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

  1. #11
    Join Date
    Jun 2012
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    Livermore, CA
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    Thanks,
    I am not sure where to touch Meter leads from your posts above. I conceptually know what rails are and what they do, but have no idea how to find them on a PCB
    Would it be possible to to show where to touch the meter leads from this pictures in post 1 and and the picture here? I know where pins 1,49,50, 100 are located
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    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. #12
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    Dwight, your still around?
    Sorry for the my another delay, again life/family stuff to do. But now back to this
    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

  3. #13

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    Sorry, I missed your post on the 28th.
    We can start on the bottom as in your last picture. The wide trace running across the width of the board is the ground( wider in the middle ). With the power off, we'll make some measurements. On the right are two large capacitors. These are mounted in parallel. The screws are both the mounting and the electrical connection. The 4 nuts are the connection to the 8 Volt's rectifiers. On the left are two smaller capacitors. One is for the +18 volt rail and the other is for the -18 volt.
    What we want to do is measure across the screws on each capacitor with an ohm meter. We only need to measure one of the capacitors on the right since they are both connected together.
    I'm not sure which on the left is the + and which it the minus so we'll just do two measurements on each. Once with the leads, red on top and black lead on the bottom and then we'll switch them. Do the measurements on the two capacitors on the left then then one of the two on the right. Report what measurements you get.
    Dwight

  4. #14
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    Thanks for the reply
    I get a number that grows from 0 to well over 1 K Ohms for the left 2 smaller caps holding the leads for about 3-4 seconds. The larger ones on the right increase in ohms also but much slower slower. I get about the same if I switch leads for each cap.
    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

  5. #15

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    The time delay difference is expected. If your meter is a digital, you can speed up the settling by momentarily switching to a lower ohms scale and then switch back.
    Still, the readings you are getting would indicate that there is no shorts. They all go to greater than 1K. This would indicate that none of the rails are shorted to ground.
    The next test is to check that there is no short from rail to rail. This is a little trickier.
    You'll notice that there is one screw on all of the capacitors that it connected to the circuit ground, by the large area of metal that almost spans the entire circuit board.
    I'd like you to measure between all of the other screws in combinations. We expect the two capacitor lead on the right to have 0 ohms but nothing should short on any of the other combinations.
    I'm trying to see why the fuse blew. So far we are not doing too good.
    There is a filter on the AC input but it doesn't look like that would take out the fuse because it is only inline with the transformer and fan. So far we've not seen problems on the secondary of the transformer. We'll keep looking. It is possible that there is no fault in the supply and that it was running with a full boat of boards when some tantalum cap decided to push it over the edge.
    We still have a couple more checks before sticking a fuse in and doing a smoke test.
    Do you have a variac or similar adjustable auto transformer?
    Anyway do those next measurements.
    Dwight

  6. #16
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    I checked all the ground combinations, they all show 0 ohms as they should. The positive combinations all show ohms growing in number. Expect for the 2 larger cap. There is zero ohms between the 2 positive screws. I do not have a "variac or similar adjustable auto transformer" (well I do bit it is for my HO model train set I have in storage. It is this
    https://www.ebay.com/itm/MRC-Tech-2-...UAAOSw3f9drRh8
    Variable dc 0-16v, and fixed ac & dc)

    This computer did have 9 cards in it. I pulled them out to clean the system up first. I did a inspection of each and did not see or smell any blown caps. 6 memory card, 1 disk, 1 VDT, 1 cpu. There could be problems with them, but we'll tackle that much later.
    Thanks
    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

  7. #17

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    It sounds like we are finding nothing to cause the fuse to blow. One thing to look at is the spacing between the 4 +8V diode studs and the chassis when the board is mounted. If it touches it will likely blow the fuse. If after putting it back into the chassis, you don't see a short under the board, lets try replacing the fuse. Do you have a variac?
    Dwight

  8. #18
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    No variac, sorry.
    There were no burn marks on the chassis anywhere under the PSB when I first took it apart. After putting the PSU PCB back I used a sheet of paper under the diodes,and they do not touch.
    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. #19

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    OK, I guess we can try a new fuse then. At 120v it is a 5 Amp fuse. Do you think you can rig up a 10 to 15W light bulb in series with the AC?

    Dwight
    Last edited by Dwight Elvey; November 1st, 2019 at 10:14 PM.

  10. #20
    Join Date
    Oct 2016
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    I would try a 100w bulb. Since the computer was out of service for a long time the large filter cap's might need what's called reforming. The inrush of power because of the filter cap's drawing lots of current could have blown the fuse. Reforming more or less means bringing the voltage to the cap's up very slowly until they can handle controlling the job of filtering the power coming out of the rectifiers. Old large filter cap's have been known to blown up if not reformed.

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