Macintosh Classic No Audio

[Otho]

Hello folks! I'm working on bringing my college 1991 Macintosh Classic back to life. It suffered from leaking capacitors and was completely dead, so I decided to learn how to solder and did a complete recap of both the logic and analog boards and a through cleaning with 99% iso. After some reflowing of my amateurish soldering it came back to life in all regards except one - a complete lack of audio. At this point I am stuck. I have checked the speaker (resistance 61.5 ohms), tried using headphones (no sound), and desoldered, cleaned, and resoldered the UA5 sound chip twice. I cannot see any broken traces - is the sound chip simply dead? I am new to this, so any advice would be most welcome! I am attaching some photos of the logic board as it currently sits.

Many thanks!

Otho

 

[bibilit]

Probably a broken trace, use a multimeter and check continuity from the chip and to any capacitor

 

[imactheknife]

You might in a worse case scenario have to remove sound chip and clean pads/legs and repair traces. Check top of legs to the pads on sound chip and then legs / pads to vias / caps. Sometimes its best to check these before new caps are put on as the broken trace can be right underneath due to cap leakage.

Good job by the way always fun to try and fix up and old mac

 

[Otho]

Probably a broken trace, use a multimeter and check continuity from the chip and to any capacitor

Will do! Haven’t found any broken traces yet, but I will stick to it- thanks!

 

[Otho]

You might in a worse case scenario have to remove sound chip and clean pads/legs and repair traces. Check top of legs to the pads on sound chip and then legs / pads to vias / caps. Sometimes its best to check these before new caps are put on as the broken trace can be right underneath due to cap leakage.

Good job by the way always fun to try and fix up and old mac

Agreed, and will do! I haven’t found any broken traces yet, but I will stick to it. I’m focusing on the C3 cap and UA5 chip just as you suggest. Many thanks - been a real trip bringing this back to life and reading through 30+ year old files on it.

 

[dschnur]

For something like this, I normally remove the chips in question, clean the pads, then take a meter and check from pad to via, or from pad to destination pad of another chip. I find that it's pretty common to have a hair line crack right between the pad and the trace. Often they are small enough to not see without magnification, but large enough to require a jumper wire instead of a solder blob. I use .1mm magnet wire to jump them.

 

[bibilit]

Hard to say from a picture but is C1 properly soldered on ? The negative side doesn’t look like it is.

 

[dschnur]

Hard to say from a picture but is C1 properly soldered on ? The negative side doesn’t look like it is.

Check it with a meter. That's the best bet.

 

[Otho]

Folks,

Many thanks for all the tips! I'll doublecheck C1, and have begun desoldering various chips and caps and checking the traces. I acquired an upgraded multimeter today to help check continuity. A good project to learn on - I will keep you posted!

Otho

 

[dschnur]

One more thing. Make sure the contacts in the audio jack are working. There are slider contacts inside the switch ( you need to remove the switch to get to them ). Those contacts can get gummed up and if they are not working, you have no audio. Try it with something plugged into the jack and see if that helps. Also, the traces leading to the audio jack and also the ASC can get damaged. Check from pad to pad via the schematic to make sure the whole trace and it's pads are all connected. I've had hair line cracks right at the pad/trace junction that have given me hard times.

 

[Otho]

Folks,

I may have found the culprit. De-soldered and removed caps C1 - C4 and the sound chip. All the traces checked out except for one running from a via by the reset button to the sound chip. When cleaning up after de-soldering the chip, the pad in question lifted, so I am hoping that was the problem all along. Should be a straightforward trace repair, so I will get to learn how to do this. Everything else checks out so far as I can determine on both the logic and analog boards.

Thanks for all of the great advice - I will keep working!

Charlie

 

[dschnur]

.01mm magnet wire, lots of flux, and clean solder. You can also just run a jumper wire from the via behind the button to the pin on the chip once it's soldered back in place. Make sure you test from the pads on the audio chip to the destination pads (where ever those may be) to verify the integrity of the other traces while you have the chip off. Also, pro bonus points, when you are cleaning pads, do not put too much pressure on your iron or solder wick. Also, don't get the pad too hot. Those are common causes of pads lifting. I curse myself to this day whenever I do that. And, I hate to admit it, I still do that.

Great detective work so far!