Mac Classic II won't start, instead purrs

[svenvendetta]

I had the logic and analog board re-capped about a year ago. Worked for about 7 months and then started doing this clicking on startup (sound on)... I did have some additional service done to the analog board and it started up fine once, then went back to doing it. Anyone have any ideas of what would make it just get stuck like this when I hit the power?

 

[Mac84]

To me it sounds like it's failing to get the power it needs to start up. I would double-check the voltages. Thankfully you can do this easily from the floppy drive port with a multimeter. Bruce has an excellent video (here) that shows the process. If the voltages are in spec, then turn your eyes to other components on the analog board or the logic board.

 

[svenvendetta]

Thanks for the advice Steve. It is indeed not getting enough voltage when I metered those pins, looked about half of what it should be on both 12v and 5v pins. I find it odd that it did indeed work fine when I installed it today and then after I closed up the case (of course) it decided to fail again. Not sure what could have happened, maybe its shorting on something? I guess I need to take a close look at the board again.

 

[SteveHere]

I was just working on a Mac Classic this week and had a similar symptom. I was reading in Larry Pina’s book “Mac Classic & SE Repair and Upgrade Secrets”. There’s a small Pot in the center of the Analog board for adjusting the voltage, look for broken solder joints and use contact cleaner on the Pot. In my case just putting DeOxit on that pot was enough to get the voltages back up where they belong. I hope that helps!

 

[ClassicHasClass]

My cat did the same thing until I replaced her power supply.

 

[svenvendetta]

So an update, kind of at an impasse with this project. I was able to get it to boot up properly one more time and when I metered it as advised in the @Branchus video on the correct pins of the disk port, I did get the proper voltages. However it went right back into "purr mode" and the voltages were once against half of what they were before.

I have to think I have failing or weak components in the analog board but still don't get the intermittent nature of a few random successful boots. I did mess with and deoxit the voltage pot, like @SteveHere mentioned too. no change. If someone has a refurb board I'm interested. I bet the current board is salvageable for someone who's got skills I don't also.

 

[3lectr1c]

There's a bunch of stuff that fails on the Classic/Classic II analog boards but I can't keep it all straight. We need a resource or thread giving a rundown on the common failures that can just be linked whenever someone's having AB trouble on one of these. If anyone makes one, I'll throw it up on my website (with permission of course).

 

[svenvendetta]

A really good idea @3lectr1c

Couple of steps I would identify (just from research) off the bat after a recap and solder joint inspection is to look at the following components:

• DP3 diode
• DP4 diode
• QP1 optocoupler
• RF2 2 KOHM 1W

A place to source the parts would be a good idea too.

 

[Retroplayer74]

I am by no means an expert on these Macs yet, but I did have a similar issue to what you are describing. What appears to be happening is that the 12V rail is rising to the cutoff point (14.8V I believe).

When that happens, you hear the sound of the power supply starting up and shutting down. Like a flub flub sound (or a purr, I guess.) There is more than just one cause of this instability of the 12V rail. In my case, it was left over leaked cap juice under components around the 12V and 5V regulation circuit. I had to rinse the board several times in alcohol.

If you unplug the hard drive, or floppy, it will likely be able to boot. But as soon as you put some type of load on that 12V, it does the flub flub again. Does that match your observations?

 

[Branchus]

Couple of steps I would identify (just from research) off the bat after a recap and solder joint inspection is to look at the following components:

• DP3 diode
• DP4 diode
• QP1 optocoupler
• RF2 2 KOHM 1W

A place to source the parts would be a good idea too.

I keep a supply of optocouplers on hand and replace them on every Classic and Classic II that comes my way these days. They fail a lot.

 

[svenvendetta]

I keep a supply of optocouplers on hand and replace them on every Classic and Classic II that comes my way these days. They fail a lot.

I have vowed to put this project away for a bit so it doesn't drive me insane but it keeps pulling me back in.

Do you have links or part numbers where you source these from @Branchus ?

 

[svenvendetta]

I am by no means an expert on these Macs yet, but I did have a similar issue to what you are describing. What appears to be happening is that the 12V rail is rising to the cutoff point (14.8V I believe).

When that happens, you hear the sound of the power supply starting up and shutting down. Like a flub flub sound (or a purr, I guess.) There is more than just one cause of this instability of the 12V rail. In my case, it was left over leaked cap juice under components around the 12V and 5V regulation circuit. I had to rinse the board several times in alcohol.

If you unplug the hard drive, or floppy, it will likely be able to boot. But as soon as you put some type of load on that 12V, it does the flub flub again. Does that match your observations?

Thanks for the suggestions, I have been trying to boot it with no components connected and getting the same result. Gonna try and replace some of the normal culprits first and see what happens from there.