Amiga 500 PSU issues

Hi all.

This is my first thread on this forum site.
Last year I decided to buy myself an Amiga 500. For the first few weeks of me having it, it worked flawlessly, no problems. However, I went to turn it on one evening and the power supply's internal glass fuse popped. I initially thought it was a problem with the mains supply so I replaced the fuse, annoyingly with the same result, another blown fuse. A tried a second PSU of the same type as the previous one . It again kept blowing fuses. Since those two I purchased a replacement PSU board which is completely different to the other two. I have wired it to the old cables from one of the other units, except for one wire in the computer to PSU cable. It is an earth wire in a black jacket and there is seemingly no place for it on this board. It works to an extent with the machine. When it is booted with a display attached, nothing is displayed and neither the power or disk LEDs light up. In addition to that, the disk drive spins when the power is on.

When no display equipment is attached, I get only a flashing power LED. I have heard that no display and flashing LEDs can mean a failed 'Denise' IC, however it has been sitting around for some time and I can't see how it or any other internal could have gotten damaged.

Does anybody know how I could go about getting the machine running, as well as confirming if the earth lead I mentioned does need to be attached?

Many thanks

[Shadowfire]

The A500 needs +5/+12/-12V supplies. There are two ground pins (1 for PSU ground, 1 for shield ground). The unconnected wire is probably the shield ground. The PSU and shield ground are tied internally in the A500 motherboard. Leaving it floating will probably work, although the power lines will be more susceptible to EMI.
If two different PSU's were blowing fuses, the problem is inside the computer, not the PSU. Check the resistance between all of the power rails and ground, and with each other (on the motherboard, without the PSU connected). If you get an exceptionally low reading (<1 ohm), the problem needs to be found and fixed. There should be three .01uf EMI capacitors by the power entry. Check across those and replace if you get a low resistance reading, as a first step.

Hi there.

Thanks for the help. I don't think the machine is the fault as I replaced fuses and tested the PSUs with and without a load. Under and without a load the fuses popped. I also bought a new PSU board (the one with the earth wire disconnected) and that itself works. However The computer doesn't seem to like it.

Many thanks

[amilo3438]

Yeah, the A500 needs +5/+12/-12V supplies ...

there is nice video that also shows power consumption on lines:

[ Show youtube player ]

Interesting part for you starts at 30:34 !!!

Btw. are you trying to run a basic A500 or its expanded with something ?!
Also, I would 1st check what are the voltage on plane PSU lines ?
Does the PSU shows the right voltage when is disconnected from A500 ?

p.s.
A little strange situation if two different PSU's were blowing fuses and you don't think the machine is the fault !!!
Also, on above video is mentioned that PSU connector on motherboard is prone to damage after repeated usage !!!
(check the above video from 27:33 -> broken joints on DIN-connector)

[Shadowfire]

Yeah, at this point its just a matter of correctly hooking up ground, +5, +12, and -12 supplies. If you swapped any of them around by accident and applied power to the machine, its time to find someone who can fix the board, since that would have blown IC's on the motherboard.