Problems with kick v1.2 rev. 5 A500 mobo.

[flaviosr]

Dear All,
I bought one of the first A500s (the ones with C= logo engraved in the case, with C= key and the red leds...).
I had to change the floppy drive and clean everything (expecially the keyboard) and everything was working...
but suddenly, some days ago, I "found" the kickstart screen of one of the photos.
When I put a floppy in the drive the Amiga sees it but does not start reading it (I tried many floppy drives, and all of them works with other A500s).
I tried to find out which is the problem around the internet and perhaps it could be one (or both) of the CIAs. I could try a change but, since I am not a technician, I would not like to ruin some more things!
Can somebody help me?
Best Regards

[arttu80]

Hi!

It's nice to have early model of legendary A500, isn't it?

Have you tried another power supply? In one case I had with A500 was fractured solder on power socket pins underside of motherboard, see picture --->

http://aijaa.com/L4uEmE

I suspect you're not getting required voltage to the drive at least..  You should check it.

[BLTCON0]

@flaviosr
Swap CIA chips
Remove and re-insert Denise, Gary, Paula, Agnus and the ROM chip.
Check for problematic RAM chips (hotter than the rest), especially the lower-left (U16) or the upper-right (U31) one.

Very nice motherboard, this is more a rev4 than a rev5 :-)

[scuzzb494]

Just out of interest ... what is the SERIAL .NO.

[ognix]

Hello!

Normally one CIA is responsible for some floppy functions, but not only: Paula, beside audio stuff, manages data transfer (if I'm not wrong - I should check schematics... it passed some time).
Anyway from the first picture it seems you have other problems there: the startup "Insert WB Disk" is broken, so I guess the problem is located elsewhere (Agnus, Denise?).

From the second picture I saw a metal clip over the Agnus chip, probably for forcing it onto socket: I never seen such thing in any Amiga 500 before.

So, since your system worked fine since some days ago and, apparently, it was quite dirty, I may suggest to check all IC contacts on the motherboard: if this system has been used/placed in a humid/dirty environment it's highly probable that the contacts between ICs and sockets are oxydated and/or dirty.

Visually check it.
You can try by removing the ICs and place them back into sockets (few times) and check Amiga behaviour (this scratches away a bit of oxyde).
If it changes, you probably have found the problem.
Of course ICs need to be extracted with some care, to prevent pin bending.

Good luck and have patience!
    Luca "OgniX"   \8^)

[scuzzb494]

Ok

Just before you kick up an almighty dust storm and never return back from it can I suggest you avoid doing too much. Very often it is a simple thing that is at fault. I have checked over your motherboard and it looks exactly the same as mine.. except mine has a Treops RAM Board with an external switch.

The metal clamp over the Agnus is normal so don't worry about that. For me I never remove motherboards from their casing unless they are busted. Especially on older kit. Sadly in replacing the floppy you unsettled something. That is assuming you didn't remove anything else. Don't remove anything else.

First up check the power supply and I mean the connector on the back of the computer.

Then check any other ports you have plugged in. And have you other cables and connectors you can try.

Check the machine with the keyboard lifted up and the shielding also lifted up. Be careful and don't touch anything.

Check the power connectors to the floppy drive and the ribbon connector to the floppy drive.

Gently press all the chips to make sure they are seated properly.

By removing stuff you are at risk of kicking up a dust storm and unless you are elctronically skilled and can check circuits you will never find your problem. Nine times out of ten its just an electrical short or poor connection.

I just had a yellow screen on a 500 and decided to go in and have a look. Seriously, all I did was lift the shielding and depress the main CPU and ROM chips and she booted no problem. Still running now. The trick is to avoid doing too much. You may think you are doing right by cleaning but in truth you are likely to generate even more issues. The trick with old kit is to get it running and keep checking but don't keep unplugging, moving about, opening up, disconnecting stuff. That may have been OK when the kit was new but in an aged state unless you are skilled at mending electronics or can afford to ship to someone who can, then leave well alone.

So small steps is what I would suggest.

I asked for the SERIAL NO. Would be useful.

[BLTCON0]

I asked for the SERIAL NO. Would be useful.

It's 74780 or so. Not particularly useful other than re-establishing this is one of the first rev5 boards made (more similar to the prototype rev4 than to subsequently produced rev5 boards).
There exist rev3 boards with higher S/Ns (in the 80000 range).

Points of interest:
1. Uses coloured RCA jacks
2. Has the Toshiba-made "buggy" Gary
3. Doesn't have the transistor-fix later rev5's had

[Oldsmobile_Mike]

From the second picture I saw a metal clip over the Agnus chip, probably for forcing it onto socket: I never seen such thing in any Amiga 500 before.

AFAIK this is a C= fix due to all of the Agnuses (Agni?) popping out of their sockets when going over a speedbump leaving the factory.  It's mentioned in many places around the web.

As others have said, clean and reseat all socketed chips, connectors, inspect caps, try a different PSU, etc.  My bet is on a failing ram chip.  Good luck!

[Speelgoedmannetje]

Try the steps from easy to difficult:
1. Reseat the chips, maybe spray them with some contact cleaner (I use Contact cleaner 390 from Kontakt Chemie for years now and it served me very well)
Make sure you get rid of any static electricity by touching the radiator with your hands or washing your hands with streaming water.

2. Check whether the PSU outputs the correct voltages with a multimeter. You can find the corresponding voltages of the pins by googling something like "amiga psu pinout"
If these are too high, don't despair, your Amiga will likely still work perfectly fine, but you most certainly found the culprit
Also if the PSU has the right voltage, it might still be the main problem, if it's plugged in into the Amiga and turned on it might not keep up the voltage needed. Anyway, a spare PSU is never a bad idea. You can get one at vesalia.de: http://www.vesalia.de/e_a1200psu[8809].htm

3. Check for leaking capacitors. Not all are easily tracable, but most often you can see it immediately. If you can't find any, try to trace leaks with an uv-lamp.
Replacing a capacitor is easy. They have only two pins and you have to replace it with a capacitor of the same capacity in (micro)Farads and type (electrolytic or not). Check whether the capacitor has a 'minus' mark printed on it. It means it is an electrolytic capacitor, and if you replace such a capacitor (with also an electrolytic capacitor) you need to remember where the capacitor pin on the side of the minus mark is located on the board (ergo write it down). Soldering such a capacitor the wrong way around and it won't work.

Aside from replacing a drive belt in the floppy drive, this should enable you to repair your Amiga (and perhaps also many other devices)

[guest11527]

From the pictures, it looks like an issue with the data bus, i.e one of this bits of *all* ram chips are flipping randomly. For that, neither CIA, nor PAULA, nor DENISE is responsible. It is probably a damage in one of the data bit lines between CPU/RAM/custom chips, probably one of the bus drivers between the two.

[BLTCON0]

From the pictures, it looks like an issue with the data bus, i.e one of this bits of *all* ram chips are flipping randomly.

Not quite - this is a rev5 board which uses 256Kx1 RAM chips (i.e. each chip has 262144 words, each word being 1 bit, and since we have a 16 bit data bus system 16 such chips are required to complete a RAM bank).
So every single word in RAM is spread across all 16 chips (each RAM chip contributes 1 bit). Since the flaky pixels are 16 pixels apart with perfect periodicity, if it's a RAM problem it's on a single chip.

For that, neither CIA, nor PAULA, nor DENISE is responsible. It is probably a damage in one of the data bit lines between CPU/RAM/custom chips, probably one of the bus drivers between the two.

Display DMA (and DMA in general, i.e. Agnus access to chip RAM) is not behind the bus drivers, only the CPU's access to chip RAM is behind drivers.
This means it's less likely to be a buffers problem but anyway, if it is, it must be a write buffers problem (74LS244) as the routine would be:
CPU reads from ROM (unbuffered) and writes (through 74LS244) to chip RAM the kickstart image, which is then DMA-ed (unbuffered) to Denise for display.

The "easy" test here is to plug in a trapdoor 512 KB RAM expansion and route RAS0 there, thus cutting-off the onboard RAM bank and using the trapdoor as chip RAM. If the visual problems disappear, the fault is with the onboard RAM.
Swapping the low-high write buffers would also reveal if there's a problem there (the vertical lines would shift by 8 pixels).

I'll agree the CIAs/Paula are much less at a chance for fault and the disk problems are more likely caused by the highly likely RAM problems (but again, if it was a drivers problem, this time it would have to be the read buffers 74LS373 as the CPU would have to read malformed data from chip RAM and thus fail to execute the bootblock. Both the LS244 and the LS373 at fault? Much simpler to blame a single RAM chip).

[Oldsmobile_Mike]

Not quite - this is a rev5 board which uses 256Kx1 RAM chips (i.e. each chip has 262144 words, each word being 1 bit, and since we have a 16 bit data bus system 16 such chips are required to complete a RAM bank).
So every single word in RAM is spread across all 16 chips (each RAM chip contributes 1 bit). Since the flaky pixels are 16 pixels apart with perfect periodicity, if it's a RAM problem it's on a single chip.

Nothing else to add, I just want to say for someone with only 14 posts on this forum... please post more info like this, more often!  Cheers!

[BLTCON0]

Nothing else to add, I just want to say for someone with only 14 posts on this forum... please post more info like this, more often!  Cheers!

I appreciate the... appreciation Cheers :pint:

[flaviosr]

Dear All,

first of all sorry for my delay in aswering!

Thank you for all your support! I am not a technician but I will try to do most of your suggestions.
Obiously I am going to look for another power supply (friends help!) and, if I remember well, the connection was "hard" so it could be really a power issue. Perhaps the connection to be resoldered (and in this case I need help... from friends!)
If this does not help I will try to check the various chips.

The S/N of the mobo is 074760 so not so new!
I have also to fix a single keyboard key, as you can see from the photo, but this is a "mechanical" issue.

Thank you again