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Thread: ATT 7300 UNIX PC Power Supply Woes

  1. #1

    Default ATT 7300 UNIX PC Power Supply Woes

    Good day everyone, I know that this is probably a topic that has come up before but I'm hopeful that this will be a new twist and will be beneficial to others as well. I am working through a recently acquired 7300 Unix PC. This is the earlier version rather than the 3b1. After a complete teardown and cleaning, the initial issue was a clicking sound when power was applied, and the unit never actually started up. This turned out the be a short on the 12v rail that was traced back to a shorted tantalum on the Microscience 40 MB hard drive. Removing that cap allowed the hard drive to spin up, and after re-installation the entire machine was able to power up and boot with no issues. Thankfully the hard drive was still readable and the operating system booted and worked as expected. After reassembly however, the power supply developed some sort of short and blew a fuse. When connected directly to mains voltage one of the large input caps released its magic smoke. I ordered replacement caps for the input side (470uf, 200V) and replaced both of them and installed new fuse. Testing with the power supply completely disconnected from the machine failed when the fuse blew again. So, unloaded the power supply blows the 5A fuse on the mains input. Measuring resistance across the input leads does NOT indicate a dead short, so I was not expecting the fuse to blow again but it did.

    At this point, I was hoping for some additional troubleshooting insights for the power supply, and/or recommendations for a modern replacement. I typically don't do that as I like to keep my restorations as original as possible but I am not opposed to getting rid of problematic power supplies as I did on my TRS-80 Model 12. I took a glance at meanwell since I have used them in the aforementioned TRS-80, but didn't see anything reasonably priced with 20A on the 5V rail. 12V3A and -12V300mA also required.

    Any tips greatly appreciated! Thanks as always and stay safe.

  2. #2
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    The primary caps blowing is an indication that something very bad has happened to the primary side.
    Remove and verify your bridge rectifier and make sure it's not allowing an AC voltage to pass.
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
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    = Excellent space heater

  3. #3

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    Great advice, thank you. This unit just uses 2 diodes as full-wave rectifier. Sure enough, one of the diodes was shorted. Unfortunately replacing the diodes still results in blown fuse, and the inrush current protector blew up. There isn't much else on the high voltage side other than a large unlabled black box marked LF1 on the PCB and a few large wattage resistors. Not sure how many more parts I want to replace just to have another blow up, but I guess that is par for the course with undocumented power supplies. Any other ideas are appreciated. Thanks!

  4. #4
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    Sometimes the inrush protectors will give out after "traumatic" instances. I likewise have seen them randomly fail and pop the fuse.
    My suspicion now is that either the primary short is now cleared or downstream something else did not like taking an AC voltage.
    [Need something to waste time on? Click here to visit my YouTube channel CelGenStudios]
    --------------------------------------------------------------------------------------------
    [No time for videos? Click here to visit my Twitter feed @CelGenStudios]

    = Excellent space heater

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