From plus to plusplus (part 2)

We left you at the end of the opening installment having explored a bit of the history of the A500+, and learned a bit about why Rob Taylor went to all the trouble of making the A500++ to make up for a particular shortcoming involving the battery. Now it’s time to look at the build itself and give you a bit of an insight into what skills are needed; perhaps even help you decide whether this project is one you could look at building yourself. Check out part 1 for the link to buy the boards or join the Discord.

What Rob will send you when you order a 500++ board is exactly what it sounds like; an empty circuit board. There’s no kit of components, and that decision helps to keep the cost down and give you the flexibility of sourcing what you need from the most cost effective place. There is a Discord server with a bunch of knowledgeable folk (and me) who can point you towards a BOM – the Bill Of Materials – that includes Mouser part numbers (Mouser are a supplier of electronic components with worldwide delivery) but equally you may find some parts cheaper elsewhere such as RS or Farnell, or for some items such as IC sockets and connectors sometimes even eBay sellers are competitive. One choice you can make is to salvage more, or less, from the donor board with items such as sockets for the custom chips, the 25-pin and 9-pin connections from the rear of the machine (the 23-pin sockets are unique to the Amiga and need to be salvaged anyway, as does the power socket), the logic chips and other minor semiconductors like the audio amp. I must admit to a quite excessive level of stinginess when I built mine, even taking the EMI protection caps (the little three legged “space rockets” that sit in rows in front of the back ports) and carefully checking the values of them with a component tester. This choice will have an impact on the final costs – at the time of building, the full BOM was around £150 where my abridged version with many more reused parts was closer to £50.

The decision to salvage more or less from the donor will also depend on the state of the machine and how much damage was done to it. Sometimes the green death can creep into sockets or up the legs of chips, and whilst they could be painstakingly cleaned there’s a consideration of cost vs effort. I must admit here to having multiple donors, as I was also helping out other builders with little parts here and there so I was able to choose the best items to make the final result from. One thing you really need to be in good condition is the custom chips themselves, Gary in particular is in the firing line from the battery leak and sometimes a bit of a clean with IPA is in order to clean the legs up so they make a good contact. However I’ve seen boards where the leak has gone much further, but at a cost of £15-20 per chip on eBay you really do need to spend the time and take care to keep them healthy.

You’ll also need some tools, and again personal preference comes into play. One of my first purchases was a vacuum desoldering pump, since I’d already decided I was going to harvest so many items and I have a hate-hate relationship with manual, spring-loaded desoldering pumps. This Duratool ZD619 set me back around £90, but on this one project alone made up for that by providing a relatively easy supply of sockets and logic chips. Again if you’d prefer not to harvest all these, you may not need to buy the pump at all. What you won’t get away without however is a good, temperature controlled soldering iron, of 60W or more due to some fairly large ground planes that will absorb the heat of a lesser iron. Good solder and flux is also essential, and with years of experience I can confidently say not to scrimp here! Get a named brand like Kester solder and Topnik flux, they really do make life easier. Again, the Discord is the place for hints and tips on what to buy. Finally get some IPA for cleaning the board down – both before you start, to help with good solder adhesion, and after you’ve finished to remove flux residue.

If you’re choosing to do a lot of salvage, you may want to invest in a component tester. This set me back about £15, but is a simple device that connects to a range of components and gives you an idea of whether they’re still in spec or not. For example, I rescued all the EMI caps which are just two capacitors connected to a common centre pin, and by using this I could make sure I wasn’t installing any faulty items. The full set of EMI caps alone was about £12 at the time of building so this item almost paid for itself, but I’m quite happy to accept other people have different views on salvaged items. There were also a few instances of either me forgetting to order a component, or it not being available and it going onto backorder, so for the odd item here and there I raided the old board and checked the item’s specs before installing it.

One item I wish I’d bought before starting is a PCB holder. As the A500++ is all through-hole there’s lots of flipping the board over – you poke items through from one side, and then hold them in place whilst you turn the board over to bend the legs and then solder into place. The PCB holder will make this much easier.

So with the components obtained, tools assembled and obviously the board in your quivering hands it’s time to make a start. There’s not really enough room here to give you a blow-by-blow assembly guide, and soldering technique is also out of scope for this blog. Needless to say, if you want some practise first then there are plenty of solder boards available on Amazon to help you get the right balance of heat and flux for your solder, and one of the best ways to learn is with YouTube videos showing what a good result looks like. But don’t be too worried – it is all through-hole so you can take it slowly and check your placements before soldering into place, and with extensive use of sockets for the ICs you don’t have to worry about damaging the more expensive chips with heat.

Your guide on where to place things is provided on the board itself, with each component outline having the value written in (as well as an indication of pin 1 where appropriate) but you may also like to use PCB Explorer at amigapcb.org for a clickable map as the A500++ is a direct copy of the Revision 8 A500+ board. I highly recommend giving the team there a few units of your local currency for their hard work if you do, we’re very lucky in the Amiga world to have such a resource.

I decided to place all the rear sockets in first, which gives a bit of support to the board and ensures components aren’t being pushed out of place when it’s flipped over for soldering. Then went in the sockets for the custom chips, before the resistors and logic IC and RAM sockets. Sometimes there appeared to be hours of work for very little gain, individual resistors for example take a lot more effort to place each one than something like the Agnus socket which is 68 pins that can be soldered quite quickly. I would highly recommend leaving the capacitors until last, purely as they’re the highest components and they can get damaged with all the turning over of the board. You can choose to lay them down – most have space alongside for this – but I prefer the original look of upright capacitors. Regardless, it will take shape, and in my case spending 2-3 hours a couple of times a week there were days when I really seemed to make a lot of progress.

Let’s assume the built went well, and fast-forward to the final phase; testing. It would be quite easy to get giddy and want to plug in the power immediately, but there’s some checks you can do first

• Do you have any dead shorts? A good test here is to put a multimeter across 5V and Ground, then 12V and Ground, and finally -12V and Ground. The first two are easily found on the floppy header, the third is on the serial port and expansion slot. You should be looking for some resistance – it may seem quite low, but in the realms of 60 ohms or more is fine. If it’s zero, stop now and check your work as damage may happen if you power it up
• Check all the chips for pin 1 placement. Pay particular attention to the custom chips, as these will cost you the most to replace! Pin 1 is indicated by a notch on one end, and a similar mark in the silk screen on the board. Match these up. The exception is Agnus, as it’s a PLCC package it has a dot halfway down one edge.
• Some other components also have a correct and incorrect way to be installed. Resistor packs have a dot on one end, and a square on the board. Diodes have a thick band one one end. Electrolytic capacitors have a stripe on the negative side and a + marked on the board, so these should be opposite sides.
• Are all the chips in the right place? Denise is behind the video connector, with the CIAs making up the rest of the top row. Paula is closer to the front left, Gary to the front right. Agnus is the only square one, bonus points for getting that in the wrong place.
• Brush the board down to get rid of any solder balls or other conductive debris like snipped legs of components.
• Now start at the top of this list and check it all again.

Here’s where it all went wrong for me, some of that list is written from experience. One of my RS232 chips that handles serial communications was in the wrong way around, and upon applying power it popped quite dramatically and blew a hole in the top. Replacing that the correct way resulted in no display and a dim power light (the power LED of an Amiga has two stages, and it going bright is normally a sign that the CPU, kickstart and memory have all responded at the very least). At this point I took some high resolution photos of the top and bottom of the board, and eagle-eyed Chrissy from the Discord almost immediately spotted both a diode and a resistor pack incorrectly installed.

With those corrected….. result!

As this is a yellow pre-production board there was one last mod to make; purely for my own comfort I corrected a fault on the floppy drive switcher pins, but did it in a way that meant both jumpers were aligned where the “simple” fix was to have them in opposite directions. This switcher forces an external drive to be DF0, and is one of the few changed that Rob made to the original design. Production boards need no modification in this area.

So there we go, a full working A500++ was born. All the wonderful features of the original, but none of the heartache caused by that battery doing its thing. I’ll leave you with a picture of the standard test that all Amigas have to pass – a game of Sensi.

• Boards available from Bobs Bits on Tindie
• Join the Retro Tinkering Discord for more information