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Home Forums Mac IIsi Power Supply Problems — #5 Mac IIsi Power Supply Problems — #5
Post #5 by equill
Source ForumMac IIsi Power Supply Problems — #5
CategoryHardware
Post DateSun, 29 Jul 2007 - 01:05
Original URLhttps://68kmla.org/bb/threads/mac-iisi-power-supply-problems.9249/
Post
The principal problem with IIcx/IIci/Q700 Mac PSUs has to do not with capacitors but a diode in the PSU. It changes value with age, and eventually will not properly bias the sawtooth oscillator in the PSU. That causes loss of the essential TRKL voltage on which soft-power switch-on depends.

There is a published remedy (in brief, putting another diode in series with the original to increase the biasing voltage drop over the pair of diodes). It's not a job for the faint of heart, especially if the PSU's filter capacitors are not completely and deliberately discharged before the work is begun. The problem is commoner by far in Astec rather than Delta PSUs.

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