Macintosh Portable Issue - Screeching Noise

[RetroTheory]

Currently working on a one of these but I am kinda stumped. Its a non backlight model.
I have fully recapped it, (every cap was leaking, including the axials)

So I can get it to Bong, and will show flashing floppy. The Connor SCSI drive is working, and it can boot, but after booting it emits a loud screeching noise. Normal audio can still work, I turn it off ASAP when it makes the noise. Then sometimes it wont switch on at all. I leave it for a while and it will power up fine. Then screech. Anyone experience anything similiar?

 

[Stephen]

Speaker may be shorting against something. The design is dangerously compact — try putting tape on the back of the speaker to rule it out and inspecting the wires.

Mine is extremely finicky and I often need to press the "reset" button on the lower-left-hand side of the portable multiple times for it to successfully boot.

There are a few projects to convert the custom SCSI connector to a standard 50-pin, I believe Zone66X (on GitHub) has a version. I also have a version but I am not happy with the outcome, it is slightly too large for the enclosure.

 

[Drake]

I agree with Stephen, sounds like a grounding issue. Similar experiences can be replicated with something like the color classic when shielding isn't properly installed. Typically I've noticed the screeching coinciding with HDD activity over the speaker.

 

[Daniel Hansen]

A little late to the party... did you resolve the issue? My first thought was whether or not the board was cleaned after recapping.

 

[RetroTheory]

A little late to the party... did you resolve the issue? My first thought was whether or not the board was cleaned after recapping.

It was cleaned in IPA but not in ultrasonic bath , but I have not gone back to it since .

 

[Bolle]

Cap goo leftovers around the sound area of the board. I had the exact same problem on my Portable. As soon as the sound chip was kicked into low power mode by the power manager (aka the 12V supply to the sound chips gets shut down) it started screeching horribly.
I desoldered everything from the board in that area, cleaned up the board and ICs and resoldered them - since then the problem was gone.

 

[RetroTheory]

That sound like what its doing. I will give it another shot at the weekend.

 

[techknight]

That sound like what its doing. I will give it another shot at the weekend.

The other thing to be aware of on these portables is the molex connector. YOU MUST make sure both B+ input connections are connected and have voltage or really bad things will happen.

 

[Nico]

Cap goo leftovers around the sound area of the board. I had the exact same problem on my Portable. As soon as the sound chip was kicked into low power mode by the power manager (aka the 12V supply to the sound chips gets shut down) it started screeching horribly.
I desoldered everything from the board in that area, cleaned up the board and ICs and resoldered them - since then the problem was gone.

Hi Bolle,

In have exactly the same problem. Can you tell me which ICs you desoldered ? It‘s pitty there are no schematics for the mac portable.
What temperature do you set your hot air station for desoldering on these boards ? I set mine at 340 celcius and speed 50%.

 

[Androda]

It‘s pitty there are no schematics for the mac portable.

Here you go: https://archive.org/details/Macintosh68kSchematics

 

[Nico]

Thanks,

I had that file but must have looked over it…
Does anyone have a schematic for the blacklight model M5126 ?

My board does look clean as I don’t see IC’s from the sound circuit with capacitor goo or corrosion on the pins but as Bolle said it’s probably under them.
Maybe Bolle has a suggestion which one‘s to start with ?

 

[pocketscience]

I had this exact same issue... about 20 seconds after boot the Portable would just start wailing from the speaker. I'd previously recapped and cleaned the board, and the board in question is super nice - no corrosion. I started looking for areas that hadn't cleaned properly and in doing so discovered that L8 was loose. It was soldered down - but the core was wiggling between it's own connectors. I removed it and it seemed ok, apart from the wiggling. So I re-soldered it back down, and made sure to use enough that it didn't wiggle any more. As soon as I booted after that the noise was 99.9% gone! There's still the slightest step change in speaker hum at the 20 second mark post-boot, but if you weren't listening for it you wouldn't notice. And I'm guessing once it's back in the case it won't be noticeable at all.

 

[RetroTheory]

Oh thanks for that, That will be next on my list to check.