A1200 motherboard stopped to work suddenly

[whitebird]

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...

[1time]

• 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.

[whitebird]

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.

[voyager]

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.

[1time]

Some more info about the boot procedure.

http://wiki.classicamiga.com/Amiga_b...r_code_colours

[voyager]

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.

[whitebird]

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?

[1time]

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.

[Hewitson]

Did you measure the voltages correctly (under load)?

[8bitbubsy]

Originally Posted by voyager

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.

[whitebird]

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.

[8bitbubsy]

Originally Posted by whitebird

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.

[Supamax]

Originally Posted by 8bitbubsy

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

[8bitbubsy]

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.

[whitebird]

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.

[Supamax]

Originally Posted by 8bitbubsy

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

[prowler]

Originally Posted by Supamax

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

[Supamax]

Originally Posted by prowler

http://en.wikipedia.org/wiki/ESR_meter

Hmmm... can a "normal" ohmeter be used to measure the ESR?

[prowler]

Originally Posted by Supamax

Hmmm... can a "normal" ohmeter be used to measure the ESR?

No.

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.)

[8bitbubsy]

Originally Posted by whitebird

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.