Image Map Image Map
Page 1 of 4 1234 LastLast
Results 1 to 10 of 31

Thread: IMSAI 8080 Power supply issues - i think...

  1. #1
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    340

    Default IMSAI 8080 Power supply issues - i think...

    Hi everyone again,
    I got a Imsai 8080 with what I think is a standard issue psu.
    I have take the whole system apart and cleaned out all the dust and debris I could find, and it was quite dirty.
    I currently have no cards or front panel installed. I shorted the front panel switch connector with a small piece of wire to have current moving. I have applied power and I get current, that i know how to measure, across the fuse. After that I am not sure where to touch the multimeter to trace current further. I get nothing at the white and black wires soldered to the buss board. There is not hum at the large transformer. The fan dose not turn.
    What should I do next?
    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

    Default

    Quote Originally Posted by KLund1 View Post
    I have applied power and I get current, that i know how to measure, across the fuse.
    I'm not quite sure how you measured current across the fuse with a multimeter.
    Can you describe where you put the leads and what level you measured?
    Dwight

  3. #3
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    340

    Default

    Thanks,
    I got 120.1vac from side to side on the fuse. So power is moving from the plug to the square box at the back, then by wire to to PCB near the red coils, then to the front of the board where the fuse is located.
    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

    Default

    That is what I thought you were doing. That means the fuse is blown. You should see '0' volts across the fuse, or at least very little.
    Don't replace the fuse until we know what caused the fuse to blow. I do recommend getting another fuse ( do not even think of jumpering the fuse with a wire. If you damage the transformer, you are unlikely to find a replacement.
    I'll walk you through what to do next.
    Remove the AC cord from the wall. We won't want to use any AC during any of the following steps.
    Put your meter on Ohms.
    I need to first ask you what type of multi meter do you have. If it is a digital, tell me that. If it is one with a meter face and needle, I need to know the manufacture.
    Next you'll need to unbolt the power supply board to make the measurements. You have to leave the wires connected but we'll need to get to the meter leads to the screws on the large capacitor bolted to these boards.
    Dwight

  5. #5
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    340

    Default

    Thanks again.
    I have a klein tools MM100 digital multimeter.
    Pulling the PSU off means a full disassembly. The wires soldered to the buss board are not long enough to move the PSU only. Also need to disconnect, then reconnect, minus the external plugs, the mess near the box at the back. It will take some time. This thing is very heavy.
    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

    Default

    You can disconnect the wires to the mother board. The reason I asked about the meters is than many cheaper analog meters reverse the polarity of the wires when measuring ohms. Digitals don't do this because it is actually more difficult to do on a digital meter.
    Yes, it is heavy. Everything is on it. If the short it there and not on the mother board, it will have to be pulled anyway.
    Dwight

  7. #7
    Join Date
    Jun 2012
    Location
    Livermore, CA
    Posts
    340

    Default

    Thanks, working on disconnect. Should be done by the time you may ses this post. Work and home have been keeping busy.
    I have a new fuse. Next testing step?
    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

  8. #8

    Default

    I'll get back to you. In the middle of stuff.
    Dwight

  9. #9

    Default

    First thing to do is to check the power rails to ground with the ohm meter. For the + rails put the black lead on ground. For the negative lead put the red lead on ground. The meter should jump to low ohms and then settle out the the value of the bleeder resistors. I forget the values. Any steady reading less then 50 ohms is a failure.
    Dwight

  10. #10

    Default

    I was just rereading my post. I should have said common return wire from the bus, rather than ground. Many think of ground as being the metal chassis and often the power return is connected to the chassis. In this case, since we've lifted the board, it may not have a connection to the chassis. The power return is the end pins of the S100 buss. That would be pins 1, 49, 50 and 100.
    If you don't find a fault on the previous, measure between the three rails. Again, we are looking for something in the low ohms range that would blow a fuse.
    Dwight

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •