Macintosh Classic - no sound after recapping analog board

[electricmonk]

Hi all,

I've been fiddling around with this Mac Classic to try to bring it back to life. I washed the motherboard and it started working again so I figured recapping the MB was the next step. So the other day I finally found some time and I recapped the MB. All was working well, it's got some goofy golf game on there from the previous owner, good times.

The image on the screen was shuddering in time with hard drive access though so I figured I should recap the analog board - although now I kind of wish I hadn't, as will be explained later. I decided to replace the following as recommended by this page http://www.maccaps.com/MacCaps/Capacitor_Reference/Entries/1990/10/15_Macintosh_Classic_Analog_Board.html

CP2

CP6

CP7

CP8

CP9

CP10

CP12

CP36

All of the above had leaked so I cleaned off the residue underneath once they were removed with isopropyl alcohol.

I also replaced CP11 because there was gunk around it like it had leaked.

So I replaced all those, and now there is no sound and the screen still flickers. Then the hard drive stopped being recognised. I read somewhere that it's good to adjust the voltage after replacing AB caps so I did that. The 5v rail gave 4.79 so I adjusted the trimpot to 5V and the hard drive spun up again. Still no sound though, and now the hard drive has stopped being recognised again.

Any thoughts on the next step? Thanks for your time.

 

[bibilit]

Sound is related to LB (i have seen very few speakers going bad, but can be also), fickering to AB.

I would replace the two diodes DP3 and DP4 wich are almost always bad (Analog Board side)

Concerning HD, can be related or dying HD

 

[dochilli]

Some of the old harddisks have some rubber parts that will become sticky. The reading arm will not work then. If I remember right, the quantum drives had this problem.

If you adjust the voltages you have to check if they are constant. If they move up with time there is another problem on the analog board.

Sound: check the caps for right polarity in LB.

Martin

 

[electricmonk]

Thanks for your suggestions guys, I'll check the diodes, I'll check whether the voltage stays constant and I'll check the polarity of the capacitors. If sound is a logic board problem it does seem odd to me that the sound was working well after I recapped the LB but stopped working after I recapped the AB. I'll let you know how it all goes.

 

[bibilit]

Capacitor goo can be trapped under the sound chip, happened to me more than required.

Is the speaker working properly ?

 

[electricmonk]

Oh dear, turns out that one of my solder joints on the logic board had broken free. Thanks for the tip-off that it was probably a logic board problem! I redid the solder joints on the LB and the sound is working again.

No luck with the hard drive yet, it's a Quantum 52 which doesn't bode well I suppose. Next step is to get another SCSI drive to test. I put the Classic on its side and tried to boot it because I thought that might free up the heads but no dice.

 

[bibilit]

The Classic is the only Macintosh which is able to boot from ROM, so you can try:

CMND+OPT+X+O

 

[dochilli]

Instead of a hdd you can buy a SCSI2SD. Then you have up to 2GB and more capacity for programs.

I opened some of the quantum harddrives. When they start, you can see that the reading arm is stuck. There is a little plastic arm that holds the reading arm. If you touch this plastic arm, the reading arm moves and sometimes the hdd is working. You can also see the rubber that became soft and sticky. Sometimes I was successful with bending the plastic arm a little bit more. Then the wind of the spinnig hdd moves the arm better and the reading arm will start to move. 

On the rubber I glued some tesa stripe. With this method I could save the data of some quantum drives.

 

[dochilli]

Here you can see a quantum 80 MB HDD.

 

[electricmonk]

Hi everyone, thanks for your help. I cracked open the HDD and put electrical tape on the sticky rubber bumpers and now the computer is working well. Sound works, HDD works, recap of AB and LB seems to be holding up. The 5V pin on the floppy port varies by 0.02V from time to time - anyone know if this is acceptable? Perhaps if I replace those diodes that will fix it. I'll get a SCSI2SD or a FloppyEMU at some point down the track so I don't have to rely on this HDD although I've read that they are reasonably robust even after you've opened them up.

The screen still wobbles sometimes with hard drive access but not nearly as much as it used to. Does anyone know if some wobble with disk access is acceptable and/or expected?

As an aside, I accidentally dropped a pair of pliers on the back of the CRT and consequently necked it (dumb!) but thankfully there's a guy in my city who had a few Compact Mac CRTs for sale - now I have a newer CRT from a Classic II installed (with the yoke from this Classic I) and after some basic adjustments it's actually a lot brighter than it was before... so I'll call this clutching victory from the jaws of self-inflicted defeat...