My old Original 512K Macintosh computer has been dead for many years. I decided to dust it off and try to get it working again. It’s not an easy task since quite a few of the original components are no longer available.
I pre-ordered this computer in April, 1984, after attending a presentation at my workplace (I think). There was no internet as we know it back then. I distinctly remember some guy touting the amazing graphics and the advanced 8MHz 68000 CPU. I pre-ordered it on the spot. It cost $2500, with another $500 for the dot-matrix printer. That was a huge chunk of my income at the time, but my wife and I thought it would be a good way to introduce our children, age 4 and 7, to modern computers. My previous personal computer was a TRS80 model 4, with a whopping 48k of RAM and a home built 5.25” floppy controller, which my wife insisted that I sell prior to receiving the new Mac.
I replaced the Fat Mac with a Mac IIci in 1990. I had enhanced the Fat Mac as far as possible before retirement: it had a New Life board attached in order to run a 50MB SCSI hard drive (cost $1200). I think at some point I had replaced the power supply board as well. It has been relegated to an attic since then. I was loath to sell or dispose of it — either sentimentality or not willing to part with something that was perceived as valuable from an earlier era.
I recently decided to retrieve the old Mac and see if it still worked. When I plugged it in and turned on the power switch it demonstrated its unhappiness with a “flub-flub-flub” from its speaker. This indicated that the power supply board was probably the culprit.
The Fat Mac Power Supply:
This is one of the strangest SMPS power supplies I’ve ever seen. The first thing I did was replace the X2 capacitor, C38, which sits across the 120 VAC even when the power switch is not turned on. The component looked like it had been cooking for a long while. But that was not the problem.
I proceeded to try to find the most obvious failed components. This would be electrolytic capacitors, rectifier diodes, etc. I replaced CR21 because the load couldn’t possibly be 34Ω, but it was. A dead end. I also replaced some 1000µF capacitors with a smaller modern equivalent — no effect.
The basic problem is that the primary side of the SMPS is not isolated from earth ground. The neutral mains wire is not really connected to GND and the common node of the primary side is swinging up and down at 180V. I did not desire to blow up my oscilloscope, or buy an expensive differential probe, so I had to come up with an alternative to running the system off the mains supply. I have two bench power supplies — a linear PS that can produce 60VDC if the two supplies are connected in series, and a crappy switching PS that can produce 32VDC @ 5A. So I stacked the three supplies and connected 60V across C35-C36, and connected the ground of the primary side to the ground of the secondary. Now I could use the scope to observe the SMPS operation without destroying the scope.
It turns out that the SMPS needs a kick to get it started. There is a kick starting circuit, but it only works when applying 120VAC. So I had to perform the kick start myself by touching the base of Q11 with a 1.2KΩ resistor connected to the top of C35. The SMPS would start to oscillate with 60VDC applied, but abruptly stopped when I increased the voltage above 64VDC. The circuit was not regulating the output, so when the +12V output exceeded 13V the SCR crowbarred the supply and stopped the SMPS operation. This phenomenon was responsible for the “Flup-flup-flup” sound out of the speaker when it was connected to 120VAC, but when supplied by DC it simply stopped the SMPS operation.
If the SMPS was not regulating the output, but still switching, then the problem pointed to either Q9 or Q10. Q9 is an ubiquitous 2n3906 pnp transistor. Q10 is a EO122Y SCR, which is no longer sourced by anybody. I ordered some BT169 SCRs from eBay, and replaced both Q9 and Q10. No luck. The SMPS would operate at 60VDC across C35, but when the voltage was increased to 64V the operation stopped. Same problem as before, but I...