On Oct 14, 2009, at 7:07 PM, Lee Tucker wrote:
> Thought I’d ask here if anyone has resurrected a Plus that upon > applying power nothing happens except a faint tick noise (2 or 3 > times a second) from the speaker. Fiddled with the power > adjustment to make sure that had not been over or under tweeked, > but no joy. This is the classic "flup, flup" of the primary side of the flyback trying to start oscillating, only to be immediately shut-down by the protective circuit. Almost all flyback-based systems work this way. In the 128, 512, 512KE, Plus and SE, the protection is an SCR in the secondary which is turned on by the +5 or the +12 rail exceeding a certain value. The SCR it pulsed, thereby turning it on (closing the path across the secondary), and the oscillator circuit in the primary is shut down, thereby clearing the SCR. The next start-up cycle again goes to a short circuit, and the whole thing starts all over, forever. The trick is finding the shorted component, and for most purposes, only the 128, 512, 512KE and Plus are amenable to a component-level repair. The biplar capacitor in the H circuit is one cause. Replace the cap with a correct one, They are now quite rare. The rectifier diodes in all dc circuits are other causes. Replace the small diodes with tab-mounted diodes. All of this is discussed in those Mac repair manuals which were available from Amazon, and others, a dozen years ago. The manual also gives sources of repair components. --~--~---------~--~----~------------~-------~--~----~ You received this message because you are a member of the Vintage Macs group. The list FAQ is at http://lowendmac.com/lists/vintagemacs.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/vintage-macs Support for older Macs: http://lowendmac.com/services/ -~----------~----~----~----~------~----~------~--~---
