IIsi audio issues with MacSD

[JC8080]

I just installed a MacSD internally in my IIsi and have some strange audio interference. The speaker makes noises corresponding with SCSI/CPU activity. It is hard to describe the noises, but it is like a combination of squeals and clicks. The noises line up with the SCSI activity, and moving the mouse around causes a buzzing sound. There is also a quiet hum when the CPU is idling. These noises aren't loud, but they are loud enough to hear with the top on the machine. I swapped the HD back in and could very faintly hear these sounds when I put my ear by the speaker, but not nearly loud enough that I had ever noticed. Has anyone run across anything like this? Here are the troubleshooting steps I have done:

• I originally bodged internal termination power from the external SCSI connector, since the IIsi does not have term power on the internal SCSI connector.
• Powered the MacSD via the mini USB port (after removing the term power bodge to ensure there was no interference from the bodge running across the bottom of the board.
• Used a long (18" or so) SCSI ribbon cable to move the drive away from the machine to see if there was some sort of RF interference. I also tried with a shorter cable, no change.

None of these had any affect on the noises. The floppy drive is not installed, and there are no PDS cards installed. This board is recapped and rather clean, the only rework needed was to bridge the leg/trace on two pins of one of the ALS245s where cap goo had corroded the connection with the pads. There was no battery damage.

 

[JC8080]

Here is a video I took with my phone, if you turn the volume up you can hear the noises pretty clearly. The buzzing sound at the beginning is what I hear when moving the mouse around, then the noises after that are a result of opening a couple folders and opening the Apple menu.

 

[Forrest]

Are you sure it’s not a feature? FYI ZuluSCSI features an option to emulate SCSI hard drives sounds.

 

[Phipli]

Have you recapped your IIsi?

Insufficient capacitance on the power rails causes noise to get back into the rails from one area to another, and sound is where you notice.

 

[JC8080]

Are you sure it’s not a feature? FYI ZuluSCSI features an option to emulate SCSI hard drives sounds.

Yes, the sound also happens/varies when there is no SCSI activity, like just moving the cursor.

 

[JC8080]

Have you recapped your IIsi?

Insufficient capacitance on the power rails causes noise to get back into the rails from one area to another, and sound is where you notice.

Yes, recapped with polymer caps.

 

[Phipli]

Yes, recapped with polymer caps.

Any bodge wires? Is there one on an audio analogue ground?

 

[JC8080]

Any bodge wires? Is there one on an audio analogue ground?

No, no bodges, the only thing I had to fix was small breaks between the trace/pad on the ALS245 closest to the cap by the power connector.

 

[ymk]

Yes, the sound also happens/varies when there is no SCSI activity, like just moving the cursor.

Insufficient capacitance on the power rails causes noise to get back into the rails from one area to another, and sound is where you notice.

This seems likely. If you're also hearing ADB noise, it seems like 5V ripple is getting coupled into the audio circuit somehow.

If the ESR on your new caps is too high, the effect is similar to insufficient capacitance.

Are you sure it’s not a feature? FYI ZuluSCSI features an option to emulate SCSI hard drives sounds.

MacSD doesn't emulate HDD noise.

 

[JC8080]

This seems likely. If you're also hearing ADB noise, it seems like 5V ripple is getting coupled into the audio circuit somehow.

If the ESR on your new caps is too high, the effect is similar to insufficient capacitance.

Thanks for the info, I'll look up the caps I ordered. Is this something that could be caused by cap electrolyte that wasn't thoroughly cleaned? I scrubbed the board with IPA and a toothbrush, and I think this one soaked in IPA for a while too, but it didn't go into an ultrasonic cleaner.

 

[JC8080]

These are the caps I used (link). The ESR for these is 30 mOhms, This seems low compared to the 800 mOhms of these tantalums (link) that seem to be the go-to for most recaps, but I know very little about caps.

 

[ymk]

These are the caps I used (link). The ESR for these is 30 mOhms, This seems low compared to the 800 mOhms of these tantalums (link) that seem to be the go-to for most recaps, but I know very little about caps.

That's a good rating. If there's electrolyte still bridging pads, bare traces or IC pins, that could easily cause problems.

 

[JC8080]

That's a good rating. If there's electrolyte still bridging pads, bare traces or IC pins, that could easily cause problems.

Do you have any thoughts on why this only happens with my MacSD, and not with a hard drive? With a hard drive hooked up if I put my ear right up to the speaker I can hear the same noises but very, very quietly. They are only loud with the MacSD.

 

[Phipli]

Do you have any thoughts on why this only happens with my MacSD, and not with a hard drive? With a hard drive hooked up if I put my ear right up to the speaker I can hear the same noises but very, very quietly. They are only loud with the MacSD.

Are you powering it from termination power, or with a molex?

 

[JC8080]

I have done both, same results. I first bodged term power from the external DB25 to the internal connector, then removed the bodge and powered the MacSD through the mini-USB plugged into a wall adapter.

Edit: I'm not sure if it is relevant, but I also do not get this sound if I am using an external BlueSCSI. Only the MacSD internally. Unfortunately I do not have an internal BlueSCSI to test with

 

[ymk]

Is the noise present at the output jack? One channel, both?

 

[JC8080]

Is the noise present at the output jack? One channel, both?

Good question, I put the machine away but I'll check tomorrow or the next day.

 

[JC8080]

I plugged in a stereo headset, the sound comes out of both sides, and does not come out of the internal speaker at all while the headset is plugged in.

 

[Phipli]

I plugged in a stereo headset, the sound comes out of both sides, and does not come out of the internal speaker at all while the headset is plugged in.

The internal speaker is physically disconnected when you plug something in externally.