Amiga 500 troubles

[mechy]

Ok guys, i've tried everything i can think of,so its time to ask for help

I am working on a rev 6A A500 board. It came to me long ago with a constant reset problem,it just stays in a reset loop,with the led blinking pretty constant. No keyboard caps lock light(it does flash once as it passes normal kbd check)but no error codes.screen wise i will get a flash of green every so many seconds on the reset.
it stays in this reset mode with or without keyboard attached.
I've socketed this whole board with the exception of the 1488/1489 rs232 chips,and used all the custom chips off my working 500 in the board, and replaced all the ttl stuff with new parts(yes, ALL),and tried several sets of 256x4 ram chips. all thats left are some transistors,capacitors,video hybrid,and misc small parts.power supply works on my good machine.i checked all sockets/traces with a ohm meter after changing sockets for continuity,since the symtoms are the same before i started,this is not the trouble)

i changed one transistor near the 555 timer,Q711,which changed nothing. i will probabaly change out the last remaining transistors tonight if i have them.

i doubt the video hybrid is at fault? since i get a screen flash.

Its pissed me off now and its personal and i am determined to fix it if i have to etch a new pcb and build it from scratch(haha not really).

anyone out there have any ideas? i'm sure it will be the last part i change!

mike

[save2600]

You know for certain your 68000 CPU and Kickstart chips are 100% okay?

[mechy]

yep, tried every version of kickstart from 1.3 to 3.1 roms. and 2 different working 68K chips.

[scuzzb494]

Strange that.. I would have gone straight to the floppy drive. Like check the ribbon, check the power, make sure there isn't a pin pushed in or the ribbon was the wrong way round. Sounds like you have something backwards thats all. Have you also taken the motherboard out of the computer and ran it safely not in the casing... Maybe even disable the floppy drive and attach an external only... Maybe just shorting somewhere... Did you disable the floppy drive and try it.... and I mean take the floppy drive out the machine. Also check wires to keyboard that they are not cut... sometimes happens from the shielding. Dunno.

[mechy]

skuzz,
   Power is good,no floppy is connected, its a bare basic board to eliminate things like the floppy,all chips are the right way round(as i said it came to me with these symtoms before i touched it).i've swapped possible ttl chips around with each other(which are new parts),no changes.i've been running it outside the case.. pressing,pushing,flexing it gently does not change the symptoms either.again,its a bare board, not even the keyboard is hooked to it(other than to test and see it was in a constant reset initially).The problem is definately on the board itself and not being influenced by outside parts like floppy,kbd,etc

my guess is somethings keep it in reset,but i have no idea what.

if someone here knows how the reset circuit works,i.e the chain of events,it would be helpfull.

Mike

Strange that.. I would have gone straight to the floppy drive. Like check the ribbon, check the power, make sure there isn't a pin pushed in or the ribbon was the wrong way round. Sounds like you have something backwards thats all. Have you also taken the motherboard out of the computer and ran it safely not in the casing... Maybe even disable the floppy drive and attach an external only... Maybe just shorting somewhere... Did you disable the floppy drive and try it.... and I mean take the floppy drive out the machine. Also check wires to keyboard that they are not cut... sometimes happens from the shielding. Dunno.

[scuzzb494]

May sound extreme but have you tried the 6" drop test on the board ... Also you can get a cleaner that disolves dust... Probably a waste of time cus you have a busted component somewhere which may not be a chip sadly. I have so many boards here I would just swap it out.

scuzz

[tokyoracer]

mechy, if you need a bare board (rev.5 or 6) PM me. Il give you one for free, just the cost of P+P.

[mechy]

I will keep that in mind its a very generous offer,thanks. This has become a quest to concour the flashing led on this board and make it run,i have it all socketed already,so i would really like to get it going. I even have a 64pin zif socket for the 68000 that i want to install,and probabaly will when/if i get this board going

mechy, if you need a bare board (rev.5 or 6) PM me. Il give you one for free, just the cost of P+P.

[guest7146]

i'm sure it will be the last part i change!

mike

Well, of course it'll be the last part you change Mike! Unless you're in the habit of continuing to change parts even after you've found the fault??? :roflmao::lol::roflmao:

Haha.  Seriously though, I hope you get this working.  I've got the same symptoms with my old A500+, which is really annoying because she has been working fine until very recently.  I thought my issue might have been related to the PCB mounted battery, since it had started weeping a little bit.  Changed it out with no joy.  I also tried my ROM chips with no luck

I'll keep an eye on this thread in case you have success.  If I have success with mine in the meantime, I'll come back and share my progress.

AppleHammer

[mechy]

Well socketed/changed all chips,still the reset problem, who knows what it could be.
and No,i dont change out parts  after its fixed

Well, of course it'll be the last part you change Mike! Unless you're in the habit of continuing to change parts even after you've found the fault??? :roflmao::lol::roflmao:

Haha.  Seriously though, I hope you get this working.  I've got the same symptoms with my old A500+, which is really annoying because she has been working fine until very recently.  I thought my issue might have been related to the PCB mounted battery, since it had started weeping a little bit.  Changed it out with no joy.  I also tried my ROM chips with no luck

I'll keep an eye on this thread in case you have success.  If I have success with mine in the meantime, I'll come back and share my progress.

AppleHammer