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Thread: IMSAI 8080 need wiring verification on PSU section

  1. #1
    Join Date
    Jun 2012
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    Livermore, CA
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    246

    Default IMSAI 8080 need wiring verification on PSU section

    This is my first IMSAI.
    I have been cleaning it up from the garage dust it was found in. Generally in good condition. The owner said he last used it about 5 years ago. But the power cord, front panel power, and fan power were all disconnected. I am not sure I connected them correctly. Also the wires were not connected to the large cap towards the read of the case. (Teh right side cap in the pics) I looked at a similar unit here
    https://www.worthpoint.com/worthoped...nal-1856017462
    I connected that large cap like the one in the inside case picture.
    I'd like if someone else could take a quick look to see if I connected everything correctly.
    Also is there any tips about testing the PSU section before and and after I power it up. (no boards attached except the front panel). I am not a EE person, but can use a Multi-meter if instructed where to test and report results.
    Many thanks
    Attached Images Attached Images
    Atari Falcon030, Atari MSTE, Atari PS3000, STACY 4, 4160STE-CosmosEX, Atari PC1, 400/800/1200XL,800XL,800XE,130XE, XEGS, Apple IIGS, TRS-80 4P, TI 99/4A, Co-Co 3, Amiga 500, C64/128D

  2. #2

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    Many removed the AC from the front panel as the particular mounting method was considered a shock hazard. Also the switch was considered a little light for such a large inductive load.
    As for the pictures, they are not clear enough to see the + sign cast into the tops of the capacitors. These capacitors can be easily rotated in the holders and it will quickly destroy them if connected backwards. I would not depend on a visual check in a photo as my IMSAI has a different power board. It would be best to disconnect one side of all the capacitors, remove all the boards, including the front panel from the buss and turn the power on. Use a meter to confirm that the polarity if the leads are correct.
    It is also recommended that if the machine has been off for some time to bring the capacitors up slowly. I recommend limiting the current to them to about 100ua until they have settled some. For the 8 volt line that would be a 100K resistor in series with them. For the 18 volt ones that would be a 200K resistor. After a day or so of charging, you should see no voltage measured across the resistors ( at least a very small voltage ). Take regular readings and when you see the reading become stable, they should be at their optimum. These large capacitors do leak a small amount.
    So, make sure the polarity is correct and make sure the capacitors have had some soak time. Once wired up, before connecting the boards, get the S-100 spec off the web for the power pins. Measure then on the connector and make sure the polarities are correct!
    Dwight

  3. #3
    Join Date
    Jun 2012
    Location
    Livermore, CA
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    Default

    Thanks you for the detailed reply.
    What you suggest make seance to me. But I have no idea how to wire that up. I have little practical experience with power supplies. I would have to have someone do it for me or watch a video on a step by step process. But until then, I will not be applying any power to it.
    Any idea where to find help here in the SF Bay Area? It must be near be me somewhere, but I do not know how to find them. I'm not more then 15 miles from where this unit was originally built!
    Again thank you for your help!
    Atari Falcon030, Atari MSTE, Atari PS3000, STACY 4, 4160STE-CosmosEX, Atari PC1, 400/800/1200XL,800XL,800XE,130XE, XEGS, Apple IIGS, TRS-80 4P, TI 99/4A, Co-Co 3, Amiga 500, C64/128D

  4. #4

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    I suspect there are at least 10 people in Livermore that can help you, if not more. Post a help request here and on things like cctalk. The stuff I posted should be clear. I don't think there is anything that should be an issue. You need a meter. Even a cheap $10 meter from Harbor Freight would work fine. Resistors can be perchased from places like Fries Electronics. Considering the low voltage used, most any wattage would be fine. 1/4 watt is a typical size.
    Dwight

  5. #5
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    246

    Default

    Thanks,
    I posted a request in the General Discussion forum. 58 views, no replies, or PM's
    Atari Falcon030, Atari MSTE, Atari PS3000, STACY 4, 4160STE-CosmosEX, Atari PC1, 400/800/1200XL,800XL,800XE,130XE, XEGS, Apple IIGS, TRS-80 4P, TI 99/4A, Co-Co 3, Amiga 500, C64/128D

  6. #6

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    May I ask what exactly your goal is with the IMSAI? You have a nice collection of early microcomputers, but the earliest like the IMSAI are far more fickle. You're going to have to get your hands in there and work on it (in my experience). Is your goal to run it with some memory and a front panel, try toggling in some programs, or build a bigger system with disks and I/O?

    There's plenty of support on this board and through many members' sites / resources to repair and restore these machines.

  7. #7

    Default

    Quote Originally Posted by ef1j91 View Post
    May I ask what exactly your goal is with the IMSAI? You have a nice collection of early microcomputers, but the earliest like the IMSAI are far more fickle. You're going to have to get your hands in there and work on it (in my experience). Is your goal to run it with some memory and a front panel, try toggling in some programs, or build a bigger system with disks and I/O?

    There's plenty of support on this board and through many members' sites / resources to repair and restore these machines.
    He needs to determine if his supply is working correctly and if the polarity of the filter capacitors are correct. The IMSAI used a number of different supply boards with different layout. He thinks using a voltmeter is only something that an engineer can do. I think he will need to learn anyway but he'd like someone to help that is local. I'm sure there are several people on the forum that are closer than I am. The pictures he sent do not even show the + signs on the caps. I did give instruction on making the measurement but it seems he can't run a meter.
    Dwight

  8. #8
    Join Date
    Jun 2012
    Location
    Livermore, CA
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    Thanks,
    I have about 50 cards for it, and several 8" and 5.25" drives, 2 video terminals that may work. but I will have to get to those much later.
    I can use a meter, but I do always know what the meaning of a read is. So I test a cap, for example. I get 16u. is that what is should be, did I get the polarity right, are other components in the circuit effecting the reading, do i have the meter set right, I do not know all the different meanings of labels/symbols on the caps themselves. Its all these small things I need to learn. I have been learning, but only with the small can, T-cap types. I can build a series or parallel circuit with battery, wires, a light build, and a switch, but all more items and I get lost quickly. But i am learning as i go. killing an Apple II+ or atari 800XL board it not a big deal, and usually easy to fix. But would rather not kill a valuable piece of history like this.

    I have add additional pics with the '+' of the caps in red.
    I will connect the power cord directly to the power PCB leads that the front switch was connected to. When I do apply power (later), it will use the surge protector's switch.
    Thanks again for all the help!!
    Attached Images Attached Images
    Atari Falcon030, Atari MSTE, Atari PS3000, STACY 4, 4160STE-CosmosEX, Atari PC1, 400/800/1200XL,800XL,800XE,130XE, XEGS, Apple IIGS, TRS-80 4P, TI 99/4A, Co-Co 3, Amiga 500, C64/128D

  9. #9

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    It looks right, again as near as I can tell. The big caps look correct and the wire colors for the 18V lines, + and - look what they'd typically use. So note, I don't have the same power supply board as you have. The only possible issue it when I was talking about bringing the capacitors up slow with some series resistors. It isn't important which end you add the resistor.
    In any case, watch the transformer for over heating. A shorted diode in the bridge can cause the transformer to over heat. They usually can take quite of bit but once they really start smoking, it is too late.
    Dwight

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