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Old 06 January 2016, 16:45   #1
tbtorro
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A4000T boot signals help needed

Hi all, I'm trying to resurrect an A4000T motherboard with no boot. (Black screen NOT grey screen).

Anyone has tips on the exact boot process (signal level) to complete my diagnosis? The board behaves as if there is no CPU card is inserted.

So far:
- All supply voltages OK
- Clocks ticking
- All RST signals are high everywhere
- MDAT lines OK
- All obvious things are checked already (physical damage, caps, connectors, burned or overheating components, pins, jumper locations etc)
- Only AV module and CPU card is on board, both known to be good.

This board has suffered a burn between the +12V line to the AV module possibly due to a bad power supply (Pictures here) but worked regardless during previous testing at least a few times. Then suddenly stopped booting (before fixing the burned track).

I'm a bit stuck with diagnosis, if you have any ideas on signals (system init) to check let me know.. I'm looking for advanced level diag tips..

Last edited by tbtorro; 10 February 2016 at 14:32.
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Old 10 February 2016, 14:17   #2
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Still working on this. My latest theory is, that since there was a burn on the 12V rail, something was maybe connected to the RS232 port (maybe PC mouse) and some current spike also affected EVEN CIA. However RST signals on both CIAs seem to be high at power on, so I'm not sure this theory is correct.

Another mystery is while testing I did notice that ODD (Not EVEN) CIA is somewhat hotter than the other, and also KBDAT is low at boot with no keyboard connected. Now the same signal is high on a known working A4000T board at boot with no keyboard connected. Any ideas anyone?

Anyone have sure way to test if CIAs are failing without first removing them? Or any other tip would be very helpful..

Last edited by tbtorro; 10 February 2016 at 14:38.
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Old 10 February 2016, 14:20   #3
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Try contacting John Hertell on Facebook - he's in the Commodore Amiga group and knows that end of things pretty well.
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Old 10 February 2016, 14:38   #4
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first question I always ask: does powerled go from half-bright -> full bright after like 1 second?
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Old 10 February 2016, 14:38   #5
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What I know the A4000T requires the SCSI board to be connected. but if it is not, what I remember it should give a WHITE screen.
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Old 10 February 2016, 14:46   #6
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Speak of the devil
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Old 10 February 2016, 14:58   #7
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Quote:
Originally Posted by Chucky View Post
What I know the A4000T requires the SCSI board to be connected. but if it is not, what I remember it should give a WHITE screen.
Without the SCSI module you get the same gray screen as without a CPU card.
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Old 10 February 2016, 17:55   #8
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Quote:
Originally Posted by hese View Post
Without the SCSI module you get the same gray screen as without a CPU card.
Actually according to my tests on a working motherbboard it would not be the same:

Without SCSI module but working CPU card = Light gray (4:3 frame)
Without SCSI module and NO CPU card = Dark gray (or black seems to me) hard to tell - but there is some video signal at least.
WITH working SCSI module and NO CPU card = Dark gray (or black seems to me) hard to tell - but there is some video signal at least.

So I would be very happy with a light gray screen, hence the bare bones setup with only AV module and CPU card. In any case does not really matter, because even with confirmed good SCSI module the behavior is the same: dark gray (or black) screen.

Will check PWR LED behavior in a sec
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Old 10 February 2016, 18:42   #9
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if powerled changes brightness, THEN you know that cpu "works" as it is one of the first instructions in the startup in rom.
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Old 10 February 2016, 19:00   #10
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Quote:
Originally Posted by Chucky View Post
I am here aswel

first question I always ask: does powerled go from half-bright -> full bright after like 1 second?
So, no it does not. It lits and stays constant brightness at power-on.
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Old 10 February 2016, 19:03   #11
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Quote:
Originally Posted by Chucky View Post
if powerled changes brightness, THEN you know that cpu "works" as it is one of the first instructions in the startup in rom.
Correct, but light gray video output would also indicate the same (CPU test passed) - or does that come after the power LED logic? Anyway, power LED is constant - no brightness change.
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Old 11 February 2016, 14:23   #12
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Can anyone confirm that the A4000T can boot without ODD CIA U550?
----------------------
OK looks like U550 ODD CIA is a must for boot, swapped to a new chip - now I have light gray video out, power LED brightness change indicating CPU test pass, but it never gets to KS screen. KB caps lock led also gets stuck after about 10-12 presses so something is still halting the system. Keyboard reboot (CTRL+CTRL+A) is working which telling me that U550 CIA is functional.

Last edited by tbtorro; 12 February 2016 at 01:03.
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Old 13 February 2016, 04:40   #13
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Well here it is, faulty Paula AND odd CIA. Board is alive and kicking well after swapping both chips.

Thank you all who contributed! Another precious board saved.
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Old 13 February 2016, 09:06   #14
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Ahh great that yet another Amiga can live again!
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