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Old 16 July 2011, 18:51   #1
whitebird
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A1200 motherboard stopped to work suddenly

Hello,

I had an unused A1200 motherboard which I decided to power up. Everything went fine, kickstart insert disk menu showed after 10s. I tried the power up sequence 2 or 3 times.

On another day, after connecting a floppy drive I started a disk with a bootloader which showed up correctly but after a few seconds, a reset occured, then green screen...

And from that point I'm no more able to start the board even when no floppy is connected.

What's strange is the random behaviour. Here is the description of the different situation I can see:

-Screen stays gray and sometimes the video output turns completely off, then restarts...
-System seems to keep rebooting permanently: when this occurs the system starts reading a disk (the bootloader on the disk shows for 2 or 3 seconds), then reboots.
-Sometimes I got a yellow, pink or green screen.

I also want to stress that before this problem, I had the following behaviour:

1 second after turning power up, a blink showed up but the screen remained gray during 10 seconds until the insert menu appeared.

I'm not sure, but shouldn't the screen be white till the insert disk menu?

Thank you for any suggestion...
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Old 16 July 2011, 19:00   #2
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  • Red - An error in the Kickstart rom as detected.
  • Green - An error in the Chip Ram was detected.
  • Blue - An error in the custom chip set was detected.
  • Yellow - The CPU encountered an error before the system's error-trapping code (the code the calls up the Guru) was in place.
  • Black - No CPU detected.
  • Grey - CPU Passed the test.
  • White - CPU failure.

Maybe this will help you.
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Old 16 July 2011, 20:33   #3
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Thanks 1time.

Strange to see some pink as this is not referenced.

I made a confusion with A500 where the screen is white after boot up.

So a gray screen means everything is OK...

Perhaps there is something wrong with the DRAM chips.
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Old 16 July 2011, 20:46   #4
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The board was in storage for some time, then you power up after years it works but then go's bad. That sounds alot like a bad capacitor or bad caps in the PSU.
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Old 16 July 2011, 21:18   #5
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Some more info about the boot procedure.

http://wiki.classicamiga.com/Amiga_b...r_code_colours
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Old 16 July 2011, 23:39   #6
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The error codes are not consistent, I don't think its a custom chip problem.
What you can do is remove the 2 kickstart roms in you system, clean the sockets with some alcohol and a toothbrush. then the legs of the 2 kickstart chips. See what happens.
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Old 17 July 2011, 00:54   #7
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Quote:
That sounds alot like a bad capacitor or bad caps in the PSU
It should have been the reason, but unfortunately voltages seem OK, at least their average values. I made a voltmeter check:

5,05
11,86
-12,04

I will check DC status by help of a scope.

I tried cleaning the ROM socket and pins but with no success.

To be more precise, the situation that happens the most is:

Black screen during 0,5s, then display gets a bit lighter and shows a constant gray color till I switch off the PSU.

What's a bit surprising is that I get same behaviour when I power up the system with no ROMS on the board. Normally I should get a red color because no ROMS is like having an error in the kickstart right?

But if the selftest is able to detect errors even on ROM, it means the test must be located not in ROM but somewhere as part of a custom chip.

Which component performs the selftest?
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Old 17 July 2011, 09:57   #8
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One of the first things the A1200 checks for is the CPU, after that it should check the ROM.

Is your CapsLock button flashing when you start the computer?

Capslock Key:
Stays on for several secs

One blink (ROM checksum failed)
Two blinks (RAM test failed)
Three blinks (Whatchdog timer)
Four blinks (A short exists between
2 lines or one of the
special keys)
Please note that the capslock 'blinks' are short in duration (about one
second)

I also found the "pink" error code:

Magenta (0xF0F) single-task or cold-start initialization failed.

Last edited by 1time; 17 July 2011 at 10:09.
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Old 17 July 2011, 10:14   #9
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Did you measure the voltages correctly (under load)?
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Old 17 July 2011, 10:39   #10
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Quote:
Originally Posted by voyager View Post
The board was in storage for some time, then you power up after years it works but then go's bad. That sounds alot like a bad capacitor or bad caps in the PSU.
I was just about to post something like this. Sounds like bad caps that broke because of "power on stress" after all the years with no power.
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Old 19 July 2011, 01:24   #11
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Quote:
Did you measure the voltages correctly (under load)?
Yes I measured them with the board connected so the PSU was correctly loaded.

I had a look at some key signals and voltages with the help of a scope.

PSU voltages are good. I checked CPUCLK (14MHz), /KB_RESET, /RST, /RESET and also status of the caps lock LED and everything seems OK.

The fact that the system behaves randomly can be the consequence of a broken track (bad contact) but also because of "bad software"

If the ROM was broken, or with inconsistent software, we should have same behaviour everytime. So I tend to think it can be due to bad RAM so when kickstart runs the value that are stored in RAM are altered which makes the system run undetermined.

When I remove the ROMs, everytime I boot I get a constant dark gray screen, I had a check at the address bus of the 68020 but it is not switching, and the /CS of the ROMs is '1'. The reason is that the 68020 is halted (by GAYLE pin 39), because the selftest didn't detect any ROM?
So this tends to confirm that the problem occurs only when the software starts to run.
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Old 19 July 2011, 04:15   #12
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Quote:
Originally Posted by whitebird View Post
The fact that the system behaves randomly can be the consequence of a broken track (bad contact) but also because of "bad software"
Or dying SMD capacitors, which is much more likely.
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Old 19 July 2011, 04:31   #13
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Quote:
Originally Posted by 8bitbubsy View Post
Or dying SMD capacitors, which is much more likely.
Is there a way to check them on-site?
Prior/Instead of replacing them all, I mean...

EDIT: I have a Panasonic VCR and a Sinclair QL which have video problems almost surely due to bad caps... but I would like to identify the "culprit/culprits" before havinh to change them all
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Old 19 July 2011, 05:01   #14
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The classic method is to see if the cap has bulged out the sides and/or on the top. Another way, which you won't see often on thru-holes ones, is if the caps had leaked their electrolytics out on the PCB.
If not, get an ESR meter and a cap µF/volt ESR table and check if the ESR is much higher than usual, that is a sign of a bad capacitor that needs to be replaced.
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Old 19 July 2011, 22:42   #15
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I had a look at the capa that can leak and found out 2 that had sign of leakage.
One had some green around the contacts but unfortunately no track was broken by corrosive effect.

The values may have changed over time but they are just "decoupling" capacitors and as already said because of good voltages on the +5, +12 and -12V, they are probably not causing the problem.
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Old 20 July 2011, 00:55   #16
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Quote:
Originally Posted by 8bitbubsy View Post
If not, get an ESR meter and a cap µF/volt ESR table
Ehm... what's ESR? And when can I find a cap µF/volt ESR table?

I have a digital multimeter with a capacimeter section... will it be useful/sufficient? I suppose not
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Old 20 July 2011, 01:11   #17
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Quote:
Originally Posted by Supamax View Post
Ehm... what's ESR? And when can I find a cap µF/volt ESR table?

I have a digital multimeter with a capacimeter section... will it be useful/sufficient? I suppose not
http://en.wikipedia.org/wiki/ESR_meter
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Old 20 July 2011, 01:32   #18
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Hmmm... can a "normal" ohmeter be used to measure the ESR?
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Old 20 July 2011, 02:50   #19
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Quote:
Originally Posted by Supamax View Post
Hmmm... can a "normal" ohmeter be used to measure the ESR?
No.

Quote:
A normal (DC) milliohmmeter cannot be used to measure ESR, as a steady direct current cannot be passed through the capacitor.
(From the Wiki page.)
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Old 20 July 2011, 18:03   #20
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Quote:
Originally Posted by whitebird View Post
I had a look at the capa that can leak and found out 2 that had sign of leakage.
One had some green around the contacts but unfortunately no track was broken by corrosive effect.

The values may have changed over time but they are just "decoupling" capacitors and as already said because of good voltages on the +5, +12 and -12V, they are probably not causing the problem.
The rest of the caps will follow the death, it's just a matter of time. As said, they can look just fine but have high ESR rating or be dried inside. Change them all, and you might be surprised of the result.
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