jansweij@swi.psy.uva.nl (Wouter Jansweijer) (01/15/90)
My four year old MacPlus is broken. Symptoms are: - a dark screen - s soft "tsjilp-tsjilp-tsjilp" sound from the transformer or its immediate neigbourhood. The transformer has the identification: "Coilcraft Taiwan 157-0047-A" and "F 4704-A". The place on the board is marked: "B20-0107-C". It's on the bottom of the sweep/power-supply board. Can anyone tell me from these symptoms what's wrong? or else is there a diagram available from this board so I can test it myself? Any help is more than welcome! Thanks very much! -- Wouter Jansweijer Phone: (31)-20-525.2152 (.... 525.2073) EMAIL: jansweij@swi.psy.uva.nl SNAIL: Department of Social Science Informatics, University of Amsterdam, Herengracht 196, NL-1016 BS Amsterdam, The Netherlands. -- Wouter Jansweijer Phone: (31)-20-525.2152 (.... 525.2073) EMAIL: jansweij@swi.psy.uva.nl SNAIL: Department of Social Science Informatics, University of Amsterdam, Herengracht 196, NL-1016 BS Amsterdam, The Netherlands.
gordon@cs.washington.edu (Gordon Davisson) (01/22/90)
[In the interests of conservation, this posting has been made with recycled materials. :-)] In article <2350@swi.swi.psy.uva.nl> jansweij@swi.psy.uva.nl () writes: >My four year old MacPlus is broken. Symptoms are: > - a dark screen > - s soft "tsjilp-tsjilp-tsjilp" sound from the transformer > or its immediate neigbourhood. The transformer has the > identification: "Coilcraft Taiwan 157-0047-A" and "F 4704-A". > The place on the board is marked: "B20-0107-C". It's on the > bottom of the sweep/power-supply board. > >Can anyone tell me from these symptoms what's wrong? or else is there >a diagram available from this board so I can test it myself? Sounds like the power supply is shutting down due to incorrect loading (and then restarting, and then shutting down, and ...). The most common causes of this are a pair of diodes (CR1 and CR5) in the video circuit that tend to fail as short circuits. They can be tested in situ, but it's a bit chancy. You're better off removing them first (be sure to remember which way around they go). CR1 is the one at the top of the analog board near the back, and CR5 is the one attached to the big heat sink just forward of the flyback transformer. Use an ohmmeter to see if either of them conducts in both directions. If one does, you've found the problem. You may have to look around a bit for a replacement -- call local electronic supply stores and ask for GI854 diodes (or GI824, a higher-current version, or MR854 or MR825, the Motorola equivalents, or...). If all else fails, you can send it to me (see .sig). I do component- level repair, which doesn't cost nearly as much as an Apple board-swap. -- Gordon Davisson Westwind Computing (206) 632-8141 4518 University Way NE, Suite 313, Seattle WA 98105