Amiga 1200 Black Screen on Boot Up

Hi all!
I was given an Amiga 1200 for Christmas in 1992.
I got a good three years of use out of it, and then it was put away.
About six months after using it originally, it stopped working, so it went back to the shop and they repaired it. I don't know what they did at the time, but it worked until I put it away.

I brought it out of retirement years ago, and switched it on and found it showed a black screen. This was around 2001, so I just put it back in the box and didn't look at it again until around 2013, and it booted up correctly, but it was missing a floppy drive and a mouse, so I couldn't use it.

Recently I brought it out of 'retirement' again, (kinda like Ozzy Osbourne), and found it wouldn't boot. Just a black screen.

I checked the motherboard and found the original repairs that the shop I bought it from.
The hard drive bracket had wore away two circuit board tracks (somehow), and a wire had been soldered from a pin on the Budgie chip, to a track terminal underneath one of the ROM chips. On the underside, another short wire was bridging a break in one of the tracks, right next to the hole where the hard drive bracket sits.
The RF modulator/shield also looked like it was unsoldered, but I'm getting a correct signal - not sure if composite and RF share a signal from the same processor???

I have checked these, and they are both not breaking any flow of voltage through them.

I replaced every capacitor today, and it still has the same problem.

Iv'e checked through composite and RF (Channel 36 on my LG telly), if you switch on the PSU, the TV clicks and shows a black screen, so it's producing a signal, but obviously nothing else.

I have a working floppy drive (modified Samsung SFD321P) - the light on the drive is permanently on (but no light on the three lights on the Amiga). If you put a floppy in, it spins forever until you switch the PSU off. The Caps Lock is permanently on, and the Power light is the only thing that is lit on the Amiga.

I've reseated the ROM chips (inserted in the correct way), and it changes nothing.

I've been soldering for years, and have recapped loads of TVs/monitors easily.

I can't think of anything else to check - assuming my 1200 is complete toast? Would love to get this thing working.

Some images:



The yellow wire is from the Budgie chip - I have an electronics repair microscope I use for soldering, and can't see a problem here.

How it sits (after 1/2 an hour):



It looks blue, but it's black, with a very faint pattern.

Replacing caps left to right:



There is no damage from any leaking caps, and they looked fine tbh, but was the next step in trying to get it working:

[dlfrsilver]

the caps are leaking from under. Sometimes it's only visible once disoldered.

And the PSU also need to be recapped.

The board is clean "AF"! I can't see any contamination from the capacitors?

Also, will check the PSU.

[nogginthenog]

Originally Posted by station

I have a working floppy drive (modified Samsung SFD321P) - the light on the drive is permanently on (but no light on the three lights on the Amiga).

Are you 100% sure the floppy connector is not backwards? It's not keyed so easy to get wrong.

The Amiga will still show the boot screen with no floppy attached. It will take a couple of minutes to timeout. Try this.

p.s. Your picture shows a light on one of the three lights :-)

No, I've attempted numerous times, with things unplugged, there's nothing left to check on that side. It never shows a boot screen.
I meant there is no floppy light on the three Amiga lights.

I'm totally sure the floppy is correctly connected (the ribbon cable has been in the same position since I've had it - it has been bent, and can only connect that way), but that's not part of the problem. I've tried it the other way, and the floppy doesn't do anything.

Checked the PSU

The caps are:

47mF 35V x 2
220mF 35V
82mF 35V
1000mF 10V
1mF 50v
47mF 35V

and a massive

33mF 400V

They are not visually expired, but on the underside (the soldered part), there is a hardened yellowish resin (like hardened tree sap). I'm sure this is adhesive (or flux) used to hold them in place prior to soldering.

I'll get some new ones and see if this does anything.

[ajk]

If the voltages (especially +5V) are stable, it's unlikely that changing the caps will make much difference.

You have tried many things already, doesn't seem like it's any obvious fault... I'd look at the reset line with a multimeter (or scope/logic analyzer if you have access to one), and perhaps burn a set of diagnostic ROM chips (e.g. http://www.diagrom.com/).

The innards:



That dried fluxy stuff - only under some of the caps.

Originally Posted by ajk

You have tried many things already, doesn't seem like it's any obvious fault... I'd look at the reset line with a multimeter (or scope/logic analyzer if you have access to one), and perhaps burn a set of diagnostic ROM chips (e.g. http://www.diagrom.com/).

I'm not sure if it's getting a constant 5V - I can't see the PSU causing this tbh, I think the Amiga itself is shagged and had it.
Have no idea what a reset line is, and can't see me putting too much time into this beyond soldering!

[ajk]

There is circuitry on the board which keeps the CPU and other chips halted for a short duration after power-up. If this is not released as it should, it'll keep the system frozen indefinitely. But it's not something that can really be visually inspected Quickly looking at the schematics (https://www.amigawiki.org/dnl/schematics/A1200_R1.pdf), it should be possible to measure at R951B, which you should find between U4 (Lisa) and U5 (Gayle).

Amiga motherboards rarely just die, unless there has been a major power surge or something that has fried several chips at once. The 5 volt line being much too high could do this, which is why measuring the PSU is important.

If the 5 volts input is good, the board can almost certainly be fixed. But it will probably require a closer look with some measurement instruments.

Ok, I will give that a go, there is a tutorial for checking the reset online.
I’m just going to replace all the caps in the PSU first.

[nikoskob]

Hi Station,

Did you manage to get your A1200 booting? I am having similar issues to yours and it wont come out of reset.

Also, do you have a link to the tutorial for checking the reset line for the A1200? I've only found one for the A600, but doesn't really apply to the A1200 due to the different components involved.

[solarmon]

Originally Posted by nikoskob

Hi Station,

Did you manage to get your A1200 booting? I am having similar issues to yours and it wont come out of reset.

Also, do you have a link to the tutorial for checking the reset line for the A1200? I've only found one for the A600, but doesn't really apply to the A1200 due to the different components involved.

I drew this diagram up of the reset signals for the A1200 rev 2B.

https://drive.google.com/open?id=1oL...cp0oIzDh2106QZ

And one for the A600 too:

https://drive.google.com/file/d/1lsK...ew?usp=sharing

[nikoskob]

Originally Posted by solarmon

I drew this diagram up of the reset signals for the A1200 rev 2B.

https://drive.google.com/open?id=1oL...cp0oIzDh2106QZ

And one for the A600 too:

https://drive.google.com/file/d/1lsK...ew?usp=sharing

Thanks mate

I'll give it a go and see where I fall over.

Cheers