Boarding School

I had advice (again) from Nophead (again) to forego the desoldering braid for removal of the dead chip on the first stepper motor driver board, and to cut the legs off as high, i.e. as close to the chip body, as possible. This I did - with the end legs, anyway, but I couldn't find anything fine enough to fit between and cut the other legs. After considering what was available that would definitely do the job with limited manoeuvre room, I decided to buy a chip-leg-cutter. You never know, I may need it in future.
This did the job well. When I lifted the broken chip off the board, a portion of the green board coating came away with it, exposing the copper underneath. The patch didn't extend to any of the pads. Carefully, I coated this with a thin protective layer of Araldite.
This was ancient - I got it in 1991, when I worked for the company! The lids were pretty stuck on, so that trying to unscrew them from the tubes resulted in highly twisted tubes, but putting some grip onto the lids worked. It is good to have some glue you know is still going to work years after first being opened.

Nophead's advice to brush the cut chip legs away with the soldering iron tip worked a treat. Each leg came away easily, and I wiped it from the iron. I used the flat tip to flatten the remaining solder on each pad, and put a thin line of paste along the row of pads on each side before soldering the new chip in place. The right way round!

I checked for bridges, and there were only bridges across legs that are connected with the trace anyway, so I didn't remove those bridges. I checked that there was continuity where there should be, and a lack of continuity where there should be, comparing with the working board. I checked the resistors, and they were all fine, too.

So I plugged in. Oops, no power LED and a very slight smoky smell. I switched straight off.
I took the power LED off, and checked it. Using my new flux pen to wet the component side of the desoldering braid, I was able to remove the solder quite easily. The LED had been fitted the right way around, and it still worked, so I soldered it back on. Hmm.

Since then, I have spent 2 days checking, rechecking and generally scratching my head. I checked all the vias going from top to bottom and did notice a couple without shiny metal rings like the working board has. At the stage I noticed, I didn't know what "vias" in electronics were!
I inserted the probe from my multimeter into the tops of the holes, and turned it around a few times. A thin green film came off, revealing shiny metal once more. I checked there was continuity between the top and bottom, and made sure there was for all such vias.

I checked the resistance for all the resistors, and they were all as rated. The only ones I couldn't check were the 0.25 ohm ones because my meter only went down as low as 200 ohm on its lowest setting.

At this stage, still scratching my head, I posted for more advice on the forum, specifically if the voltage regulator could be checked, and how.

I talked to my brother, who works in electronics, as he happened to 'phone, and he advised that I should check anywhere that a trace has been placed between the pads for a surface-mount component, because there may be a short here. I knew there were a couple of these on the boards, so I checked these, too. No problems found. He also suggested checking every pin, on the headers having 2 rows, against its neighbours. So I did that. No different from the working board. He also recommended connecting the power supply via a 12V bulb rated for 5 or 6 W to the board. If the board were shorted, the bulb would light more brightly compared with doing the same thing on the working board. I put this off because I couldn't locate a suitable bulb or pair of bulbs.

The advice from Nophead was simply to connect 12V power to the regulator and check that 5V came out.
Referring to this website, the power supplied to a 7805 regulator need only be 7V or more; a-ha, say a 9V battery, then, since I didn't want to connect the whole circuit again.
I roped in assistance, and connected wires to a PP3 battery with the ol' trusty Blu-Tack and held the +ve end against the reglator pin next to the "rrrf" text on the board, with the -ve end against the flat plate of the regulator, whilst my assistant measured the voltage off the pin next to the "make" text against the ground pin on the Molex-style connector. Oh. No voltage output. That would be the problem, then. No wonder the power LED didn't light, as it wasn't getting any power at all, it was all just going straight to ground!
That means the rest of the circuit hasn't been tested under voltage since the chip blew!

So for my next order, a new 7805DT voltage regulator..... And I said I might need the chip-leg cutter again, looks like it was sooner than I expected/hoped!

Nophead also explained that I could check the 0.25 ohm resistors by reading the resistance on the meter with the probes touched together, and subtracting that reading from the one I get on the low-value resistor. I'm going to try that next.

In the last 3 days, I have learnt a huge amount about checking boards.
I shall update the project costs soon.
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On the surface

It has taken me about an hour to get together all the surface-mount parts for the first stepper motor driver board, and I haven't even put them (nor even the solder paste) onto the board yet. I'm very glad I used my own identifiers when I submitted my order to Farnell, because one rating of ceramic capacitor has no useful information other than "capacitor" on the Farnell label, so I was glad of being able to cross-reference. Also, having a spreadsheet of suppliers for each component per board was useful, so that I knew I was looking for a RS part for this and a RapidOnline part for that, for example.

Here is a picture of different packaging, of resistors: same idea but this time mostly soft card with transparent film windows in it.
Here are all the surface-mount parts laid out around the board. You can see one capacitor still in its packaging towards the bottom of the picture - that is to distinguish it from the others.


I had forgotten that I hadn't been able to find everything as surface mount, and now I look at the 0.25 ohm resistor, and go, "Oh my, how on earth am I going to mount this on the surface? It's (relatively) huge".
Now I do remember BodgeIt giving a link to his non-surface-mounted component done as surface-mounted, so I'm hunting that down. I shall have to stand this resistor vertically because it won't fit in any other way. So I'll do that when I do the manual soldering for the through-hole components.

Seeing more clearly

I had bought a set of magnifying glasses to help with the assembly/checking of the circuit boards. It suddenly occurred to me yesterday evening that I could perhaps try taking the photographs with the magnifying glass, since my camera really doesn't like very close-up work, despite having a close-up setting.

Tried it out this morning. Holding both items steady with the magnifying glass against the zoomed out camera lens, and trying to avoid casting one huge shadow over the subject was tricky, and I had to use Blu-Tak to hold the subject, but I am quite pleased with the results. I shall use this technique in future.
So here is the stepper motor driver board, again,


and then the strip of capacitors showing the opened end if you look really carefully (it is the invisiblest film - do you like the new word?), and then the front of the capacitors' packaging.


You can see the scale of this against the camera strap on the right of the photo'!

First stepper

I started assembling the components for the first of my stepper motor driver boards today. The one shown on the instructions on the reprap site is red, but mine are all green. The printing on mine is much less clear, too, so I have the website page saved as a file on my laptop, and I am scrutinising the pictures and text on screen rather closely.
Also, none of my boards have been printed or perhaps cut square, so the fixing holes in the corners don't line up.
I shall have to mark each fixing hole individually to mount the boards on Faith.


This is the first board on which I shall be soldering surface mount components. I began with the 0.22 uF ceramic capacitor (as it says in the text), also known as the 220 nF capacitor as it is marked on the board. Apparently. I can't make it out on my own board!

For those of you new to surface mount, this little capacitor (which I am assured is large - ha!) comes in a little strip of packaging. I tried to take a photograph, but the packaging is clear, and the components tiny, and the camera couldn't find it to focus!
You have to peel back the clear plastic film from the back of the packaging to get at the capacitor. And then it jumps right out...
it is a fawn-coloured thing, tiny as I said before (!) and I am wearing grey trousers. It jumped towards me. I had to search for it; I could barely spot it on my lap!

The instructions say to colour it with a red or black pen, to distinguish it from the other similar but differently rated capacitors, but as I had only a green marker pen, mine is green. Colouring it wasn't easy, either!

Here it is. You can just about see it in the photo' (the camera clearly couldn't, or couldn't clearly!)

Oh this is going to be fun, isn't it?!

On a lighter note, I've got my soldering tutorial up on the Builders' Wiki, with a lot of practical technical help from Renoir, and my purple RJ45 cable arrived.
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