Advice wanted Amiga A500 power issue

[gksonnier]

I recently acquired and Amiga 500 through E-Bay.  It came with only the main unit.  No PSU or anything else included.  It was sold for parts or repair.  I was able to put together a PSU using the ATX hack.  I do have correct voltages (+12V, -12V, 5V) at the Amiga connector.  I also checked the connector to make sure I didn't have a short.  The issue I am seeing is as follows:

When I attempt to power up the Amiga, the CAPS LOCK key will do a quick flash, then turn off.  The ATX PSU will then stop working all together.  I have to unplug the PSU from the Amiga and plug it back in to get the PSU to power up again.  If I disconnect the 5V lead and power it up, the ATX PSU does continue to run.  Obviously the Amiga will not start like this.  I also took it a step further by hooking up the 5V side (both +5 and Ground) to a separate power source.  Once I power up the 5V side, the ATX PSU will shut down again.  Another thing I notice that when this happens, if I unplug the power lead to the ATX PSU, then pull and replace the connector on the Amiga, the PSU fan will briefly spin.  It is like there is a capacitor somewhere discharging. 

I have also reseated all chips on the board.  The motherboard appears in perfect condition.  The battery has been removed from the expansion board and shows no evidence of prior leaking.  I also tried booting without the expansion board, floppy drive, and even keyboard.  All three produced the same result. 

I am afraid I may have a bad motherboard.  I am unsure what to check next.  This is an Amiga 500 Rev 5.  Any help would be greatly appreciated!  I really look forward to getting this computer working.  I had one of these years ago, and regretted letting it go. 

Thanks in advance!

[scuzzb494]

Shame you can't use an Amiga PSU. Without powering the board you could do a continuity check on the circuits and see if any of the tracks are faulty. I've had a similar problem with a bent pin to a chip particularly the main processor. Also check for damage to pin seats inside the chip socket. Look for corrosion around the Agnus. Do not pull chips without the special tool. Fully remove the board and have a close look underneath. Check also for dry solder joints at say the power supply etc where excessive plugging in and plugging out can break jounts.

Did the floppy kick in. Sometimes you get a click. Also depressing one of the switch pins may roll the drive round. Get an external drive and check also with that. Make sure all the chips are the right way round. You didn't mention what the power light was doing. Did that flicker on and off.

By the way they build aeroplane flight recorders out of Amiga 500's cus they are impossible to break. My guess is a circuit is busted from usage from one of the back ports. Could even be a ROM. I've had all kinds of problems preventing an A500 from not working. That's why I keep a big box of spares of everything so I can swap stuff out.

Have fun.

[gksonnier]

Really appreciate the reply, thank you!  I did uncover another detail through my investigation.  On a hunch, I wanted to check for ground at the power connector.  The pin that should be 5V is shorted to ground.  This explains the behavior with my ATX PSU.  I tried another PSU (before testing for ground at the power connector) which also did not work.  As for the power LED, it does not light at all.  I did pull all chips one by one to test further.  I see no physical signs of corrosion or damage, but did not check with a microscope or anything like that. 

I will continue to troubleshoot.  I imagine I may have a shorted capacitor or an issue with a chip/socket as you mentioned.  I may go ahead an find extra chips to have something to swap and test. 

Again, thanks for your reply.  It has been years since I have owned an Amiga 500.  I really want to get this one going again. 

[mechy]

Its a long shot but badly shorted ram may cause this. sometimes the ram chip(s) in question will get hot. I've never seen one trip the psu but it might be possible.
maybe u can catch a chip heating up before it does it.

if it has a bottom metal shield, make sure the insulating plastic is in there and its not shorting to it.

[gksonnier]

Appreciate the response about possible shorted RAM.  I am still working on this.  Haven't solved it yet, but haven't given up either.  I think I may have to remove the RAM to really test it.  I may go ahead and socket those so that they can easily be replaced in the future.  Thanks again for the reply!  I really hope I can get this thing going again. 

[dalek]

You might need to load up your 12V line some to get the ATX voltages stable - add a spare old hard drive as a dummy load and see if that helps.

[gksonnier]

That is a good idea.  I will try that when I get home.  Thanks for the reply!