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Old 10 November 2018, 20:15   #1
alphagemini
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A1200 power supply

The power supply to the A1200 has five pins, obviously different voltages for different purposes. Does anyone know what the different voltages are used for?

Thanks for any help
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Old 10 November 2018, 20:42   #2
elenachen
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+12V for things like Floppy, seriell (and OP-Amps?)
-12V for OP-Amps
+5V for the rest
and a common GND
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Old 10 November 2018, 20:46   #3
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Originally Posted by elenachen View Post
+12V for things like Floppy, seriell (and OP-Amps?)
-12V for OP-Amps
+5V for the rest
and a common GND
Thanks for your quick response. I take it that OP stands for output? Would this include output to monitor?
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Old 10 November 2018, 20:47   #4
elenachen
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No, operational amplifiers

https://en.wikipedia.org/wiki/Operational_amplifier

I think there are only OP-Amps for the audio circuitry present on the A1200 Board
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Old 10 November 2018, 21:04   #5
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I am trying to trace a fault that stops monitor output. The machine boots and the presence of a SCART connection is shown on screen but nothing else. I have tried using another HD and booting from floppy but no luck. I was wondering if the power supply had a fault.
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Old 10 November 2018, 21:25   #6
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+12V and -12V are both used for the audio output and for serial. +12V is used for floppy drives (though not all require it). All the video circuitry uses the 5V supply, so check the supply to the DAC (U30, between pins 1 (GND) and 22 (5V).
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Old 10 November 2018, 21:34   #7
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+12V and -12V are not used in the video circuitry as mentioned above. If you can see signs of life (floppy clicking, will read a disk when you turn it on, or hard disk access) then your problem is not is probably not with the power supply.

I usually start with the easiest thing to test. Do you have another SCART cable lying around to test?

Last edited by Shadowfire; 10 November 2018 at 22:37.
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Old 10 November 2018, 21:55   #8
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Thanks for the response. I have tried new SCART cables and swapped the HD. It flashes the right lights. I have booted from a recent floppy. But no display. When it came back from Amigakit it worked for a day. They said it was OK, all checked etc.. But just one day ......
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Old 10 November 2018, 22:32   #9
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I know you just sent it out for repair, but they said that it was working when they shipped it back to you, and it DID in fact work for a few hours when you got it back. This is proof that they were not lying.

But you should realize that just because something was serviced (and working), that doesn't magically prevent something else from breaking on it (even if its only minutes later).

I'm going to retract my earlier post saying that the power supply isn't the problem, and amend it to say that the power supply "probably" isn't the problem, since I've visited your other thread and read about the other symptoms. You definitely have something on the 5V rail, since the processor is working. Does the video come back if you remove the accelerator?
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Old 10 November 2018, 23:13   #10
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I know you just sent it out for repair, but they said that it was working when they shipped it back to you, and it DID in fact work for a few hours when you got it back. This is proof that they were not lying.

But you should realize that just because something was serviced (and working), that doesn't magically prevent something else from breaking on it (even if its only minutes later).

I'm going to retract my earlier post saying that the power supply isn't the problem, and amend it to say that the power supply "probably" isn't the problem, since I've visited your other thread and read about the other symptoms. You definitely have something on the 5V rail, since the processor is working. Does the video come back if you remove the accelerator?
I never implied that anyone lied, please!! This is a fault that comes and goes. Always a problem with complex tech and the worst to solve. It makes no difference if the accelerator is removed. My experience with Amigas is very good, I had a 4000 that ran for decades from new, with a mechanical HD, no probs.. So I have no complaints about Commodore.
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Old 11 November 2018, 14:33   #11
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Quote:
Originally Posted by Daedalus View Post
+12V and -12V are both used for the audio output and for serial. +12V is used for floppy drives (though not all require it). All the video circuitry uses the 5V supply, so check the supply to the DAC (U30, between pins 1 (GND) and 22 (5V).
Yes, but Scart cables do use the +12V to activate the switch signal.
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Old 11 November 2018, 14:40   #12
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Originally Posted by idrougge View Post
Yes, but Scart cables do use the +12V to activate the switch signal.
Yes, that's true as well, but the same fault is happening with composite so it doesn't appear to be a SCART issue.
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Old 11 November 2018, 14:48   #13
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Where does it say that he used composite?
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Old 11 November 2018, 15:58   #14
alphagemini
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Yes, I tried composite, no solution.
I have left the 1200 with Steve Clifford who has found faults in the mother board near the power input. He will rebuild everything on a reconditioned board, recapped etc.. He is confident that the problems can be fixed and the board reused.

Thank you everybody for your help.
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Old 11 November 2018, 23:35   #15
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Sorry idrougge, this conversation is spread over two threads - I thought it was this one but it's the other one.

@OP
Might be an idea to keep threads on the same subject together to avoid confusion. But good that it's getting looked at.
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