Sunday, May 29, 2016

A1200 Capacitor Problem (but not the ones you think)

So, a couple of weeks ago I went and got myself a Micronik A1200 tower thanks to ebay (and my superb wife). This tower had an A1200 motherboard in it - it was sold as a working unit after all - but I actually transferred my own A1200 into it.

The reason for this is that my own A1200 was re-capped by me. The soldering standards aren't high but at least the caps are only a year or two old. I don't know the history of this new A1200 board other than it's been in the tower for some time.

Anyway, during the changeover, I noticed something strange. On the opaque plastic sheet under the motherboard (the new one) there was a small, oval hole that shouldn't have been there...

A cigarette burn? A micrometeorite?

Closer inspection revealed that it had been melted by something. A hasty look at the back of the motherboard showed me this:

Aw, crap.

Something bad has happened here. After a look at the A1200 schematics, this capacitor is a de-coupling capacitor for U18 (a RAM chip). Most likely, it has failed, gone short circuit and then pulled a load of current directly from the +5v rail to ground. Release the magic smoke!

One quick order to Element14 for a strip of new caps and we're off on the next repair. :)

Here's a close up of the problem. From this it should be obvious that one end of the cap is likely to have damaged the solder pad on the board. It needs to come off.

It doesn't look any better close up.
As this next picture shows, the pad WAS destroyed. In fact, as soon as I touched the cap with the soldering iron, it basically fell off the board. The picture shows the scene after a cleanup.

Wherefore art thou solder pad?

Here's the offender in all his 1206 glory (1206 means 12 thousandths of an inch long by 06 thousandths of an inch wide in case you're wondering how flippin small these things are).

Ah. There's my pad. Tell you what, you keep it...

I don't have the tools, time or patience to carry out a proper repair. I'd need copper sheet, suitably rated epoxy for under-bonding, appropriate conformal coating etc etc which is all stuff I don't have. So here's my solution:

Nailed it.

The replacement cap was soldered to the good pad and a small jumper wire (about 4mm long) was soldered between the end of the good cap at C18B and the end of the replacement cap at C18A. So far, everything seems OK. I have the A1200 playing the demo for Elite II over and over to give it a bit of a workout and it all seems OK.

Tomorrow I shall take it all apart again and check to make sure it has not repeated.

Thursday, May 19, 2016

A500+ Update - A Bit Disappointing....

Depending on your point of view, this update on my broken A500+ is a bit disappointing.

I dismantled my own A500+ so I could pinch the floppy drive and keyboard for some more rigorous testing. I noticed straight away that my own A500+ has a very different motherboard to the broken one. Well, I say different. They are actually different colours. Have a look:

My original A500+ is very green

This one is very brown(ish) - note the missing trapdoor connector
There's no difference in the components, just the colour of the actual PCB. Strange. 

Anyway, after connecting up the floppy and keyboard I turned the broken board on and.... worked. Perfectly. Everything I threw at it it just worked. This is outrageous. I demand a more damaged motherboard that takes me longer than half an hour to fix. 

Pictures to prove that the Amiga is a hardy beast:

Running a test program

Running OctaMED on a super untidy desk...
So that's it. It just works. Anyone need a fully working A500+ motherboard?

Wednesday, May 18, 2016

Death by Rechargeable Battery

Any Amiga afficiandos amongst you will know that, back in the day, Commodore had some funny ideas when it came to upgrading the Amiga. First, the A500 came out with Kickstart 1.2 which was upgraded some time later to Kickstart 1.3. This was the de-facto Amiga A500 for a good number of years.

Then, in 1991, like all good buses, two upgrades (well one upgrade and a cancellation really) came in quick succession. First, the A500 had a boost to the A500+. It came with a whole 1Mb of RAM and the shiny new Kickstart 2.04 which was similar to the big brother A3000s ROM. Very soon after, for some bizarre upper management reason (i.e. complete incompetence) the A500 was scrapped and replaced with the A600.

I won't debate the merits of the A600 (or A300 as it SHOULD have been - I seem to recall the manager responsible for the A600 was fired) but instead focus on the A500+. This machine holds a special place for me since it was my first Amiga. I had an early A500+ and the white box it had said 'A500' (no 'plus') in the Cartoon Classics pack so I had no idea it was a A500+ until I actually took it out of the box.

One additional feature of the A500+ was the built in real time clock. This may seem quite quaint these days but back then, this was the height of 'ooooh!'. To power the clock a small battery was factory fitted to the motherboard. For the market life of the machine there were no problems and we all enjoyed having accurate dates and times on our files.

Fast forward 25 years.

Retro computing is gaining in popularity. Amiga computers are popular with collectors and retro gamers as they're pretty reliable and, well, cool. But there's a problem. Remember that little barrel battery on the motherboards of the A500+? Well, what do you think batteries do if they're left for years with no use, no voltage, no current and no charging? They leak. What do batteries contain? I will give you a clue. They do not contain rainbows and unicorns. They contain liquid death.

The story normally goes something like this. You see some article on how retro gaming is popular. Prices for systems are rising. Hmmm. You used to have an Amiga. Maybe it's in the loft/garage/shed? You dig it out. Yes, an Amiga A500+. You plug it in. You turn it on to see if it still works. Green screen. Or black screen. Or slowly flashing power LED. At this point a lot of people might throw it in the bin (NO DON'T!) or pop it on the table at a car boot sale for a couple of quid. The braver souls might open the case and peer inside. They might see something like this:

What's that green stuff?

More green stuff..

That green stuff is corrosion caused by the battery spewing its contents all over the motherboard. The first thing is to remove the battery then cover the affected parts of the board in lemon juice (really). It neutralises the corrosion and fizzes nicely while doing it. Then, wash the area with isopropyl alcohol and leave to dry. Sometimes the board has been lucky and the damage is limited. Sometimes it's too late. :(

Just to be clear, the batteries in these things have a life of up to about 6 or 7 years if you're lucky so there is no blame here. These things are now 25 years old. It's just a fact of life if you have an Amiga A500+ (or 4000 or 2000 actually).

The point of all this is that I took a gamble on a cheap A500+ motherboard from ebay. The seller said that it didn't work as the battery had leaked. Looking at the photos the damage didn't look too bad - not that you should trust the photos. So, a few days ago I received a package.

After cautiously unwrapping it, I found this:

Shielding included (didn't realise it was!)

The Patient
First things first. Let's check what the damage really looks like. At first glance, it's actually not too bad. Someone has attempted to clean up the board and the battery has been removed. The damage is still obvious though. 

Corrosion everywhere :(
Next, lets plug this baby in. The screen turns white then almost instantly shows solid green. That's not good. It means that there's a problem with the chip RAM but could mean that it's the RAM that's faulty or any of the components between the RAM, CPU, ROM and custom chips... This will take a bit of work. 

OK, so now we inspect the board a lot more closely. I cleaned off as much of the green corrosion as I could, partly by scraping with a scalpel and partly by using isopropyl alcohol. Not as good as lemon juice but it did the job for what I needed. Next, a lot closer look at the tracks that are in the firing line:

There's one definite break (possibly more)

Close up of 4th pad down on top picture - definitely unhappy

A clean break - no copper left here at all

By using the continuity function on a multimeter I confirmed that there were two traces that definitely had problems. These were from the 'south' end of R101 to the via and from the 'south' end of R112 to the via. As it turns out, there was a third issue from the 'south' side of E102, as it runs past the leg of the EVIL KILLER FROM HELL - er I mean battery. 

For the resistor R101, I could not find where the break was. It was most likely that the trace had multiple tiny breaks, making it impossible to just use a sliver of wire as a track repair. In the end, I installed a jumper from R101 to the via. 

For the resistor R112, things were a bit more straightforward. I used a stripped piece of 30awg wrapping wire, about 6mm long and bent into the right shape to bridge the gap from the track to the via.

Finally, I used a straight piece of the same wire, again about 6mm long, to repair the straight trace that had been damaged next to the battery leg.

The results look like this:

Wire added from broken track to via - jumper visible back to R101

Connection to R101 - not the neatest but it's connected now

Track repair next to the battery leg (battery removed)

Before I could test it I realised that the trapdoor connector had suffered badly too at the hands of the evil battery. The pins had corroded badly and there was nothing I could do to save them. I had to remove the entire connector. :(  Fortunately, they are available new. Yay! :)

Connector still attached but looking poorly...

Connector partly removed - not looking any better

Pins cleaned up to illustrate the damage batteries can cause
So, with all this done, time to plug it in and see if there is any improvement. Why, yes, Yes there is. :) 

Another A500+ saved from the bin

There is still a lot to do on this but the worst of the problems are out of the way. Stuff that needs doing:
  • Test the whole thing
    • Floppy drive
    • Sound
    • Keyboard
    • All ports
  • Replace the trapdoor connector
  • Buy a case, keyboard and floppy...(remember I only bought a faulty motherboard!)
I will update you all soon once I've carried out more tests, but so far, it's looking good.