VAXstation 4000 VLC Power Supply

-----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk- bounces at classiccmp.org] On Behalf Of Tony Duell Sent: 12 April 2010 21:28 To: cctalk at classiccmp.org Subject: Re: VAXstation 4000 VLC Power Supply (clicks on here resistor No. If the startup resisotr fails, the supply simply fails to start up. The outputs sit at 0V. A PSU that 'tweets' or clicks is starting up, detecting a problem and shutting down to protect itself and the load. And then trying again. This may be due to a fault in the overcurrent circuitry (but whatever yo do, don't disable said protection to see what happens unless you like scraping bits of transistor off the ceiling!). It may be a shorted rectifier or capactior on the secondary side. It may be tghat the crowbar circuit is tripping and shorting out one of the outputs, either because of a fault in the crowbar circuit or becuase there is a problem with the regulation loop and the output voltages are genuinely getting too high. Or it may be high ESR capacitor on the secondary side so there are spikes on the otuptus to trip the crowbar. It might even be an open capacitor on the primary side so the chopper control circuit can't get a steady supply (had that one once, took me a long time to find it!). I asusme you have a suitable load on this supply. Some PSUs will not regualte correctly (and thys trip the crowbar) if there is insufficient load. -tony