Boarding school, term 3. Muddling through the end of year tests....

Hoorah! Hooray! Yippee! Wonderful! Marvellous! Brilliant! Fantastic! (Can you tell I'm happy?)

After a whole week of head-scratching, web browsing, and advice from my brother, my partner and Nophead, I have finally FIXED my bad stepper motor driver board. Unfortunately, this was the board with near-perfect soldering, of which I was very proud. Now it looks a mess! Anyway, here is the proof it works:

OK, it's not pretty.

The voltage regulator had been tested whilst still fixed to the board (but only by its ground pad). The regulator was definitely faulty. Unfortunately, once I'd removed that, everything else in the 5V path gave readings of zero ohm when tested on a 20 k ohm setting on the multimeter with the +ve probe on the path and the -ve probe on the ground pin of the Molex-style connector. This meant there was a short to ground somewhere, still.

The first thing I checked was the power-indicating LED, as this was what I had replaced twice, once because the first one was dead (although it turned out that that was incidental to the board not working), and once because I checked it for correct orientation when the board still didn't work. The result was that this part of the circuit certainly didn't look tidy, and was a cause for suspicion. I cleaned it very carefully and checked the gap between it and its protective resistor, but it looked separated. I wondered if I had damaged the pad on the board, but without disconnecting it from the circuit there was no way to check.

Next candidate was the damaged track alongside the regulator's space. I decided to cut the track close by the 5V pin' pad,so I scraped off the protective Araldite I had applied, and scraped back to the copper track. Then I very carefully cut the track, and made sure there was no continuity across the cut. Now, using the diode setting on the multimeter, I could light the power LED, so that side of the circuit was sound.
I cut the track in a similar way towards the rear of the regulator's space. Checking on resistance setting 20 k ohm again, the part of the track alongside the regulator gave reading of 1 against ground, meaning no connection to ground, ie no short there.
O.K., they were my top candidates! And they were fine. And I now had 2 unnecessary cuts in my track!

Oh well, nothing ventured, nothing gained.....
Following the track, it goes from the regulator to the electrolytic capacitor from where it splits, with part then heading to the 100 nF ceramic capacitor and to the second RJ45 socket, and the rest going down a "via" under the electrolytic capacitor and off to various other parts of the board.
In order to test this, I cut the track in a clear section underneath the board. Testing the chip end of the circuit to ground, the meter gave 1.
Ah-ha, now the problem is narrowed down to 2 capacitors, the "via" or the RJ45 socket.

The RJ45 socket seemed unlikely to have a problem, and ditto the large capacitor. Having plenty of spare 100 nF ceramic capacitors, thanks to supplies from Nophead after I cracked (a different) one on this board right at the start, I decided to simply swap it.
Some little desoldering and resoldering later, and voila! we have readings of 1 from everywhere on the 5V path to ground. So the problem WAS the capacitor.

Now my problem was the breaks I'd deliberately put in the track....
First I used the conductive pen to fill in the gap in the track on the underside of the board. It takes 10 minutes to get conductivity through the "ink", and testing afterwards showed I had good conductivity between the copper ends.
Unfortunately, when I tried this on the top of the board, I found that I also had re-shorting to ground! Obviously the cuts on the upper side of the board reached the ground connection in the board. So, using a trusty cocktail stick (which should also be added to the parts requirements!) I could paste Araldite (ditto) into the cuts in the copper track, but while that dried, I used the conductive pen to rejoin the breaks going around the cut section. If the conductive pen only works temporarily, I shall solder wires between the track breaks.

Then I soldered a new voltage regulator in place, with the help of Blu-Tack, again. Unfortunately, I knocked the electrolytic capacitor loose, causing more burning plastic due to access issues with my soldering iron as I fixed it back on. I tested it, and the voltage regulator, and then resoldered the first (removed) RJ45 socket. I tested everything exhaustively.

And now we have Blackpool at Christmas! (For those non-UKers, that means all the lights came on.)

My partner thought the board was a gonner. I thought if it didn't work anyway, there was nothing to lose by trying to resuscitate it, and it was going to take another 3 weeks to order a fresh board from the States anyway. And it works.

(PHEW!)

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.
.

Step(per) 1 finished!

I finished the first stepper motor driver board today. My second mega-resistor went better than the first. I am amazed how often I end up needing to use Blu-Tack to hold things in place; it should be named on the parts list under tools!

So here is the front of the board:

and here is the back, with its near-perfect soldering:

I just wish I could test it, but I don't yet have a 12V power supply, nor do I think I can mock one up, as I did for the 5V previously.

I am very grateful to Nophead who has promised me a replacement 100 nF ceramic capacitor and surface-mount green LED, so I should be able to do the reflow work on the other 2 of these boards towards the end of this week.

I'm hoping to read Barney's blogging of his reflow experiences before I do more, to see if I can pick up any tips.

I really appreciate the help and encouragement and practical assistance given freely on the forum and in people's blogs and comments. It's something special.

I have learnt that these boards that have been giving me quite a challenge are now available ready-assembled! Oh no, they're not are they? I could have waited for that had I known it was in the offing.

Oh well, at least at the end, I can say "I made all of that", with some pride (especially if I have a working 3D printer!) because I knew how to do NONE of this 6 months ago! And I couldn't have done it without the RepRap "community" being what it is.

Meanwhile, I can tackle the cabling, or I've still to try out those penny washers on my mechanical build.