Macintosh Portable: "pop" noise every 10s from Speaker & Headphones

[JDW]

NOTE! The motherboard has been ultrasonic cleaned, recapped, traces repaired, and cleaned again via ultrasonic. The Apple AC Power Adapter I am using has also been cleaned, recapped, and cleaned again.

PROBLEM: I'm hearing a soft pop sound from the speaker every 10 seconds when my 5126 Backlit Macintosh Portable is powered, regardless of backlight usage and regardless of the HDD being spun up or down. Audio below was recorded with my iPhone's mic sitting directly atop the speaker with the Volume set to 7 (max), backlight OFF, HDD spun down. I boosted the audio slightly in the video below so you can hear it better.

The little "pop" sound is not too loud sitting at a normal distance from the machine, but I am curious if you Mac Portable owners have ever heard anything like this. Yes, I hear the same thing with headphones connected to the headphone jack, and of course, the noise is more noticeable with headphones. By the way, all the noise you hear (not just the pop sound) is coming from the speaker. I recorded that in a very quiet room.

When I set the Speaker Volume to 0 (OFF), then I cannot hear the popping at all. Ditto when I use Headphones at Volume-0. However, I still hear a bit of noise through headphones when Volume=0.

Not sure what is normal, as I have no experience with the Portable until now. @Branchus , I'd love to hear your thoughts.

NOTE: I am using a Portable Battery Eliminator, which allows me to power the machine without a battery. Sadly, I have no battery to test with, and this is my first experience using a Portable, so I would love to hear from you Portable owners regarding this repetitive noise through the speaker and headphones. But I've determined that the super capacitors in the Battery Eliminator can keep my Portable live and running with backlight (but no RAM card and HDD spun down) for more than 10 seconds. I yanked the power cord out 1 second before the expected speaker pop, and I heard a pop, then I heard the pop again 10s later -- all while the machine was running off super capacitor power. This leads me to believe that even if I did have a lead acid battery, the speaker pops would remain.

Tagging some fellow Portable owners who might be willing to listen via speaker & headphones to see if theirs has the same issue:
@Sideburn @Garrett @Paolo B @techknight @wottle @ScutBoy @pocketscience @alxlab @Androda @Branchus

 

[JDW]

I will admit I am disappointed that although this thread has been read 78 times, I am the only person thus far who has replied. But maybe this reply of mine will bump this thread into view and generate a little feedback from some of you.

I'm getting replies on FaceBook (Vintage Apple Macintosh Enthusiasts group), and I shall now share a little of that with you.

I still have no idea what the source of the soft "pop" sound every 10 seconds is. But I am examining the audio circuit.

One person on FaceBook contends that Solid Tantalum capacitors used as replacements for C9, C10, and C11 might be causing the problem, but it's hard for me to believe that because @Branchus and many others use solid tantalum replacements in the audio circuit, and yet I've heard nothing from them about a soft "pop" sound every 10 seconds. So either that pop doesn't exist on the Portables they recap, or Bruce and others never listened to the audio line for more than 10 seconds. I have no idea. I really wish someone would do a test to confirm though. It's fast and easy, especially with headphones. So if anyone could bless with me with a listening test of your own Portable, I would be forever grateful.

Next...

The audio circuits of the 5120 and 5126 are slightly different when it comes to how "SONY_+12" is routed to the two SONY audio chips, as shown on the marked up photos below (original photos courtesy of the @Branchus Creations website):

The absence of Q5, Q6 & Q7 on the 5126 (backlit model) indicates R92, R88, R63 & R62 serve no purpose. It is therefore surprising they are included on the 5126 motherboard.

Anyway, on the 5120 (non-backlit model), Q5 is a P-channel power MOSFET that connects its pins 2 & 3 together when its pin-1 (the Gate) is grounded by Q7 or Q6. See schematic below.

When the MOSFET is ON, +12V gets routed and filtered through L6 and sent out to all pins marked SONY_+12, which is to the two SONY chips. Since there should be +12V send to the two SONY audio chips even on the 5126, it means +12V is getting to those pins another way on the 5126.

Not sure why Apple eliminated Q5, Q6 & Q7 on the 5126.

---

Anyway, my intention is to power my 5126 motherboard headless so I can probe various points in the audio section with my oscilloscope, as shown in the photo below:

The card you see installed is the Portable Battery Eliminator, designed and sold by @Androda and tested by @techknight . That nifty device will make it easier to power than the stock way, especially for me, since I don't have a working battery (and is why I purchased the Battery Eliminator).

I'm busy this weekend and won't have time for that scope probing work until Monday, but if any of you have ideas, thoughts or audio test results from your Portable, I would really and truly appreciate hearing from you!

Thanks!

 

[Paolo B]

I don’t have this issue on my non backlit Portable. Or maybe I never noticed it.

 

[JDW]

I don’t have this issue on my non backlit Portable. Or maybe I never noticed it.

I wish to humbly thank you and praise you for being the first person who has so kind take time to do a listening test on your 5120 Portable. You have confirmed there is no soft "pop" sound every 10 seconds.

I suspect that is true on the 5126 Portable as well, but I would appreciate feedback from someone with a 5126 to confirm that, especially through the headphone jack where it can be more easily heard.

In the meantime, @Paolo B , I would like to ask you about the base noise level you hear through the headphone jack when the Volume is set to 0. Do you hear nothing at all, or do you hear a little noise/hiss?

When I set volume to zero on my 5126, the "pop" is gone but I hear a noise floor that is higher than what I hear on my other vintage Macs, and that is why I am curious what the noise floor is like on your 5120, as heard at the headphone hack, with volume set to 0.

Thanks!

 

[Paolo B]

Unfortunately, I am not exactly the best guy to refer to when it comes to “hearing” something, but I will give my best shot at it. I need to get my Portable out of the closet, will see what I can do.

 

[JDW]

Unfortunately, I am not exactly the best guy to refer to when it comes to “hearing” something, but I will give my best shot at it. I need to get my Portable out of the closet, will see what I can do.

When you test, please note:

1. I cannot hear the soft "pop" at all if the Volume is set to 0 via Control Panel.
2. I can still hear background noise through headphones even if Volume is set to 0.
3. You need to listen for 30 seconds to see if the noise changes. In my case, it does. There's a slight increase in noise after 10 seconds or so -- something I would define as a swirling noisy sound on top of the noise floor. (You can hear the swirling sound in the video I posted in my opening post. Sounds almost like a soft car engine running.)
4. Set Volume to 7 when you listen to headphones (with no sounds playing, of course). Then set it to 0. Compare the noise floor.

 

[SuperSVGA]

Since there should be +12V send to the two SONY audio chips even on the 5126, it means +12V is getting to those pins another way on the 5126.

On the M5126, the source and drain pins of Q5 are directly connected, so +12V is always supplied, and Q5 would be pretty useless.
My guess was they decided that being able to turn off the sound wasn't necessary, like many other questionable decisions they made when making the M5126.

As to what's going on, I'm not entirely sure you have a hardware issue, because that sound I believe is software generated. Can you try doing something to generate constant floppy disk activity and see if you still hear the sound?

 

[JDW]

On the M5126, the source and drain pins of Q5 are directly connected, so +12V is always supplied, and Q5 would be pretty useless.

Can you try doing something to generate constant floppy disk activity and see if you still hear the sound?

Thank you for making time to reply and for confirming that the source & drain on that P-channel MOSFET are connected together on the 5126 Portable.

In answer to your question, my first steps will be to test the sound with my 5126 Portable in the headless condition as shown in the photo in my earlier post, where only the keyboard is attached to the motherboard. It will be powered by the Androda/TechKnight Battery Eliminator card. After that, connecting an external floppy drive would probably be the safest and easiest thing to do, at which time I could offer you feedback.

The reason for removing the motherboard from the plastic case and running at headless is so that I can use my oscilloscope to probe various points in the audio circuit in hopes of possibly tracking down the source of that little “pop“ sound that occurs every 10 seconds, so long as the control panel volume is set between 1 and 7.

Because there is no Pram battery attached, the default volume setting should reset to 3 when I do my testing in the headless condition.

 

[JDW]

By the way, the following is a close-up photo of the two 10-ohm resistors

The resistors look fine, but take note of how the 2 traces that connect those resistors in parallel have been blackened. I’ve highlighted those connections in yellow below:

I was able to use an X-Acto knife to very gently and carefully scrape off the blackened solder mask to reveal that the traces below are just fine and perfectly intact. I confirmed that using my handheld meter in Continuity check mode as well.

I can only interpret that to mean a fair amount of current must have passed through those 2 resistors, either momentarily or on a consistent basis. Yet it was clearly an amount of current that those two resistors could handle, but which clearly overheated those two traces.

The sound works, by the way. It also gets quite loud at the maximum control panel setting of seven. So I’m not complaining about the audio not working.

I did test with headphones and remember that the balance wasn’t perfectly the same between the left and the right, but I don’t remember it being too terribly off center. I booted into System 6, so I didn’t have as much control panel flexibility as I would if I had booted into system 7, to change the balance.

The main reason I started this thread is because I’m trying to determine what’s normal for the 5126 portable in terms of the noise floor, even when the control panel volume is set to zero. And I’m also trying to determine if that soft pop every 10 seconds which you hear in the video in my opening post is normal, and if not normal, what is the source of it.

 

[Paolo B]

When you test, please note:

1. I cannot hear the soft "pop" at all if the Volume is set to 0 via Control Panel.
2. I can still hear background noise through headphones even if Volume is set to 0.
3. You need to listen for 30 seconds to see if the noise changes. In my case, it does. There's a slight increase in noise after 10 seconds or so -- something I would define as a swirling noisy sound on top of the noise floor. (You can hear the swirling sound in the video I posted in my opening post. Sounds almost like a soft car engine running.)
4. Set Volume to 7 when you listen to headphones (with no sounds playing, of course). Then set it to 0. Compare the noise floor.

Guess what? The battery in my Portable is totally floored, it will take many hours before it gets back to a decent voltage.
Anyhow, it will be a good opportunity for getting out of the closet some vintage recording equipment, too...

 

[JDW]

Guess what? The battery in my Portable is totally floored, it will take many hours before it gets back to a decent voltage.
Anyhow, it will be a good opportunity for getting out of the closet some vintage recording equipment, too...

Honestly, I strongly recommend you purchase the following product, which eliminates the need for a battery entirely:

Macintosh Portable Battery Eliminator – Tech by Androda

androda.work

I also strongly recommend that everyone recap the Portable’sAC adapter too, because all of them have leaked, and they’ve leaked a lot.

 

[JDW]

The more I look at R62 & R63, The more inclined I am to believe that somebody must have swapped them out in the past.

For example, look at the solder on either side of those two resistors in the photo in my earlier post. It comes up in the shape of a little mountain, just as if it would if your soldering iron had applied the solder and then you were pulling up on it. The solder on either side of any other SMD component in that area does not look like that.

Also, with all of the blackening in that area, they have those resistors look as pristine as they do gifts for their evidence they must’ve been replaced.

 

[SuperSVGA]

And I’m also trying to determine if that soft pop every 10 seconds which you hear in the video in my opening post is normal, and if not normal, what is the source of it.

Well as to "normal" if my guess as of the cause is correct I would say "yes, but also no".


Take a look at the SndWatch VBL task from the Macintosh Portable ROM:

SndWatch:
        movea.l     PowerMgrVars,A2
        lea         (SwVBLTask,A2),A0
        move.w      #600,(vblCount,A0)
        subq.w      #4,SP
        movea.l     SP,A0
        move.w      #soundRead,D0
        bsr.w       PowerMngr
        move.b      (A0),D2
        beq.b       .L2
        btst.l      1,D2
        bne.b       .L1
        clr.b       (A0)
        moveq       #1,D1
        move.w      #soundSet,D0
        bsr.w       PMGRsend
        bra.b       .L2
.L1:
        move.b      #3,(A0)
        moveq       #1,D1
        move.w      #soundSet,D0
        bsr.w       PMGRsend
        _IdleUpdate
.L2:
        addq.w      #4,SP
        rts

The #600 literal there is the timer for the VBL task. 60Hz * 10 seconds = 600.
This VBL task basically just turns the sound off when not in use, and then turns it on when something is using it.

Now take a look at this excerpt about VIA registers from the Guide to the Macintosh® Family Hardware, Second Edition, specifically the second paragraph:

Bit	Direction	Bit name	Description
7	Input Output	vSCCWrReq vSndEnb	0 = SCC Wait/Request, channel A or B 0 = sound enabled
6	Input	vSndExt	0 = plug is inserted in external sound jack
5	Output	vHeadSel	Floppy disk state-control line SEL
4	Output	vDriveSel	0 = upper internal floppy disk, 1 = lower
3	Output	vSync	1 = synchronous modem support, channel A
2	Output	vTest	Test signal
1	Input	vPMAck	Handshake signal for Power Manager IC
0	Output	vPMReq	Handshake signal for Power Manager IC

Bit 7 of Data register B in the Macintosh Portable serves two functions. When the CPU's interrtupts from the SCC are disabled, this bit monitors the /W/REQA and /W/REQB lines from the SCC, which are wired together on the main logic board ("wire ORed"). The Macintosh Operating System uses the vSCCWrReq bit to determine when the SCC has received a character (byte), thus enabling the operating system to maintain serial communications during floppy disk accesses, when SCC interrupts are disabled. The SCC is discussed in Chapter 10.

When SCC interrupts are not disabled, bit 7 can be monitored by software as an aid in controlling the sound circuit. Once every 60th of a second (VBL interval), the operating system checks to see if this bit has been cleared. If it has, the Sound Manager sets up the ASC and sends a pulse to click the speaker. When used in this fashion, bit 7 emulates the function of the same bit in the VIA of the Macintosh SE and classic Macintosh computers and thereby helps to maintain compatibility with software written for those machines.

I haven't yet found the VBL responsible for this in the ROM, but I assume it only plays the sound if the bit was previously found to be set, and then on the next check it was cleared (sound on).
This would imply SndWatch is having to turn the sound on every time it checks (every 10 seconds) or something else is clearing VIA bit 7.
Maybe you could look at the pull-up for bit 7, which should be R15, located in area G-11.

 

[Paolo B]

So, I took the opportunity "kindly" offered by the ugly weather here in Aichi for playing around a bit with my Portable and vintage audio system.

I can confirm the headphone output is dead quiet, regardless of the output volume level. Totally equivalent to my “modern” MacPro.

As for the loudspeaker popping, that one is also not present. The only noise I could catch with a microphone positioned right on top of the loudspeaker location is a 60 Hz noise (and harmonics) which I guess may come from the power line.

Additional remarks. The battery took one day to get recharged to the point I could start the machine.

I have System 7.1.1 I got a few odd crashes, as the machine hung up waking up from sleep mode. I remember it working flawlessly, going to sleep and waking up without issues.

Final remark. Believe it or not, only today I realised that my beloved trashcan MacPro has indeed no sort of audio in. If anyone has any suggestion about an audio interface (preferably, with digital in/out), I’d appreciate.

 

[JDW]

Thank you for your kindness in doing that audio testing. That confirms my suspicions. For truly, the base noise floor with the volume turn to 0 is unusually high, and such as not there on any of my other message Macs.

Nevertheless, the audio works and is quite loud at the maximum volume setting of 7. And of course I get that soft Pop sound every 10 seconds too.

That raises the question about what could possibly cause those two things on my machine. Again the two things are a high noise floor, and the pop every 10 seconds.

If one of the two Sony audio chips were bad or if the Apple audio chip was bad, I would assume that I wouldn’t get any audio at all or get some kind of strangeness beyond which I’m seeing now. Or maybe the maximum volume would be low even when the control panel is set to a maximum volume of 7. But I am not seeing that. Everything is normal except for the two things I just mentioned.

It’s a very perplexing problem!

 

[YMK]

Did you find out what powers the sound chips?

I don't have 5126 schematics.

 

[JDW]

Did you find out what powers the sound chips?

I don't have 5126 schematics.

According to @SuperSVGA in his post here, he says pads marked 3 & 2 for the missing MOSFET Q5 on the 5126 backlit Portable are connected together. In other words, that would mean the following should be true...

NOTE: I don't yet know if the SOUND_CS circuit above is just left floating due to the absence of Q7, or if its R93 output is actually connected to something.

As you can see from the 5120 schematic Sound Interface section below, the above means that +12V is getting sent through the L6 ferrite bead and through R63 & R62 in parallel (yielded 5Ω total resistance) to pins 12 & 13 on both SONY sound chips, marked "SONY_+12":

Not only that, but as you can see, that same "SONY_+12" also goes through a 330kΩ resistor to pin 15 on each of the SONY sound chips too.

The full Macintosh Portable 5120 schematic can be downloaded from our Resources section here:

https://tinkerdifferent.com/resources/macintosh-portable-5120-schematic.133/

Nobody has 5126 schematics. Even so, the audio circuit is largely the same on the 5126 as on the 5120, except for the missing Q5, Q6 & Q7. All of the component silkscreens that I am seeing so far are the same on my 5126 as I am seeing on the 5120 schematic.

 

[JDW]

Okay, friends, here are the results of my first HEADLESS test of my 5126 Portable:

Everything was disconnected from the Motherboard except:
(1) Keyboard (required for power-on)
(2) Battery Eliminator from Androda
(3) SONY PCM-D100 Audio Recorder with headphones attached.

High quality audio was recorded in 24-bits 48kHz Linear PCM (not compressed). Audio sounded exactly the same even when I yanked the AC adapter cord and left it powered for several seconds via the Battery Eliminator's super capacitors. So no one can say it is wall socket generated noise.

There are the following 2 points of interest in this test regarding NOISE:

1. There is no "pop" every 10 seconds in this test, although there is when the machine is fully assembled.
2. There is a constant whine in the right channel after 5.5 seconds into the test (see my video above). I don't remember that whine when the machine was fully assembled.

Other points of interest:

• As you can see in my video, after pressing a keyboard key, I get a bong, which is normal.
  
  
• Voltage measured across axial capacitor C8 is SONY_+12 (see schematic pic in my earlier post). That voltage level is initially 1 V before I press any key, and then it rises to 11.83 V after I press a key and hear the bong.
  
  
• The computer shuts itself down approximately 2 minutes after I press the key to start it up. There is no sound whatsoever coming through the headphones after that. And the voltage measured across capacitor C8 drops to zero after capacitors on the motherboard discharge.
  
  
• SONY_VREF = 4.95v stable
  
  
• Cannot hear any whine when the stock speaker is attached and headphones disconnected. I only hear the same kind of hiss that I hear in my left ear when wearing headphones.
  
  
• When I measure the PWM signal on pin 18 of either of the Sony sound chips, I measure the same 22.254 kHz 5Vp-p squarewave on both chips (oscilloscope Probe in 10x mode):
  
  
  
• When I put my oscilloscope probe in 1x mode to better measure small signals, and then put the probe on capacitor C10 (the 47µF output cap for the Right channel coming out of pin-11 of U15C), I measure the waveform you see below. (Ignore the light colored spikes, as my probe's ground was the normal long one, and those spikes result from that longer ground lead.) In other words, that whine in the right channel seems to be 1.1kHz, which sounds about right to the ears when you listen to the whine in my video above. You can hear it via your computer's speakers when you watch my video, but using headphones will let you hear everything more clearly.
  

I am now letting the super capacitors discharge so I can test again tomorrow night after work with the LCD attached, to see if there is any difference in the audio. I will leave drives disconnected. But I have one question for @Androda ...

What voltage level, when measured at the 4-wire power connector on the motherboard, is safe for me to connect or disconnect the LCD, do you think? After yanking the AC Power Adapter, it takes an extremely long time for the voltage to drop, even when I use the little discharge jumper on the Battery Eliminator.

 

[Androda]

What voltage level, when measured at the 4-wire power connector on the motherboard, is safe for me to connect or disconnect the LCD, do you think? After yanking the AC Power Adapter, it takes an extremely long time for the voltage to drop, even when I use the little discharge jumper on the Battery Eliminator.

Generally speaking it's safe to connect or disconnect things from the motherboard when the main 5v rail drops to a solid zero volts. My listing states to wait awhile because these are supercapacitors and if you accidentally bump the legs with something conductive you're going to get a huge blast of current across whatever touched the legs.

Q16 is the main regulator mosfet (you can tell which it is because of the metal heatsink). One of the pins will be very close to the battery voltage, another pin is the regulator control, and the last pin (and metal tab) are the main 5.2v output.

 

[JDW]

Okay, I did a little testing on my lunch break today.

By the way, the no-load (disconnected) voltage level of my fully recapped Macintosh Portable AC Adapter measures 7.568 V. I used that Adapter to power the Battery Eliminator card. Not sure what the normal output voltage of that Adapter is, so I just thought it best to mention the no-load voltage of mine.

@Androda , you may find this interesting. I measured the 5v line at C16 and found it to be 5.28v initially, and then after power on it slowly rose to 5.30v and stayed there until the unit automatically powered off after 2 minutes, and then the voltage measured 5.29v. (My meter's ground was on the ground lead of capacitor C8, a 470µF cap in the audio circuit.)

The above schematic is for the 5120. My unit is a 5126. No, I've not checked all the differences, but C16, L10 and Q16 are all there on my backlit model.

The photo below was taken just after the 2 minutes expired and the machine powered off by itself. As you can see, 5.29v. Is that voltage level normal for all Macintosh Portables when the Battery Eliminator is used?

What's interesting is that there was no change to the audio. I heard exactly what you hear in my video. No "pop" every 10 seconds, but the whine only in the right channel of my headphones.

The next step in testing Audio would need to be to re-attach the Trackball and then the drives.

Again, the audio works. That's not the issue. The original issue in the fully assembled Portable was that I could hear a soft "pop" sound every 10 seconds. It never went away. Then I disassembled and did the headless test only to find that issue gone, and now I hear a whine in the right ear when using headphones. Not sure what the deal is with this machine!