VAXstation 4000 VLC Power Supply

Tony Duell ard at
Mon Apr 12 15:27:32 CDT 2010

> On 4/11/10 6:37 AM, Rob Jarratt wrote:
> > I have a faulty power supply in one of my VAXstation 4000 VLCs (clicks on
> > and off more than once a second).
>    This failure mode of switching power supplies has been discussed here 
> before.  What was the common cause of this, Tony?  The startup resistor 
> or something, right?

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.


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