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

  1. #21
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    DeltaDon,
    Thanks for the input. But I do not have a way to slowing bring up the a/c voltage to reform the caps. But yes, from what I have learned from Dwight here and other threads, bring them up slowly would be the BEST option. Bit I do not have such equipment. Multi-meter, soldering iron, hot air gun, screwdrivers, electrical tape, and these very helpful forums are just about all the tools I have.
    Dwight,
    I can put a light between the two power switch connections? Is that a ok place in the circuit?
    The fuse that was originally with this unit was 3A, 250V. That us all that I have on hand. Will that be sufficient for testing?
    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. #22

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    That supply was suppose to run on a 5 amp fuse for 120V and a 2.5 for 220 wiring.
    If you'd like to bring up the caps slowly, you can use some resistors but that will require removing them from the PC board for the plus and minus 18V caps. The 8V line, we can unbolt the 4 diode studs and put a resistor in series one of with them. If you remove the 18V caps, use a sharpie to mark both the board and the caps so you'll get them back in the right place and oriented correctly. You know something like a stripe down the side and onto the board for one and two stripes for the other.
    For the 8 volt line, I recommend using a 2K resistor.
    For the other +/- caps, you can put a series resistor for bot of them of 4.7K. You'll need to run wires for these two. Make sure to match the polarities to the what they ar in the board. You can use some nuts and bolts to make the wire connections to the board. If you do it this way, you shouldn't need any series limiter to the primary. Check the voltage every no and then. When the 8V cap reaches 5V or so, you should be safe to install it.
    For the +/- caps you'd like them to be about 10 or 12V.
    It may take an hour to a couple days, depending on the state of the caps.
    Dwight
    Last edited by Dwight Elvey; November 4th, 2019 at 11:27 AM.

  3. #23

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    Quote Originally Posted by KLund1 View Post
    I can put a light between the two power switch connections? Is that a ok place in the circuit?
    The fuse that was originally with this unit was 3A, 250V. That us all that I have on hand. Will that be sufficient for testing?
    One issue with these types of supplies is that due to the massive capacity of the filter capacitors, at turn on there is a large initial surge current. The power transformer's secondary is effectively shorted out for a while, until the capacitor's terminal voltage climbs up. The resistance of the transformer's primary and its leakage reactance helps to some degree to limit the turn on surge current. For this reason it is important to have a SLOW blow fuse, I'd suggest using a 3A one at least initially until the condition of the capacitors have improved a little, probably though they are ok.

    All you need to do is wire a 25 or 40W light bulb in series with the power feed. Once its up and running leave it on for half a day or so. Then you should find with just a 2.5A or 3A slow fuse and no bulb limiter it will be fine at turn on.

  4. #24

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    ...I should have said a 5A slow fuse after the capacitors are back to good order (I keep thinking of 230V systems) but as Dwight noted you should have a 5A fuse for 120V and a 2.5A fuse for 220V.

  5. #25
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    Jun 2012
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    Thanks everyone. I understand the need to slowly bring up the power to the large caps. I have have a line a veritable A/C transformer I might be able to borrow for a while. I'll post a pic of it, if I get it tonight, to see if it will work with our need.
    If not, could I put the light bulb in the circuit where the power switch would be connected. If not, I need a near exact location from the preciously posted pisc as to where to add the bulb to the circuit. Sorry, but I can't translate many circuit terms and descriptions to actual locations on a PCB.
    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. #26

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    I always used a variable transformer.
    I'd look at the 18V caps and bring them up to 1 V. Id the step them up by 1 v every 4 or longer hours( I had to sleep some time ). I never had a good cap blow up and even the bad ones never blew. The bad ones never had enough capacitance but they never blew.
    That was only true for electorlytics. Tantalums still blew. Bringing them up slowly didn't seem to make a difference.
    Dwight

  7. #27
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    I finally was able to borrow a variable a/v transformer.
    It dose not gradients to show what voltage it is putting out. I guess I can use a multimeter to see voltage.
    Should I start at about 1 volt, or another amount? Then increase 1v /hr as you mentioned.
    What should I be looking for while doing this. Excessive heat somewhere? Buzzing/muming?
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  8. #28

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    It looks like the dial may be backwards so check that you are controlling it right.
    Look on one of the the 18V lines start them at 1V. Check both the + and -. They should be withing 0.5V of each other.
    There may not be any voltage on the 8V with this first setting. That is OK. It will start a little slower than the higher voltage rails.
    Step it by 1 volt after about 4 hours or more. Even if you don't step for 8 hours, make the steps 1V. By the end of the 4 hours, you may see a slight rise in the voltage. this is OK.
    It is just a matter of taking your time. If anything gets overly hot stop.
    Dwight

  9. #29
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    Thanks,
    I put the psu back properly in the case to avoid any possible bad contacts. Powered it up to 1v A/C. ( I check the variable transformer output with the meter first) I checked the black and white wires connected to buss board. I did not see any volts DC. I did see 1v a/c on the aux a/c plug coming from the pcb. I saw 1v a/c going into the large transformer, and very small 100's v coming out. but that cold be the jumping I see on the meter every time I move it's probe wires. The small cap nearest to the 'front' of the case is a bit warmer then all the rest after about 1 hour on.
    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

  10. #30

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    Please reread my post. I said to bring the AC voltage to the point that you had 1V on the 18V lines. It is not likely that 1V AC will even forward bias the supply diodes. Caps should not be warm. It sounds like one of the input RFI filter caps may be shorted. If it is actually getting warm with 1V, you should remove it.
    Dwight

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