Category Archives: Macintosh 512k

Mac back on track

Despite the little set of ceramic capacitors I put in having a combined value in an acceptable range for C1, it would appear they were either breaking down or otherwise incapable of sustaining the horizontal frequency of the screen scan.

caps being meaured

So, a different set of capacitors was used and that brought the screen back to life.

working again

That appears to overall conclude the work on this machine.

Broken Macintosh

Very short-lived, this one. I took everything apart and adjusted up the screen so it was central.

Nice and even

Set it on the side to do a bit of a burn-in test, and the decoupling capacitor for the horizontal yoke let out all its magic smoke, the screen went a bit funny and I switched it all off quickly.

c1

Decided to build the little capacitor network suggested in the manual, of several ceramic capacitors in parallel to build up the capacitance but reduce the ESR.

c1c1c1c1

Put it back together and again we have screen but.. the horizontal isn’t aligned any more. The two interlaced pieces of the screen, all stretched out – do not line up.

interlaced

I’m going through the schematic, I’ll check all values and make sure that the capacitor failing did not draw too much current through the flyback and fry one of the windings.

Rejuvenated Macintosh

I decided that the wobbly-screen Macintosh needed to have a bit of work done to make it a bit more trustworthy and reliable. The electrolytic capacitors had been installed in it since 1985 (8113th device produced in the 24th week of 1985 in Fremont, per the serial, so a real early 512k).

mac on table

Took it to pieces after ordering a bunch of spares (total about $15, so not bad), with the replacement caps rated at least one voltage rating over what was fitted and 105C rather than 85C as fitted originally to try and keep them a little more stable.

logic board

Not many capacitors on the logic board, though it is a 4-layer PCB so extra care had to be taken. I’ll give them this, it’s a well made PCB.

logic board closeup

Tried to keep the solder-work neat. Don’t think I did too bad a job, to be honest.

work in progress

Got stuck in doing the power and sweep board, which has the majority of the capacitors.

powered up

First test, and things were promising until the brightness control decided to stop working.

broken brightness pot

Turns out the center carbon track had completely fallen apart. Checked the schematic, fairly standard 16mm 1MOhm linear pot, so went on the hunt for something similar in the electronics catalogs online.

new pot

Closest modern variant I could find. The board is the right size and the connecting pins the correct length, important in this application because of the way the computer is assembled.

take apart

Carefully undid the new piece, as the thing was essentially backwards from how it needed to be.

sweep

This side is where the old one had the handle coming out. I removed the crimped section and split it all down, had to file the ears of the board a little to make it fit but managed to put it back together again, and have it test good and smooth operation across its range of travel.

reassembled pot

Put that back in the circuit and the brightness is now adjustable again. Checked the voltages and the 5V rail (the master) was a bit low at 4.72V so that was tweaked up to 5.00V. The 12V rail, slaved from it sat happily at 12.2V so that’s all well within limits.

5 volts

The screen became a bit intermittent- traced that eventually down to a couple broken solder joints on the connector to the end cap of the tube.

broken joints

Removed the old solder and re-added new.

working now

All up and working again. Need to adjust the centering of the screen a little, I think I must have knocked the centering rings when I put the wiring back in. Overall, a lot better than it was.