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Old 15 May 2017, 20:01   #41
Daedalus
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Is it under load? That is, is your Amiga turned on when you tested it? If so, then yes, there's something wrong there and I wouldn't trust that PSU.
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Old 15 May 2017, 22:37   #42
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The PSU is plugged in and turned on and I have the multimeter connected to the relevent pins on the power cable.
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Old 16 May 2017, 09:35   #43
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Hmmm, if the output of the PSU isn't connected to anything other than the multimeter then it's not under load and so the readings may or may not be reliable. There is a risk involved, but you could try it plugged into your Amiga and see what the voltages are while it's running.
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Old 16 May 2017, 10:33   #44
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To have 12V a bit too low won't cause any problems, however it might be a false reading if the voltage is not stable. Cheap multimeters are very inaccurate when measuring unstable voltages and can exhibit all kinds of weird behaviour.
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Old 16 May 2017, 11:31   #45
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I tested other PSUs and am getting good readings, it's just one PSU I'm getting low voltage readings. The multimeter I'm using is a Fluke 117, I think Fluke are a reliable brand?
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Old 16 May 2017, 12:28   #46
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Yep, they're pretty good generally. As Demolition says, there's very little risk if the reading's too low - chances are it will only go lower when it's in use. Just make sure your +5V isn't something crazy before you test it. Personally I would rig up a dummy load like an old hard drive or something and test it that way to be safe, but that's tricky with the non-standard connectors used.

Not all PSUs work the same way - linear PSUs will generally have good regulation when there's little or no load, whereas SMPS types will have poor regulation. However, many SMPS units have small dummy loads for cases where they're not connected to anything, but you can't guarantee it.

Commodore are known to have used both types, and even combinations of the two, in their power supplies.
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Old 16 May 2017, 14:02   #47
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If the +12 voltage does decrease over time what's the worst that could happen?
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Old 16 May 2017, 14:26   #48
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Floppy drives start to misbehave, audio gets distorted and serial reliability goes down. Not the end of the world - the rest of the machine should work fine.
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Old 16 May 2017, 15:28   #49
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I think many floppy drives only use 5V so in that case only the audio and serial port remains. Early floppy drives use 12V for the motor, but I'm not sure when they stopped doing that. A Samsung SFD321B for example only needs 5V: http://www.techtravels.org/wp-conten...21B-070103.pdf

The audio should not start to distort significantly until the 12V line drops very low, perhaps 6V or so since the voltage swing on the audio is only a couple of volts and the center voltage is 2.5V. It might work fine from +/-5V but there is no -5V available and since 12V is not really used for much else, it is also much less noisy and that is something people tend to like with audio.
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