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AuthorTopic: Amiga 1000 PSU, Bad Voltages?  (Read 58 times)

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Offline AmmoJammo

Amiga 1000 PSU, Bad Voltages?
« on: December 06, 2016, 06:37:54 PM »
I purchased a very used Amiga 1000 a few months back, and before simply plugging it in, I decided to test the PSU.

The 5volt rail, is a bit low, at 4.9volts, while the 12volt measures at around 9.5volts, and the -5volt is at -4....

Looking in side, the +12volt, and -5volt both appear to use linear regulators? In fact, the -5volt rail uses a positive voltage regulator doing something weird...

There's some weird ripple on the 12volt line, suggesting around 8volt minimum, 12.5volts maximum from memory, but with an average of 9.5volts.

It didn't seem like a capacitor issue, but I replaced all the output capacitors anyway, and sure enough, the voltages are still off.

Should I just give up? Or do the rails need a load to properly regulate?

Thanks!
 

Offline Castellen

Re: Amiga 1000 PSU, Bad Voltages?
« Reply #1 on: December 07, 2016, 06:21:12 PM »
Quote from: AmmoJammo;817387
Or do the rails need a load to properly regulate?



Yes, most computer switchmode supplies are designed to work with a minimum load.  Without that, it's normal to see output voltages somewhat out of range as you're seeing.

Things are pretty close, suggesting that nothing is critically unhappy, so you're not going to damage the A1000 by connecting it.

Under load, the various outputs should be within about 5% of the nominal voltage.  i.e. 4.75-5.25V for the 5V line.
 

Offline AmmoJammo

Re: Amiga 1000 PSU, Bad Voltages?
« Reply #2 on: December 07, 2016, 07:54:21 PM »
The +12volt and -5volt are both linear regulators off the switchmode supply, so as long as there's above the drop out voltage being fed in, they should regulate.

Unless the 5volt rail needs a load, so the switchmode controller is running harder to keep that supply up (its all one transformer, but with multiple isolated outputs)
 

Offline Amiwest

Re: Amiga 1000 PSU, Bad Voltages?
« Reply #3 on: December 08, 2016, 01:06:39 AM »
I have not worked on a A1000. (Info I have saved.)
http://amiga.serveftp.net/Schematics/A1000_schematics/A1000_PSU_Euro_schematics.pdf
It is a 220v 50 Hz supply so they might be the same.
Note in the PC0 device( 5V), one half in the primary and the other in secondary. ( need a load) There are two variable resistors shown (RV01-02) I would not adjust these without the correct load.
 

Offline AmmoJammo

Re: Amiga 1000 PSU, Bad Voltages?
« Reply #4 on: December 08, 2016, 05:10:43 AM »
I stuck two 15ohm resistors in parallel across the 5volt rail (600ma load), and sure enough, the other two rails came up to a more reasonable level.

Thanks for the help!
 

Offline gregthecanuck

Re: Amiga 1000 PSU, Bad Voltages?
« Reply #5 on: December 08, 2016, 07:41:47 AM »
Thanks for this thread. I have an A1000 to revive that has been in storage a while. First testing will be of the power supply. This is good information. :)