Help resurrect my Xerox Alto

Given what you're seeing with the 5v rail, my advice would be to first isolate the 5v supply and troubleshoot it. Either it is defective or it is trying to deliver into a significant sink. Chances are it is defective. I don't know for sure but given the era of this drive I'd suspect the power supply is a switcher. Run it into a lightbulb to provide a load. If you aren't getting 5v into a dummy load, consider changing out the filter caps - on a machine of this era, they're almost certainly bad unless they've been replaced along the way (look at the date codes). Of course, there's also the possibility of a bad pass regulator or other active component, but my experience has been that it's most often the filter caps that go south. None of this advice is really much different for a linear supply - if you have a big ol' transformer in the box - but you don't have to have a load just to get it to run. A load is still a good idea so you can see if the filter caps collapse when the power supply is actually trying to supply current. If you find you have a perfectly good 5v into a dummy (and look at it with a scope, too), then I'd first look at filter caps on the boards fed by the 5v. I had a power supply in a Nova 1200 that was actually working just fine but was trying to deliver 15v into a dead short caused by a filter cap on one of the core memory rails. The current limiter in the power supply worked admirably, but it looked like the 15v supply was not generating, until I isolated it with a dummy load.... It looks like a really nice machine! Congratulations on a fabulous acquisition -- Ian ________________________________________ From: cctalk-bounces at classiccmp.org [cctalk-bounces at classiccmp.org] On Behalf Of Nick Allen [nick.allen at comcast.net] Sent: Saturday, September 11, 2010 4:18 PM To: cctech at classiccmp.org Subject: Help resurrect my Xerox Alto I thought a video might be easier for you all to see the issue I am having with restoring my Xerox Alto. The issue I am having is the Diablo Disk drive does not power on fully (I am measuring +15 and -15V on the drive, but +5V is only measuring at around 2.5V. The lamps on the front of the drive do not light-up, and the LOAD/RUN button does nothing, hence the door lock does not unlock. Here is a link to the video: http://www.youtube.com/watch?v=mx80CvjuvyE I have visually inspected all PCB boards and components, nothing seem to be damaged. Also, the power supplies seem to be outputting power properly (as seen via the RED voltage LEDS in the Alto card cage. Any recommendations on how to troubleshoot the disk drive further?