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Old 03 November 2018, 22:42   #1
ddewaele
 
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Amiga 500 switched 5v and gnd. Amiga toast ?

Hi,

I have an amiga 500 where the PSU got lost and I was able to bring it back up using an ATX power supply (by using the 5v / 12v / -12v / GND pins).

Overwhelmed with joy and in a hurry getting everything back together in the case, when I fired it up I noticed the PSU fan halting and noticed a smell coming from the Amiga.

My only guess is that I reversed the 5V / GND pins.

I'm not seeing any burn marks or blown components on the board, but I'm not an electronics expert (given what got me here in the first place). I do smell something "burnt" around component LF1 (the line filter, part of the power supply).

Now some questions
  • Can someone more knowledgeable with electronics assess what kind of damage the board (the chips) received here ?
  • What would be a good way going forward in diagnosing what damage there is and what can be fixed.
  • I have a second fullu working Amiga here with an original Amiga PSU this time. Is it a good idea to start swapping components from the bad machine to the good machine (or vice versa) to see what is broken and what still works ? Or could potential faulty ICs damage a good board even more ?

Thanks a lot in advance.
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Old 03 November 2018, 22:55   #2
bebek
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What do you mean by guess? Did you reverse 5V and GND or not? Have you tried good PSU?
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Old 03 November 2018, 23:12   #3
ddewaele
 
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Quote:
Originally Posted by bebek View Post
What do you mean by guess? Did you reverse 5V and GND or not? Have you tried good PSU?
The amiga doesn't do anything anymore on the ATX PSU I was using and the PSU fan switches off immediately.

The ATX PSU is good as it runs fine on a PC and the voltages check out ok.

Haven't tried it with my original Amiga PSU I have now cause I'm afraid to blow it up.

I'm assuming I mixed GND and 5V cause AFAIK it's the only thing that could have gone wrong.
 
Old 04 November 2018, 00:57   #4
ddewaele
 
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I checked with my multimeter and notice there is a short between the 5V and GND pin (measured on the amiga power connector).

Could it be that the line filter LF1 is causing the short ? Would it be an option to try and remove and replace the LF1 component with a similar one ? (assuming such components exist and can be modded onto the board) ?
 
Old 04 November 2018, 01:26   #5
project23
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You mean a short on the actual motherboard?

Continuity between the 5v rail and GND on the amiga? (not on the PSU)
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Old 04 November 2018, 01:37   #6
ddewaele
 
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Correct. Measured the pins on the power input connector on the board and I have continuity between 5v and gnd.
 
Old 05 November 2018, 00:18   #7
ddewaele
 
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Quote:
Originally Posted by project23 View Post
You mean a short on the actual motherboard?

Continuity between the 5v rail and GND on the amiga? (not on the PSU)
I'm assuming a faulty component (caused by the original 5v / gnd mixup) is now responsible for this short ?

I tested it both with and without the big ICs mounted in their sockets but there is always continuity between 5v and gnd.

As I noticed a smell coming from the line converter, would it be a good idea to desolder this one and see if the short is still there ?

Or what would be another way of diagnosing this issue ?
 
Old 05 November 2018, 21:59   #8
ddewaele
 
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For those reading this thread and who might experience a similar issue here's what I did :

After noticing the short between 5V and GND (there was constant continuity, not the 1sec continuity you might get by charging and discharging a capacitor via the multimeter)

I focussed on the line filter component (LF1) because I believed that this component blew when the problem occurred and it still smelled funny.

I removed the component and verified that there was no longer continuity between 5V and GND.

I then proceeded to wire the pins directly (without a coil) to test.

The short was gone and the amiga was able to start and load up disks. So audio / video / IO was tested. I assume the Amiga is back up and running and no other components were harmed.

I still believe that the root cause for this issue was me powering the amiga with the 5V and GND leads from the PSU switched.
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Old 07 November 2018, 21:27   #9
project23
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Since you're using an ATX supply the removal of the toroid might not actually be that bad.

It's fitted for RFI reasons as i'm sure you know. Maybe you're radiating RF at a couple mW at around 7 megs now, which is a ham band, so wouldn't be ideal - but we're talking mW like I said.

The problem - if there is one - will arise when you try to put an accelerator on it, or run something for a long period of time. It may now take interference *in* between the ATX supply and the board.

I'm not sure of the specs of LF1. Maybe somebody has them? If you have any ferrites about it'll be incredibly easy to build a replacement should you encounter issues.

John


EDIT:


The following site is of great use and should be shared as much as possible:

http://www.amigapcb.org/

I suppose its possible that something allowed a real high 5v current through LF1, burning the cabling and creating the short? Maybe the above resource provides the answer.

Anyway, glad it worked, and sorry for the late reply.

John

EDIT AGAIN:

On second thought they only have rev 8A (the plus, really) and yours is rev... 5? So maybe not worth the bother trying to trace how it happened. If its working, its working. Just replace that toroid if you care at all about RFI.

Last edited by project23; 07 November 2018 at 22:02.
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