quality one but it will have to be near-future and not today. I'll take another look and see if someone is selling something different/better before I go get that one, but it may have to do (if it claims to do the things you mentioned are required)

As far as SA I was using an app. I have a buddy with a real one (that has a mic and an independent box that he carries around) and it honestly didn't measure much differently than the app (+-5%) so I've used it when needed.

Thanks so much for the help. I have downloaded the schematics and will report back when I have tools in hand to take the measurements. Anything I should do in the mean-time in terms of familiarization or setup?
 
Ok. Good news. I didn't have to get an absolutely horrible cheap-o MM. I did get a low end Klein (MM600). No TRMS and it is autoranging but I couldn't find a manual range posted anywhere and this one was still new/unopened so here we are lol. I've put an upgrade on my list of things to pickup later.

Now, I've downloaded the schematic and service manual and am currently viewing them. I'm fairly familiar with engineering drawings, wiring diagrams, and schematics (for things much larger so this is much more compact) but it's been years since I've looked at them. I also notice it goes through a thorough testing regimen and doesn't seem to touch on the power supply portion other than the layout.

So I'm going to take AC & DC measurements from the power supply solder points? I'm looking for these on the schematic but don't see them.
 
The schematic is a representation of the electrical circuit, but does not help much with physical location. You can find circuit board foil-side drawings, and major chassis component location in the SM.
The first places to measure are the main dc power supply points.
Cover the probe tip so only the very end is exposed, so you do not short something out. We have all made that mistake in the past. Clip the black led to the chassis / ground, measure with the red lead. Set meter to dc volts.
The first measurements should be on the main PCB, GWM-114.
Measure dc volts on the main filter caps, + side of C413 (6800µF, 35v) should be about +28vdc. The - side of C414 (6800µF, 35v) should be about -28vdc. The - side of C415 (220µF, 35v) should be about -26.6vdc. These are unregulated dc supplies, and may not measure exactly as expected.
Measure AC volts at the same point, looking for high ripple. Some cheap meters do not do well when trying to measure AC while it is riding on a dc voltage, so you may have to put a small cap in series with the probe to block the dc.
Measure dcv on the emitter of Q401, sb about +13.6vdc.
Measure dcv on the emitter of Q402, sb about +41vdc.
Measure vdc on the - side of C416, sb about -28.2vdc.
Report your findings . . .
 
Just to clarify a dumb question, measure from the component side or the "bottom" solder side of the board?

Disregard this question. I wish I could unpost it it's so dumb lol
 
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if you look on the schematic for the marked voltages then see where the nearest convenient place to measure from should be fine ..
example 13.6v cathode d505 .less likely to cause a short than measuring q401 emitter .
 
Below are the findings. Only concern is should I have opened the speakers to get these measurements? I applied power and turned on the receiver, but didn't open any speaker "circuits" so to speak (maybe I used that correctly, maybe i didn't but i think you know what i mean). Anyway:

DC
c413 +31.3 vdc
c414 -31.27 vdc
c415 +10.95 vdc
Q401 +13.78 vdc
Q402 +45.7 vdc
Q416 -30.04 vdc

AC - I basically got 3 consistent readings and I'll post the 3 for each
c413 +161v, +18.35v, +3.84v
c414 +161v, +18.35v, +3.84v
C415 +56.5v,+6.4v

Some of these are obviously off the expectation, but if they're off the expectation in an odd or unexpected way (even for a failure) let me know and I'll remeasure any of them.
 
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Sorry, but your posts are not understandable.
With a - between the component and the number, makes it impossible to tell if it is negative or positive.
Please post positive dc voltage format as: +31.8vdc
Negative dc voltages as: -31vdc
The AC voltages are probably in millivolts or volts, format as: 193mVAC or 17VAC

That is one of the problems with Autorange meters, you have to check the range for every measurement.

Since I cannot see what you do, a description of "top to bottom" does not tell me anything.

The Service Manual or transistor datasheet (you will have to download it) will show which lead is which. Correct transistor lead identification is an important skill to develop.
Just part of the learning process, every technical field has its 'secret language' and notational conventions to increase understanding and communication quality.

Please don't take this as harsh criticism, but as information to learn to get help with remote troubleshooting. We all started at the same place you are in, new to electronics troubleshooting.
 
FWIW, power supply troubleshooting is normally done with no signal input and no load on the output.
 
All good points and I appreciate the feedback. I'll take another reading in the morning and update. Thanks for your help on this.

Is the AC understandable in how it was posted (with the three readings) or should I go get a cap to put in series for it?
 
A meter that has a problem with reading AC on a DC level will give inaccurate results, the only way to know is have a different meter or try it with a cap. 0.1 to 1µF works, as long as it is non-polarized and has a high enough voltage rating. Not a big deal at this time . . .

And you are welcome!
 
Ok, I updated the initial post with the properly indicated values and the measurements at the emitter. The AC readings are without a cap in series which I'll go get tomorrow. I just decided to go ahead and get what readings I could tonight.
 
DC voltages look good except for C415 -should be -26.6vdc, not +10.95vdc. Measure (with power off) R410, it should be 100Ω. I suspect it is open. Is it covered by a sheath tube? You may have to unsolder and lift one end. But even in circuit is should not be over 100Ω.

J A said:
AC - I basically got 3 consistent readings and I'll post the 3 for each
c413 +161v, +18.35v, +3.84v
c414 +161v, +18.35v, +3.84v
C415 +56.5v,+6.4v
Click to expand...

Why 3 readings? How do they differ from each other when taking the readings? If those are the actual readings, your AC range does not like DC voltage.
 
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