My Macitnosh Classic's Quantum SCSI Hard Drive accelerates

[Roberto]

I recently performed a recap of the capacitors of the analog card of my Macintosh Classic (the critical ones); all right for the first few seconds from power on: load the operating system correctly from HD Quantum 20 MB SCSI (the original Macintosh Classic to be clear). After a few minutes the HD starts to accelerate; you hear that the number of revolutions begin to increase until the system crashes. What could it be?

 

[bibilit]

The Optoisolator can do that. 

 

[Roberto]

Thanks for your answer. Could you tell me where it is?

 

[Roberto]

or is it in the hd controller?

 

[bibilit]

OP1 part CNY 17 G3

in the middle of the Analog Board. 

Just above capacitors ties in your picture. 

 

[AndiS]

Check the voltages of the 5V and 12V rail. It can be done from the Floppy port and is described in the Classic Macintosh Repair Guide. The menioned Optoisolator can be the culprit, but so can the nearby PP1 potentioeter. In any case - checking the voltages while the machine is running, will give you an idea if this part of the analog board is the issue.

 

[Roberto]

thanks, tomorrow I'll try to replace it with a card that I already have recovery. In fact I have tried the hd on another macintosh and it works perfectly.

 

[Roberto]

Thanks bibility, I changed the Optoisolator and it worked. Thanks to you too AndiS; now i have a more general idea about this problem.

 

[bibilit]

No problem