Clinging on like a Limpet (Board) - Torch Triple X Part 14

The Torch Triple X was working. Then it wasn't. Then it was. Then it wasn't again. 

I've been having some issues with the Torch. This has been a persistent problem for a little while (Future Paul here - looking back it's been an intermittent issue for two years or so!!) and I'm not really sure why. Every so often there is no boot and I get left staring at a pale blue screen. 

Sometimes it gets a bit further and I end up at the dark blue screen with 'CARETAKER1.3' at the top but then it stops with an error trap.

Trap.

Another, older trap.

Sometimes it will get to the Torch logo and I get an error trap. 

Well that's not right...

Sometimes it will start to load OpenTop and then freeze.

Don't panic! Too late..

You get the picture. Reliability is (R)Not Good.

I put this down to the Limpet board. This has been a thorn in the side ever since I managed to break off one of the pins on one of the stupid connectors it uses. I repaired it but that was, I think, the start of all the issues. I have tried multiple things since then to improve the connectors. I've replace the sockets for both of them on the motherboard, I've installed a totally different connector type, I've used multiple turned pin sockets stacked to act as a more stable riser etc etc.

Original Limpet board.

Stupid riser connectors..

After looking at the issue I decided to do something drastic. The original Limpet board is in a bit of a sorry state having had RAM sockets installed, then removed, then re-installed (all to try and solve this bloody fault) so I thought, why not just make a new Limpet board? It's double sided, relatively small and relatively (!) simple. Surely this is the sort of thing one of the PCB manufacturers could make?

Many, many trace repairs...

So that's what I did.

First, I had to create a schematic for the Limpet in Kicad. I have used it before to recreate the schematics of the Torch itself but when I did those I was a bit ham-fisted with the symbols and I wasn't too bothered about getting everything nice and resolved for PCB manufacture. 

I had to do it properly this time.

Fortunately, the majority of what I needed to do was fairly standard and I was already au-fait with a lot of the functions in Ki-cad. All of the chips were standard and in the regular library of symbols, except for the RAM. I had to take a previously defined chip and change it into one of the 'ancient' 505256 DRAM chips. This was a bit finnicky but once I'd done it, it worked - which is a good job as the RAM accounts for most chips on this thing!

Actually creating the schematic took a lot of hours. The first bit was fairly straightforward. Plonk all the components  on to the Ki-cad sheet and put then roughly were they are on the board. Then I took the original board and removed all of the components on it so I could see all the traces.  I was left with the bare PCB. I did notice that once all the components were removed the board has a massive bend in it. This could be why it was, or appeared to be, so unreliable.. With the bare PCB I could visually follow the tracks across the board and use my multimeter ('..in continuity." - StezStix) to verify I'd followed it correctly. 

Then it was a simple case of joining up the pins from the various components as per the tracks on the old board. This resulted in a slightly untidy schematic. After many hours of tweaking and fiddling it finally started to look reasonable. 

Pah. Simple.

I should note that this board also had four bodge wires on it that came from the factory which I also had to take into account when creating the schematic. This was fine for adding to the schematic, but it did raise one question. Should I try and 100% recreate the board as it was, or should I just do it to get a working board? Hmmm. 

Anyway, with the schematic now done it was time to start work on the PCB. This was also relatively straightforward. When you switch to PCB mode from the schematic, Ki-cad automatically dumps all of the components onto the board with all the pins linked as per the schematic but only conceptually. The fun part of this was starting to actually route traces between the components.

To start this I drew out the boundary of the board so that I could then approximately place the components where they would need to go. Then, before I even started drawing in the traces, I measured as closely as I could the positions of the connectors. These are key because there is a limited footprint for the board to fit, and all the connectors had to be in the right place or they wouldn't all er... connect.

All present and (more or less) correct.

So, finally, I managed to start routing the traces. And this took a LOT of hours. The DRAM bit was fairly easy as a lot of the pins connected through to the equivalent pins on chips further up or down the board. There were some very tricky bits too, partly due to space constraints but there was also one instance around the Limpet's LED, where my brain just wouldn't brain properly and it took me far longer than I care to recall to get the tracks correct.. 

And then, once I had done one side full of tracks, I got to flip the view and do the same again for the other side. By this point I had also decided I just wanted a working board, so I incorporated the four bodge wires into the design.

Finally, after nearly two weeks worth of spare time, I had a finished schematic and PCB layout. I printed it out at 100% scale so I could compare it to the original Limpet board. And realised one of the connectors was in the wrong place. Gah!

Top and bottom layers switched on.
Routing is fun...

And, of course, moving the connector altered a lot of the previously carefully routed traces.. Double Gah!

Take 2. This time, the connectors were all aligned with the original board. Phew!

Next step. Pick a PCB manufacturer. I went for JLCPCB as they actually seemed to be significantly cheaper (cheapskate) and had fewer issues with customs charges etc.

Their website was pretty good and considering I was a complete beginner at this I found it pretty straightforward to use. I had exported my design as 'gerber' files as exported from Ki-cad and I uploaded these to the website. A price was given - minimum order quantity of 5 boards - and then the files started processing to check for any 'gotchas'. 

Back Side Gerber

Font Side Gerber

After a few minutes the site came back with some potential pitfalls e.g. pads very close to tracks that could merge during manufacture. None of these looked significant (foreshadowing) so I accepted them and continued to the next step. Confirming the order and paying. I went for the regular shipping since this was only £8 or so. Word on the internet was that this would also avoid potentially overzealous charges from other 'carriers'...

Then it was just a case of sitting back and waiting. I ordered them on 18th May and they arrived on 29th May. So only 11 days, all the way from Singapore. Nice.

New boards left and middle, original right.

The quality of these new boards is astonishing for the price. I paid $8.90 for the five boards and $12.77 in shipping and customs charges making a total of $21.67 which ended up being £16. That's just over £3 per board. I could not even begin to buy the stuff I might need to make a board the old fashioned way for that price. Amazing.

But the big question is, would it work? Before I even started putting components on I did a quick check between ground and Vcc. And guess what. There was a short. A full on, 100% straight short between gnd and Vcc. What. The. Heck.

Well, you remember that warning that JLCPCBs website had highlighted about tracks being very close to pads? Yep. That's exactly what it was. For capacitor C45, the ground track was supposed to go to the first leg of the cap and then bend around underneath the cap and carry on its merry way. Unfortunately, it was too close to the other pad of the cap and so was attached to both sides of the same cap. Worse, the other side of the board at that point had Vcc, hence the short.

A quick 'update' with a craft knife removed the link. (I ended up soldering the required cap directly to the bottom of the board rather than risk trying to use the now sliced pad.)

Minor engineering fix...

Now it was time to install the components. I had bought some new capacitors and some new sockets to fit to the shiny new board. I did re-use a few original components i.e. the two tantalum caps but only because I didn't have any, and the LED as it looked a better quality than the ones I had.. 

Line 'em up.

Socket to 'em.

Lookin' good.

Several more hours involving a soldering iron later, I had the answer as to whether it would work.

No.

Darnit.

Break out the oscilloscope.

I started by focussing on all the links between the RAM. First up the RAS and CAS lines. These all showed a nice and strong repeating pulse, exactly as would be expected. All of the data lines also checked out, or at least they were all at the same value but not really showing any activity. Then I went to the write enable pins.

Oh. 

Bugger.

They were different across the columns of RAM chips but they should all be linked together. What has happened here? I'll tell you what happened. I forgot to add a PCB track to each of the rows of RAM that connected the /WE signals together and that also connected them to the rest of the board. 

DOH.

Fortunately, a simple bodge wire fixed this and gave me the chance to use my recently purchased solder mask and UV light kit to glue it down. You'll also notice from the picture that there are a few capacitors soldered to the bottom. There were only supposed to be two which, as per the original board, were not included. On the original these had been soldered across the legs of the chips on the top (very neatly) but since I added sockets I put them on the bottom (not quite so neatly). It makes no difference really but makes it easier to swap chips quickly.

Ooop, Another engineering fix...

Does it work now?

No. I just get the pale blue screen. Darnit (again).

But after a bit of fettling I managed to get to the dark blue caretaker screen. And then it kept going! All the way to the 'insert key disk' screen - which was because I'd forgotten to plug the SCSI cable back in. 

I did this but when I re-started it, it got stuck at the pale blue screen again. Hmm. Some more fiddling and try again. And this time it got to the Torch logo and crashed. By this point I had realised that there was something wedged under the board, and given the weird nature of the faults both now and previously, I began to wonder..is there some sort of mechanical fault causing the problems?

I wedged a red handled tool under the right hand side of the board and placed a weight on it (my old HP laptop). Then I turned it on. And it booted past the pale blue screen. And it booted past the dark blue screen. And it booted past the torch logo and the box telling me I'd fitted a new battery. In fact, it booted all the way up to the point where it complains about not being shutdown properly (the more things change the more they stay the same..). I carefully maneuvered the mouse and clicked 'X' to clear the window. 

And it kept booting all the way to OpenTop. Yay!

Most importantly, this confirmed two things. First, and most important, the Limpet board recreation works perfectly. Second, and far more problematic, this confirmed that the issues I have been having are almost certainly on the motherboard. *Activate Conflicting Feelings*

A bend in the road...
(I can't look at this any more!)

So, now what?

Well, the issue is clearly mechanical. This must mean that a connection or connections i.e. a trace or plated through hole(s) are broken but are re-connecting when the board has that worrying bend applied.

But the question remains, now what?

At this point, if this was an Amiga A500 or C64 I'd be looking for a different motherboard i.e. just replace it. The broken board would then become a parts board for stripping components for future repairs. But this is just not feasible with the Torch. I know only one other person with a Torch Triple X (excluding any museums out there). They're not exactly what you'd call common.

The options I have are:

1. Do nothing. It was a good run. I managed to get it to work but just call it a day.
2. Find where the breaks are and try and fix them.

Of course, it's option 2 all the way. :)

More next time!

Coming up....