Mac Plus: no audio

[superpantoufle]

I have made myself a nearly perfect Mac Plus out of a dead one with help from a dead 512. I had a couple of strange Ram issues which are discussed here.

Just when I thought it was all solved, I got no more audio from the speaker. No startup beep, and no system sound. It does work well through the jack output, though. Any suggestion?

 

[tomlee59]

The most likely thing is a broken wire/disconnected cable. If you recently worked on the machine, perhaps you didn't securely reconnect the speaker cable?

The fact that you get sound from the jack means that the amplifier is working. There isn't much between the jack and the speaker, so it reduces to very few possibilities: bad jack, bad wiring, bad speaker. I no longer recall if there is a large coupling capacitor along the way, but if there is, I believe that the jack is downstream of it. Given that the jack provides output, that would tend to absolve the capacitor (if there is one).

The speaker itself is easy to test. A standard ohmmeter will reveal continuity -- it should be a couple of dozen ohms. The exact value isn't critical. If it reads only one or two ohms, or infinity, then the speaker is bad. If it turns out to be the speaker, that would be the first one I'd ever heard of going bad in a compact Mac.

 

[superpantoufle]

What bothers me is that the speaker is on the analog board, which I haven't touched. I did indeed switch logic boards several times.

 

[Dennis Nedry]

Does the speaker work with one logic board and not the other, or does it work with neither?

 

[superpantoufle]

Does the speaker work with one logic board and not the other, or does it work with neither?

Neither, sadly. And as long as I can see the speaker's cable on the analog board, it looks ok.

 

[Dennis Nedry]

Well then the problem is not on the logic board; I think that's a good thing. As I see it, the problem must lie in one of these spots:

Connectors & Cable between Logic board and Analog Board

Components on the Analog Board

The speaker

The first thing I would do is disconnect and reconnect all 3 connections several times to clean the connections. Then I would take something non-conductive to reach in and press around on each connector while playing sounds and see if you hear anything. If so, there is likely a bad solder joint. If that doesn't work, I'd closely examine the analog board and follow where the speaker goes and test/replace components along the way. I can't imagine this being too incredibly difficult to fix even in a worst-case that I can think of if you're willing to do some soldering.

 

[superpantoufle]

Thanks for caring!

Sorry, I don't really understand… which 3 connections are you talking about? And which connectors should I press?

There's only one big multi-cables connector between the logic and analog boards…

 

[Dennis Nedry]

Thanks for caring!
Sorry, I don't really understand… which 3 connections are you talking about? And which connectors should I press?

There's only one big multi-cables connector between the logic and analog boards…

The cable that connects the logic board to the analog board has a connector on both ends. Also the speaker probably has a connector (yet I'm not certain, it may be soldered directly, I can't remember)

 

[tomlee59]

According to the schematic, http://dyne.org/museum/apple/stuff/mac/andreas.kann/MacPlus3.GIF, the speaker is connected to the headphone jack through a single 10-ohm resistor and a connector (J8-4). That's all. So, troubleshooting should be trivial -- all you need is an ohmmeter to check continuity, and your eyeballs.

 

[Dennis Nedry]

Wow, this should be very easy to fix then.

I bet you could troubleshoot the resistor by simply shorting across it for a moment while playing sound. I can't imagine that would really hurt anything.

As always, be careful not to touch anything on the analog board while the Mac is on. There are life-threatening charges on there. For extra safety, always keep one hand in your back pocket and work with the other. This reduces current across your heart if you are to get shocked which can save your life.

 

[tomlee59]

Don't short the resistor. First, the likelihood of a resistor going bad in some subtle way is exceedingly low. If it's been overheated or mechanically damaged, it will be obvious. Second, the sound chip's output driver is not particularly robust. That 10-ohm resistor is actually important.

To test the speaker itself is trivial, either with an ohmmeter, or with a DC source (eg., a battery) in series with a suitable current-limiting resistor (say, 1kilohm or so). Contact will cause the emission of a click/scratch sound out of a working speaker.