A1200: now a PSU issue...

[starf81]

Well... just solved the Apollo's issue, my "A1200 project" goes ahead. And, obviously, I've another problem  :-D

Up to now, I've tested my configuration with an A500+ power supply but now I want to adapt an ATX one.

I had an old working 200W ATX power supply so I've adapted it using the original cable taken from the original A1200 PSU. I've found on the net some schematics to wire the ATX PSU to che Amiga cable: +12v, -12v, +5v and GND. After I've tested it with the A1200 and all work correctly.
But, as I attach the Apollo 1240, something goes wrong. The green power led is less strong, the screen is black and the fan wired to the Apollo's board turns slower.
I can't understand... I've tried two different ATX that work correctly with my PC. In addition, when I use the A500+ PSU, the Amiga and the Apollo work correctly.

Last note: to start the ATX PSU, I've linked the green "PS ON" cable with a black GND wire.

Could you help me?  :-D

THANKS!
Alex

[Karlos]

Is it just the 1240 card and no other expansions that this happens with?

A slower fan implies to me that there is a voltage drop on one of the rails. As far as I recall the fan on the Apollo1240 is a 12VDC one. You are definately sure the +12VDC line is correctly wired up?

[starf81]

Yes Karlos, only the 1240 and the mouse, no other devices or exansions.

I've taken the +12v line from the 4-wires molex as the +5v, while the -12v line comes from the 20 wires plug. Obviously I've tested them with a meter and are good.

In addition, as I said, I've tested two different ATX PSU, the old 200w and the 400w that I use everyday into my PC.

It's so strange... voltages are right and without the Apollo all work correctly. The Apollo works only with the A500 PSU.

Any ideas?  :-?
Alex

[Karlos]

Can you test the levels with the base A1200 and with the apollo attached? If possible chech the levels on the pins powering the fan and also on the floppy power header on the motherboard. If the fan is slower I'd expect the 12VDC line to  be low.

I wonder if something is pulling too much current through some  weak link?

[Doobrey]

Karlos wrote:
 If the fan is slower I'd expect the 12VDC line to  be low.

Or the ground could be high  :-o

I'm scratching my head on this one, but since it all works with the a500 PSU I'd point to the re-wiring job.
 Maybe something silly like the +/- 12v lines swapped, or a bad ground connection.  Is the audio output distorted when using the ATX with no Apollo?

I was thinking about the +5v being low on the LED, but then remembered it only goes bright when after it's passed the startup checks in the ROM...which points to the CPU not working...which only confuses me more
 :-x

[blobrana]

You will need a AT Power unit ...
A ATX power unit will not work usually.
(power saving/stand by)

[starf81]

Tomorrow I'll measure voltages inside the Amiga. This is the only test I've skipped.

After I'll try also the audio quality and I'll let you know...

Thanks for your interest... this is a very strange case  :-o

Alex

[starf81]

Hi BlobRana. Why you said this? Help me to understand because I'm very confused  :-?

I've found several schematics on the net to use an ATX PSU. There are also circuits to turn on it with a switch as you can  do in a PC.
In addition, you can find on Aminet little software to turn-off the computer directly from the shell only with ATX PSU...

Thanks
Alex

[Karlos]

My A1200D is running from a cannibalised ATX PSU with no problems. You just have to ground the PS_ON line (grey I think, but seriously do not take my word for that).

[Doobrey]

Never trust wiring loom colours..
I just double checked on my mini ATX, and its PS_ON is purple, but according to the ATX spec it should be green :pissed:
 Since my A1200 is laid bare on the bench with it's top casing off, I just stuck a simple toggle switch between PS_ON and GND. I dare say when I bung it into a proper case I'll use something like the circuit on Stedys site to use a nicer non-latching push switch.
 Thinking about it, I might use a PIC to replace the debounce and flipflop, with the added bonus that it could time the duration the switch was held in and do a reset if it was held for >2-3 secs, or even have an input from the miggy for a soft power off too..
But now I'm waffling out loud and not helping at all...  :insane:

[Karlos]

I never trust any of these particular so called "colours". Nobody ever offered me any tangible proof they exist :lol:

[starf81]

I've just measured voltages from PSU and inside the A1200.

These are the values:

ATX PSU
-------

OUTPUT:
+ 11,68V
+ 5,15V

A1200 (without Apollo):
+ 11,95V
+ 4,58 V

A1200 (with Apollo):
+ 11,86V
+ 4,24V


A500+ PSU
---------

OUTPUT:
+ 12,00V
+ 5,31V

A1200 (without Apollo):
+ 12,35V
+ 5,15V

A1200 (with Apollo):
+ 12,61V
+ 5,00V

It's so strange...  :-?  :-o  :-?

The A500 PSU provides higher voltages than the 200W ATX PSU. In addition the ATX works perfectly inside a PC and, as I just said, I've tested another ATX PSU, that was a 400W.

Looking to the values, the main problem seems to be the +5V, that is too low when I attach the Apollo. Probably it needs more energy so the Amiga doesn't start.

... I can't understand  :pissed:

Alex

[Karlos]

Most likely your PC is drawing most juice from the 3.3v rail so that the 5VDC one has less impact on reliability.

[Framiga]

unfortunately, the problem still with the UNUSED 3.3volt output, when used on an Amiga.

Some "cheap" ATX PSUs, when used on Amiga, gives unreliable output on the 5 volt rail (see at your 4.24 volt with Apollo installed).

Often (see at the ATX PSU specs) the 3.3 and 5 v power output, are given together (as total output Amperes)

When we use it on an Amiga, those ATX PSUs, works in an unbalanced way.

The solution, would be to use more reliable (more expensive ATX PSUs, with indipendently regulated output) or give to the PSU a "dummy" charge to the 3.3 volt output.

EDIT- erm . . . about how to achieve a reliable charge on the 3.3 volts line, any help wiil be apreciated :-)

A little personal note:

i own a PowerTower4000 (but its the same even on a PowerTower1200) with those little PCB with row leds-2 drive transistors and few polarization resistors).

Seen that i had the 3.3volt ATX output line there doing nothing, i have used it instead of the 5 volt to power the leds PCB (an easy way to dimm all those leds light).

Result . . . those very, very low power drain, from the 3.3volt ATX output, screws the whole PSU output (not so much but it does).

[starf81]

It seemed a reasonable solution so I've tried but it didn't work.

As "dummy charge" I've used some car lights with a value from 5w to 45w. Nothing happens  :-(

I'll look for a better ATX supply or for an old AT...

Thanks for your interest.
Alex