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Amiga 500 power LED and screen dims when external floppy drive is accessed

gonk23

Experienced Member
Joined
Jun 15, 2013
Messages
199
Location
Melbourne, Australia
I recently pulled an Amiga 500 (Rev 6A) out from storage. It seems to work well, except for a strange problem when an external floppy drive is connected. The 500's power LED and the screen (on an 1084S monitor) dims in sync with the external floppy drive access. When the drive isn't being accessed there's no dimming, although the normal drive clicking every few seconds (when the drive is empty) does result in dimming for the duration of the click.

I tried 2 different external floppy drives and they both do it, although one seems to cause slightly greater dimming than the other. The internal floppy drive doesn't cause the same problem.

Any ideas what might be the cause and how to fix it? I suspect it might be power supply (+12V?) related.
 
I'm just reflecting on how someone in this very forum flipped out when I called something a "Rail." :)

Thanks for using common terminology. :)

Yeah, attach a scope or a DMM to the voltage rail and see how badly it dips.
 
Weird ... when I took the supply apart so that I could attach the multimeter leads, the problem went away and hasn't come back yet (even after reassembling the supply). Maybe I dislodged a cockroach shorting something in the supply :)
 
You probably had a high resistance joint in one of the connectors somewhere and moving and tugging at the wires has dislodged it and you are now OK. Having a slightly higher than normal resistance in a connector would cause the voltage drop across the joint to increase the more current that flowed (e.g. when you operated the disk drive). This would cause less voltage to appear for the logic downstream of the joint (hence the dimming LED). As the resistance (or current) increased - the voltage downstream would fall until such a level that the logic would fail to operate correctly. Usual things apply - if it is old, waggle it about a bit (or disconnect and reconnect things) and you should be good-to-go for a few more years!

Don't worry about calling things rails. We have +5V 'rails' in our process computers at work. They are each 3/4" wide, 1/4" thick and approximately 5' in length. At points along the 'rails' they have capacitors big enough to make the end of your screwdriver tip disappear "as if by magic" if you accidentally short them out! Now they are rails... and yes, they do look like parallel train rails!

Dave
 
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