A few months ago my wife brought home her childhood Mac 512k, but she didn't know about RIFA caps and the Magic Smoke, so she booted it up and was playing a game for about five minutes before one of the RIFA caps blew.
Last month I ordered a bunch of capacitors and re-capped it. There was also corrosion damage from the battery which I cleaned up as best I could and replaced the corroded fuse clips. I connected my FloppyEmu, and for three days we enjoyed a Happy Mac running off the Floppy Emu.
Then it made an awful noise when she turned it on. (See next post for a video.) I opened up the Mac again and saw that the LF1 line filter right under the battery compartment had burned through one of the solder joints where there had been a lot of corrosion. I desoldered the line filter, cleaned that area up, resoldered it and added a bodge wire between the burned leg and the fuse. But it still makes the same noise when I turn it on.
I'm hoping for some guidance, or someone in/near Essex County NJ who can look at it.
Thanks,
Michael
trim.4D04E2FA-B5D8-4A90-AD1D-52D66A3837DF.mp4
I can't offer any specific guidance, but The Dead Mac Scrolls book is an excellent reference for troubleshooting issues like this:
https://vintageapple.org/macbooks/pdf/The_Dead_Mac_Scrolls_1992.pdf
Thanks for that PDF. The book points to the flyback transformer. I need to do the resistance checks, but these parts are pricey. I could buy another Mac for the price of this one part, including shipping in some cases. Oh well, I was hoping I'd get it working before Mother's Day. Ah the joys of retro collecting!
The Dead Mac Scrolls says "With the power off, check resistance across CR5. If the reading indicates 44/45 Ω (not 3.6K/∞), replace the flyback transformer (Apple part# 157-0026-8) at board reference T1."
With the Mac unplugged and the flyback transformer discharged, I put my multimeter in diode mode and measure the resistance across the diode at CR5. It displays "446". There is no decimal shown.
Does this mean 446 ohms or 44.6 ohms? And should the flyback transformer be discharged before this test? Thanks for any help.
Michael
Diode mode on a DMM measures voltage, not resistance. The "446" may mean 0.446 V, which is close to the normal forward voltage drop of a silicon diode.
This page of the Dead Mac Scrolls is mysterious. He is taking a measurement across one component (CR5) and calling it a test of something else (T1). Evidently the intention is to test T1 for shorted windings, but this seems like a really bad way to do it. I'm not even certain what "3.6K/∞" is supposed to mean, unless he is talking about the resistance readings he saw when probing CR5 in both directions.
A semiconductor does not have a "resistance", so the voltage measured by the meter when forcing the test current through the circuit is displayed, instead of converting it into a resistance reading, as is done in Ohms mode. In a working semiconductor, the forward direction (from P- to N-type silicon) will have a voltage drop of about 0.6 V, and the reverse direction (from N- to P-type silicon) will show "OL" to indicate that the voltage is above what can be measured in that mode (the reverse breakdown can be thousands of volts, but the diode mode range is limited to about 3 V).
A flyback transformer cannot become charged because it does not have any capacitance. The capacitance is in the bell of the cathode ray tube. But this high voltage charge is blocked from flowing back to the analog board via the flyback.
If you are just hearing a distorted "startup bong" sound, none of the pages of The Dead Mac Scrolls that mention the flyback match that condition. Note the words "Symptoms: There is no startup bong. The display is dark."
The "bong" sound is a success tone generated by the Macintosh ROM that means "all is well". It has passed the hardware tests performed by the ROM at startup time. It sounds distorted because there is something wrong with the speaker circuit; the most common cause is that the foil RFI shield is making contact with the logic board (see Dead Mac Scrolls, page 111).