Second term at boarding school!

Just when you think you've got everything settled..........

.....My brother pointed out that just because I wasn't measuring 5V out of the voltage regulator whilst it was connected in circuit on the stepper motor driver board didn't mean it wasn't working; with a short-circuit anywhere in the 5V section of the circuit, the current would flow straight to ground, meaning no voltage to measure. However, there were no obvious shorts, no solder bridging two components and I had previously checked that all the resistors apart from the 0.25 ohm ones were sound. After Nophead's advive on checking low value resistors, I noted that my multimeter read 0.5 ohm when the probes were touched together on the lowest resistance range setting, 200 ohm. The resistors gave a reading of 0.8, meaning they are rated at ~0.3 ohm, which is about right allowing for slight inaccuracy of the meter at such low levels compared with range setting.
In circuit, there was no reliable way of measuring the capacitance of any of the small capacitors, so I was going on guesswork that the problem still lay, in fact, with the regulator.

I desoldered one pin on the regulator, and then the other.
I am getting on fine now using the solder braid with flux drawn onto it from a pen. This helps the solder to reflow. I placed the braid onto the top of the pin, acrosswise if possible, and applied the soldering iron tip on top of the braid for a few seconds. I found it easiest to position the braid and hold it in exactly the right orientation, with no twisting etc, with my hand, but once held in place by the soldering iron, I swapped to holding the braid with tweezers so that I didn't burn my hand when it got hot. Access was awkward, and I decided to desolder one of the RJ45 connectors to reduce the amount of melting and burning plastic where the large screws on the soldering iron shaft would catch! I still caught the edge of the Molex connector from time to time. I cut off the used end of desoldering braid holding the absorbed solder and repeated with clean and freshly-fluxed braid until the regulator's pins looked to be clear of solder. The pins still seemed tacked to the board, possibly with sticky burnt flux, so I cleaned around them and prised them up off the board. The pads below are fine.
Desoldering the through-hole pins of the connector was easier, still, after cutting the end of the braid into a point and using the fine iron tip, pressing the braid slightly down and into the hole as the solder wicked up.

Next I tried to desolder the regulator's large ground pad - that was just too tough! I managed to remove most of the solder, but there was some visible under the centre of the pad, but the gap between the part and the board was too thin (top to bottom) to be able to slide the desoldering braid in, and although I could melt some of it, because of the proximity of other components I couldn't use the big flat soldering iron tip, which would have been ideal to melt the lot all at once and allow the regulator to be released.
Eventually my partner went for the brute force option! and created more damage than was intended! Some of the board's contact pad lifted clean away, but the remainder still has contact to ground, and there is damage to the green coating over the track that leads from the 5V pin adjacent to the side of the regulator. There still seems to be continuity everywhere (there should be), though, so I have Araldited a thin protective layer over the shiny copper that is visible.
We tested the removed regulator - it is most definitely dead! Luckily, the minimum order for these had been 5, when I (thought I) needed 3, so there are a couple of spares after all (which is a relief because this was the part that came from RS, who were absolutely useless, and from whom I have no wish to order again)!

My brother rang just as all this was done, and said that I could test this board before putting another regulator on (just in case there is still a short on the board as well) by connecting the working board to power, and connecting a 5V contact on that board to a 5V contact on this board, and a ground contact on that board to a ground contact on this one. The regulator on the first board should easily be able to handle the current for both boards together. At least the power LED should come on for both boards. And if I do Nophead's thing of getting the other LEDs to work, too, I should get proper indication on both boards as well if this board is working. I need to look that up again, but meanwhile I'm letting the Araldite cure.

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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