Amiga A500 + Motherboard - Part 5 - The End

Oh dear.

A black screen.

Having been stopped dead in my tracks by the black screen of despair, I stopped and had a long think about what to do next. I could throw the board in the bin after salvaging the custom chips etc. Or I could run around the room waving a ladle and screaming 'the transmission of the Austin Allegro is mounted transversely to reduce gyroscopic torque'.

Fortunately I decided to go back to basics.

Check the traces. Again.

First up, the 68000. As the screen is black and black normally means a dud processor, I did two things. I put in a spare good CPU but still got the black screen. Then I checked all of the traces from the 68000 out onto the board. No problems found.

Next, I went back to the logic section. I tested all of the pins on U10. All traces good. Then U11 and 12. All traces good. Then, finally U13. All traces good BUT I noticed something that looked odd. A trace runs up to R111 (which was fine) and then from R111 to pin 25 of Gary. I measured this too expecting a resistance of 68 ohms. I got nothing. No continuity from the end of the resistor to pin 25.

This track is only about 5mm long so this was surprising and I was sure at some point I had checked this previously. Time to get a picture (or two)..

Is that a break?

After examining this I thought it was a straightforward break in the track. The green line through the trace looks like it is a break but this was a red herring and was actually a piece of the solder mask still in place.

Let's look a bit closer at the pad...

Well, there's your problem...

Ah. That looks like a bit of an issue. The track has disappeared from the point at which it should join the pad. I suspect what has happened is that removing the temporary resistors that I previously installed defeated the few specks of copper that were left of this trace. A proper clean and new picture shows it a little more clearly. The dull circle around the edge of the solder is basically bare substrate.

Track broken at pad - oh dear

To repair it I put a blob of solder on the leg of the resistor and dragged it across to the track. Relatively simple if not the neatest.

Track now present

New solder track close up

So, does this fix it?

NO. But we're now back to a green screen. This is the equivalent of getting the first 'chug' out of an ancient engine that's on the verge of starting for the first time in years..

After having another think about the basics again and, just for a laugh, I decided to re-seat all the chips including Agnus. Would you believe, as I removed Agnus, a chunk of corrosion fell out of the socket which was presumably accidentally inserted as I put the chip back into the new socket. A salutary lesson in keeping everything clean and tidy if ever there was one.

Back to the board. Does it power up and will it get past the green screen?

So there you have it. Another A500+ motherboard saved from the indignity of spare parts (or worse).

This board will eventually get a floppy drive and keyboard (and maybe an new expansion connector too) so I can test that it ALL actually works but, for now, I am overjoyed that I managed to get to the boot screen,

To everyone out there with an A500+ motherboard in this condition, keep at it. If I can do this, then so can you!

Here's a couple of pics of the success:

The recovering patient

The glorious purple screen

A list of parts used:

Resistors:
  2 off 28 ohm 1/4 watt
  1 off 47 ohm 1/4 watt
  3 off 68 ohm 1/4 watt
  2 off 1K ohm 1/4 watt

Capacitors
   1 off 10nf ceramic disk (not quite as original but OK)

Chips
  2 off 74LS244
  2 off 74LS373

Sockets
  4 off 20 pin DIP sockets
  1 off 84 pin PLCC socket
  3 off 36 pin sockets sacrificed for their pins for Gary and Paula (their loss will be remembered)

Misc
  Lots of solder
  About four inches of green wire wrap which I had lying around

Time
  Lots but the total is irrelevant if I can keep bringing them back from the brink...


Thanks for reading. 

Amiga A500+ Motherboard - Part 4

Standby for action.

First, that broken track. I wasn't happy with the 'temporary' repair as it looked more like I'd thrown the wire at the board in the vain hope it would stick. One touch of the soldering iron and it was gone quicker than my wages the day after I get paid...

The permanent repair is, to be fair, pretty much like the old one but with a bit more care. I don't have any fancy replacement PCB copper traces as these are monumentally expensive and this is, after all, a bit of a hobby and not my profession. More's the pity as I'd love to spend all day fixing these things. Anyway, the track is fixed, on to the next thing.

Can you see the join? (Just to the left of the socket)

Resistors. Nice and cheap from Tandy in the UK. It's a simple job of desoldering the old ones (including the temporary replacements I installed that may or may not have been the totally correct values) and then inserting the new ones. One advantage with new resistors is that they have nice long legs which makes it easy to get them into the board and then, by folding the legs out (or in if you prefer) underneath the board, they remain in place. This means you can get them all into the board in one go and solder them all in one go. The excess legs just need to be trimmed when it's done.

Folding the legs over 

Resistors Replaced - and now correct values!

Next.

Sockets. Each of these sockets cost 15p. Some purists may say the 500+ never came with sockets so why put them in now when a) they could cause connection issues that wouldn't happen if the logic chips were soldered to the board and b) it's a bit late. Well, these logic chips are still relatively cheap, but I'd much rather lose a total of 60p on sockets rather than the few quid I spent on the chips themselves if the board is beyond repair. Did I also mention I'm a bit of a cheapskate? Monetarily efficient is what I prefer to think. :)

New sockets

From the photo above you'll also notice something is missing. The expansion connector was too far gone to be useful. It wasn't the worst I'd seen but for the sake of a few quid for a new one it wasn't worth the pain of trying to clean up the old one. New connectors are available still (which is more than can be said for the female A1200 expansion connector) but I won't order one until I know this board is fixed.

Sockets flush with the board

A big hint for anyone starting out with soldering this type of component, be it socket or multi pinned chip. Hold them in place and solder opposite corners before you wade in and do the whole lot. If you don't your socket will end up at a funny angle and won't be flush with the board. I used a piece of masking tape on these which worked well. On my personal A500+, which I repaired a few months ago, I failed to do this and as a result there is a 74LS244 that appears to be making a break for freedom as one end is significantly higher than the other. Ooops.

Finally, I re-instated a link wire that I'd put on from one of the vias near the expansion connector to the jumper near the battery, to fix a potential additional track issue.

Link wire added

So with the sockets in place and the resistors now the correct value it's time to get the chips in this board and plug it in.

Can we get past the green screen?

I recorded the moment for posterity. (Do I really sound like that? Crumbs.)

Amiga A500+ Motherboard - Part 3

A short post this time.

If this were a film, this would be the point at which everything goes wrong for the hero but then turns around at the last minute. Racing to save the heroine, delayed by the evil bad guy, he loses her seemingly forever only to be reunited in a climactic final act which results in the bad guy dying and the good guy gets the girl forever.

Sadly, this part of my adventure involves a multimeter and a lot of traces. A lot of traces. Continuity checking is dull. I checked all of the traces from Agnus. I checked all of the traces through the logic section.

So many traces.... 

I Quit! (Not really)

I still have to check all the traces from Gary but I shall save that until I feel like shoving a multimeter probe in my ear...

On the plus side, the postman has been! Yay!

A500+ Essential Supplies

I would have ordered from one of the larger suppliers but due to their lack of stock or extortionate postage charges for small orders I went elsewhere. I ended up ordering from Tandy electronics which is the reborn UK version of Radio Shack. I have no idea whether they have any connection to Radio Shack now but their stuff was pretty cheap, they had most of the bits I needed and they only charged £1 for delivery.

Anyway, I will be fitting sockets and resistors etc when I  have a few spare minutes. Other than repairing a couple of broken traces, I really don't know what else there is to do before I start poking it with a hot stick again...

Amiga A500+ Motherboard - Part 2

So, I managed to replace both Paula and Gary's sockets by re-cycling the plastic frames from the original sockets and replacing the pins with new ones pinched from new sockets that had the wrong number of pins. This was particularly tedious and shall not be repeated. I will order new sockets if there's another time...

On to more pressing matters. Does this board now work?

No. Black screen. Still. Bugger.

Right. After poking around a bit I removed U12 by carefully cutting it off the board. Then I decided to install a temporary socket made up of off-cuts of my Paula/Gary escapades. This would enable me to install a replacement chip to see if it was just a simple logic chip replacement required. While I was there I also noticed a bit of a problem...

Is this a broken track I see before me?

Yes.

A track had broken, most likely due to my cleaning efforts which, from the state of the board, looks to have dislodged a piece of the substrate which was probably damaged by the battery leak. This has left a small but significant 'pit' which obviously made it easy for me to catch the metal track as I continued cleaning. This is no big deal and I installed a temporary repair just while I carried on troubleshooting. This was a tiny sliver of wire, soldered across the two ends of the broken track to form a bridge. Unfortunately I forgot to take a photo but, as I am bound to say, it looked like new*.

*did not look like new.

Anyway. At this point I thought I should check all the tracks from Gary through to the logic section seeing as Gary took the brunt of the corrosion. All was OK except for one track. For some reason I could not get continuity from pin 2 of Gary to the 'north' side of R102. I thought that this might be due to corrosion on the solder joint of R102 so diverted off for a couple of hours to de-solder all of the resistors and capacitors in that area.

De-soldering this components was easy. Except when they disintegrated. Like R102. And R111. Oh, and R114. And, in fact, most of the resistors in the two little banks of components located below Gary on the motherboard.

I think the phrase rhymes with 'clucking bell'.

To cut a long story short, I scraped together most of the resistor values I needed and cobbled together a couple of bodges (resistors in series) to make the values I didn't have. New resistors are on their way but I haven't got them yet...

Temporary Resistor Repairs

Then, once I'd finished faffing with resistors I turned my attention back to the non-continuity of the track from pin 2 of Gary. Even with the new soldering in place, there was still no joy so I resorted to a short jumper wire to get around the problem. It's fairly discrete and I suspect I will leave this one on the board as it is not visible when the board is installed and does the job quite nicely.

Jumper on Gary

With all this done it was time to try and plug him in again.

Does he work? No. Green screen. BUT that is an improvement and shows we're getting there. Green screen is shown when there is a problem with the chip memory. It can be caused by a multitude of faults but the most common are a bad memory chip, bad logic chips or a broken Agnus.

I have ordered new logic chips (74LS244 and 74LS373) as they are the cheapest and simplest to replace and they should be here early next week; also included should be the resistors and some sockets for the logic chips. If they don't make any difference and I end up having to scrap the board at least I get to take back the new chips.

In the immortal words of The Carpenters, "Please, please, Mr Postman (Why's it takin' such a long time?)"...