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One dead amiga 500 rev5 board


Pinball freak

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Hello guys,

 

The title says it all, and i hope someone can help me to get my Amiga 500 back up and running after about 30 years :)

It brought me so much fun in my younger years, but the younger me also helped it to die i think, but i can't remember it anymore.

What i saw when i opened the Amiga from collecting it at the attic of my mothers house were the loose cables at the side of the floppy drive.

 

The bad soldering i did in my younger years came loose and i can almost surely say that this caused a short and probably blew something, after that i probably gave up and put it aside 30 years ago.

 

Now, i have a rgb to scart cable that came with it and connected that to my old lcd tv, i do not know how well this cable works.... 

But, if i flip the switch, the caps lock light lits up and goes out, and i get a white or greyish screen ( can't say for sure ), but that is it.

 

If i press caps lock on and off it works about ten times, then it does not lit up anymore.

There is no heartbeat from the floppy, so maybe that one is dead also.

The cpu get's warm, not hot, fat agnus get's warm, but i notice chip U38 get's pretty hot.

 

I measured with the multimeter at the pins of the floppy on the amiga board, and i do get proper 12.3 and 5 volts.

 

That is it for now, i did not do much testing because i do not wan't to do any damage further along the way, before asking people here what they can make out of this.

 

If anyone can point me in any direction, that would be great.

I will try everythig to get it back up running again.

 

Oh, my soldering skill has been improved greatly over the years, you gotta start somewhere ;)

 

Thanks !!!!

 

Edited by Pinball freak
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Okay, the caps lock thing is an indication that the CPU isn't running. Either it has detected a serious fault and halted, something is holding the reset line low, or there's something more significant going on with the board. Without the CPU running the Kickstart code, the floppy drive won't do its ticking thing either, so don't worry about that for now - it could be fine.

 

I'm not sure what diagnostic gear you have there, but the first thing to look at is the /Reset line to see if it's being held low. It should be held low for maybe 1 second after power on, then go high. If that's the case, the next thing to look at is the /Halt line. That's similar, it should go high right after startup and stay there, but if it's low it means some fundamental issue occurred, like the CPU can't read the ROM properly or something.

 

U38 is one of the serial driver chips IIRC. If that's damaged, it could be causing issues for one of the CIAs too, which in turn prevent the CPU from running. I don't think they get particularly hot, especially when idle, so that's a good place to start. You can remove it completely to eliminate it; the machine doesn't need it present to work.

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Thanks for the reply !

 

As i am just starting with this i do not have much more than a multimeter at hand.

But i can order what i would need to get it up and running.

 

I will google on the reset line, i have no idea how to measure this. But i will look up as much as possible for this.

 

Removing the U38 is no problem, so that is one things i will try first ! It is not that hot that i can't touch it, but it get's way more hot than all other chips. It is beyond just warm.

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Cool, well a multimeter can tell you what level these signals are sitting at, it just can't show you any rapid changes that occur. It's possible as well that you have a short from what you're saying, and anything like that on the address or data buses will likely prevent the machine from booting. Check the CIAs (and all the chips) for correct fitting and alignment too, in case any previous work has resulted in one of them being positioned incorrectly.

 

For the reset and halt lines, these can be checked at the CPU, pins 18 and 17 respectively. Both should be high (close to 5V) during normal operation. If either one is low (close to 0V), it will prevent the machine from starting.

 

You can find some schematics for the Rev. 6 A500 board here: https://www.amigawiki.org/doku.php?id=en:service:schematics

 

Note that there are significant differences between the Rev. 5 and Rev. 6, particularly when it comes to ROM and RAM, but the key signals at the CPU and between the custom chips will generally be the same so it can be used to check pin numbers for example.

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Hey Daedalus,

 

I will probably have time on sunday to work on the board, I will firstly remove the U38.

Chips seem to be in the richt place, all have been investigated if they are seated well, but i will have a look to be sure !

 

Thanks for the schematics, will check that and measure the pins !

 

Will keep you updated on this.

 

Appreciate your time !

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I measured today on the board, the pins 17 and 18 give 5.24 V each.

Als the U38 gets what seem to me normal voltages of 5 volt on some pins, i am doubting it get's to hot....

 

Just probing some chips on the board give me mostly 5.xx voltages, so current flows all over the board.... not that that says much..

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On 3/16/2022 at 2:58 AM, Pinball freak said:

 

But, if i flip the switch, the caps lock light lits up and goes out, and i get a white or greyish screen ( can't say for sure ), but that is it.

 Some points.

 

1.If it's blinking at 1 burst per second, then that indicates a kb rom checksum failure.

 

2.Otherwise consider the kb self-test passes with a single flash in a normal working system.

 

3. Note that all Amigas will boot without a keyboard plugged in or if the kb controller is faulty.

 

4. As mentioned check CIAs, swap them over and see if your symptoms change to something else.

 

I would also swap the kickstart rom for quick troubleshooting if you have a spare lying around.

 

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Hmmm, to be honest, 5.24V is too high. As a supply voltage, it's borderline within spec but realistically it should be a bit lower, and most signals will be a bit lower than the supply voltage. Can you check the exact 5V level at the floppy power connector?

 

U38 has a +12V and -12V supply rather than a 5V supply.

 

We've already established that the keyboard is working, and it's the CPU that's not servicing the keyboard's requests. With both /Reset and /Halt signals high and a grey or white screen, I'm next pointed towards an interrupts failure. Without interrupts, the machine will boot to a grey or white screen and then hang just before showing the insert floppy screen. Swapping the CIA chips around as suggested above is worth doing to see if the problem changes, or it could also be a problem with Paula or the connections to Paula. Check all the power pins to Paula, and the Paula socket for any tarnished or damaged contacts.

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Hey Shoestring,

Thanks for the reply, it is not blinking 1 steady blink.

Do i need a donor board to have the proper cia's , because they are hard to find... Or can the chips bought on the internet ?

Is it possible to work with a diagrom instead ? I have seen on YT that you can read it out with a pc connected with putty.

 

@Daedalus2097   

I've done a quick measure on the pins of the U38 at the nudge the +12 gives me steady 12.74

The minus on the other side gives me a fluctuating reading between -11,73 up to -11,8   all the minus pins fluctuate..

+edited+  and the floppy reads 5,27 volts steady

So maybe this chip is problematic ?

If so, where can i find it in europe ?

 

As mentioned, i do not have a spare board nor any chips....

 

Paula seems fine to me, but i will have a look if i can find a diagram with corresponding voltages....

 

Thanks !

Edited by Pinball freak
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The schematics I linked to before will show you which pins are for power to Paula.

 

Another option to look at is getting a DiagROM, which is a replacement ROM that you fit in the Kickstart socket for diagnostics. It's excellent because it can work even when most of the computer is dead. It includes interrupt checking functions.

 

The serial driver chips can be replaced by functionally equivalent chips if needed, but if it's faulty and causing the issue, simply removing the chip will allow the machine to work (without serial of course). You don't need to source a chip until you know it's faulty.

 

The specifications for the 5V rail include a 5% tolerance, which means 4.75V to 5.25V. Now, it's always a good idea to not completely trust the readings from a multimeter unless it's been calibrated, but chances are the voltage on the 5V rail is a little high, which may or may not be causing problems.

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3 hours ago, Pinball freak said:

Do i need a donor board to have the proper cia's , because they are hard to find... Or can the chips bought on the internet ?

You have two 8520 CIAs in your current Amiga ( odd / even ). If only one is faulty you might see something different after swapping the two around in the machine. If you're lucky, the machine might even boot.

 

CIA responsible for I/O like kb, parallel, serial ..etc  and also generating timings and interrupts.

 

image.thumb.png.fbbf8f4ad7704e9c6e2645710c9c52d5.png 

 

More here: https://www.amigalove.com/viewtopic.php?t=324

 

I'd say your ks rom should be OK since you get a whitish/greyish screen, some code has run to a point but then the machine can't go any further due to some other h/w issue. A completely dead ks prior to v3.1 will give you a black screen.

 

 

 

 

 

Edited by shoestring
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Without some proper tools ( logic probe or scope, eprom device programmer, ram/logic tester ) or at least a second working system, you're going to have a hard time and will probably need to shotgun it, and that's going to be very expensive looking at the price of custom chips these days.

 

Do you recall where the bad solder job was located ?, this might indicate where the short took place. Some photos of the board at location might help someone see something

 

With a short, it could be as simple as a blown resistor,  logic package or even a blown trace.  I've seen resistors blow in A500s just from normal use.

 

The schematics should help

https://amiga.net.au/files/Tech_Amiga/Commodore_A500_System_Schematics.pdf

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Hello Shoestring, 

Yeah, maybe i will not be able to fix it, i probabaly will buy a second hand working one..

But i will try with what i got.

 

I will post some pictures later on, i have done some visual inspection now, and i see now that the connector of the keyboard pins on the right side are somewhat molten or damaged.

After probing it with the mm, the second pin from the right ( blue ) jumps like crazy with 0.07 to 4.8 with all kind of voltages inbetween...

 

The short will be caused on the floppydrive power connector, VCC the 5 volt has been shorted bij the next wire, what will be ground...

 

Pictures will follow later.

 

Thanks !

Edited by Pinball freak
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If you're worried about a short at the connector, then you can dial the dmm in continuity mode  and look for a dead short between pins 1,2,3 and 4. ( with the machine switched off of course ).

 

Pin 4 is vcc so it should be 5v. Clock and data are held high ( 5v ) before any keyboard transmission begins.

The _KBCLOCK signal ( pin 1 )  is driven by the keyboard itself, _kbdata ( pin 2 ) is driven by both keyboard and the Amiga ( bi-directional ).  I'd assume that _KBCLOCK should float ( not measure anything ) with the keyboard unplugged since it's uni-directional

 

The keyboard will only pulse _kbreset low when the user presses the ctl-amiga2 combo. So when gary sees low signal, it will assert _reset and _halt to initiate the reset process. _kbreset ( pin 3 ) should be held high by default if the user is not pressing the 3 key combo. 

 

I don't know if a white/grey screen is with enough confidence to assume that the ks rom is passing software checks, what version of kickstart do you have ?

White or black is screen is common value in color0 register when the Amiga ( denise ) powers up and that seems to depend on the revision too. Lots of other faulty parts of the chipset can cause the machine to boot to white, grey or black screen.

 

Ideally I would start my troubleshooting at the reset circuit, at the 555.

 

After probing it with the mm, the second pin from the right ( blue ) jumps like crazy with 0.07 to 4.8 with all kind of voltages in between...

 

It should probably be flashing. Have you checked the keyboard LED and resistor R6 on the keyboard pcb ? Might be blown or loose.

 

 

 

 

 

Edited by shoestring
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22 hours ago, Pinball freak said:

And to add to this, on the first four, count from the left so first is black wire i measure a steady 5.24 volt on each.

As i look at the diagram, is that normal ?

Yes, that's normal, though as I said before, that voltage is a little high for comfort.

 

3 hours ago, shoestring said:

The _KBCLOCK signal ( pin 1 )  is driven by the keyboard itself, _kbdata ( pin 2 ) is driven by both keyboard and the Amiga ( bi-directional ).  I'd assume that _KBCLOCK should float ( not measure anything ) with the keyboard unplugged since it's uni-directional

The clock and data signals are both held high on the motherboard.

 

4 hours ago, shoestring said:

I don't know if a white/grey screen is with enough confidence to assume that the ks rom is passing software checks, what version of kickstart do you have ?

White or black is screen is common value in color0 register when the Amiga ( denise ) powers up and that seems to depend on the revision too. Lots of other faulty parts of the chipset can cause the machine to boot to white, grey or black screen.

It's possible I guess, but not something I've ever seen. I've seen lots of pre-KS3 machines stall at a grey/white screen because of faulty interrupts though, and the keyboard behaviour (other than the power LED) is consistent with an interrupt failure.

 

4 hours ago, shoestring said:

Ideally I would start my troubleshooting at the reset circuit, at the 555.

Yep, but /Reset isn't being asserted so it's unlikely to be the reset circuit.

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14 hours ago, Daedalus2097 said:

Yep, but /Reset isn't being asserted so it's unlikely to be the reset circuit.

 

Needs to be confirmed.

 

Initial power on reset is also triggered by the 555 during power up. GARY will then pulse _RST and _HLT briefly to various custom chips and U37 ( 74LS32 ) which buffers the _RESET to both CIA chips and some of the IO such as the parallel port and the external floppy. I'd check GARY , the 74LS32 @ U37 and 4.7k resistor at RP101

 

 

 

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OK Guys,


First and foremost i want to apologize to you for taking time of you before checking this out first.

As i mentioned in the first post, i did not know how well this cable works.....

 

Well, i bought a nice working Amiga 500 rev.5 yesterday, the guy showed it to me working.

He told me he fixed it because he bought it more than 20 years ago..... And it was not working.

I asked what was wrong, one of the legs of a chip was broken, i do not know wich one. But again the amiga booted fine, all lights working Caps lock has a green led, where mine has a red .... Did not know they used several colors for that.

 

Now, today all happy i was eager to get some nostalgia hours to play me old games.... hooked it up, fired the thing..... same white screen. Nooooooooooooooooooooooooooooo arghhhhh.

 

Can't tell you how embarrassed i feel right now, so truly sorry for this.
But yet again, the mainboard is still dead, that hasn't changed.

 

At the end i will show you pics with my board, the one completely dissassembled, and the amiga open i bought yesterday.
however both rev 5 , there seems to be many differences. But again, i haven't touched an amiga for over 25 years or so.

 

So bare with me, i am again really noobish, have to learn a lot.

 

The question, i have an older 32'' LCD TV pal with, amiga's also pal,  if i buy online any Amiga rated rgb to scart cable > will this always work ?

Or does the tv have to be a match like monitors have to be 15hz ?

 

I found this online : http://www.pavouk.org/hw/en_rgbscart.html

I opened my cable at the rgb side and found out that the cable that i have found with my amiga stuff has only 5 wires going to the scart AND it seems that the audio cables are coming from the scart end and not the rgb end.

So i have no clue where i did get this cable from or if i ever used it, because as from what i can remember is that i have hooked my Amiga 500 in a amiga monitor... wich i do not have anymore.... ugh.

 

So here are the pictures, now i am first going the buy me a decent cable, then we will see what colors the bad board will give.

 

 

 

IMG_20220403_122112.jpg

IMG_20220403_122126.jpg

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If it has a SKART input then it should accept the Amigas video signal. Only 5 wires are needed on the 23 pin connector ( video ground, red,green,blue and csync ). 

 

The rca audio cables from the SKART end of the cavle just taps the output from the back of your Amiga into your TV which has internal speakers.

 

Does the new machine have any drive activity despite the white screen?

 

No drive click then. Are you still using the old power supply ?

 

 

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