Sunday, 20 September 2009

Rice Krispies

Apparently, Churchill said, "Success is going from failure to failure without a loss of enthusiasm".
I must be heading for success then!

Firstly, I must thank Nophead for sending me those spare parts - I have practice material! More later.

The instructions for the stepper motor driver board say to plug it in to see if it works, but I didn't have a 12V power supply. Maplin, as usual, didn't have what I wanted, so I ordered from Farnell again and ordered spare surface-mount LEDS, too. I was underwhelmed to find the power supply didn't come with a lead! I dug around in the long-since-abandoned-shelving upstairs, and found the power lead for either my old PC or its CRT monitor. It had a 5 A fuse, so that seemed OK. Plugged it into the power supply - and nothing. Checked all the switches were on and the socket was live, all OK. But no power from the PSU. Wondered about that old lead? Then I remembered somebody telling me something about how ATX power supplies differed from AT power supplies, so some internet hunting later, and I found out how to short out pins in the power header for testing purposes.
Usually the power header is a 20 pin affair, but mine looks bigger. It has a separate 4 pin block slid* onto the end of the larger connector.

Here is a picture, with the power header shorted between pins14 and 17:

Thank goodness for the internet and the wealth of information available on various websites.

This done, I switched back on - and it was a good job I was watching the fan closely, because I watched it disappear as it spun at speed, but I couldn't hear a thing. Wow, I know it is called a "silent" PSU, but I didn't expect that they meant it!

Barney reported a blown chip, with the advice to check continuity on the board before plugging in. My meter has dead batteries....

Now I had a working PSU, I switched off, plugged one of the Molex-type leads into the board until it
snapped into place and switched on with my finger over the off switch. Here was the crackle and pop. I switched off very quickly with a slight burning smell developing. What was that, you said, Barney? Something about checking continuity everywhere?

Ooops, I have given the chip a contour! but it doesn't look like the board is damaged.

This time following Barney's advice (!) I shall nip the legs with the tin-snips if I can to remove the dead chip. And I'm going to Screwfix now for some replacement meter batteries - open on a Sunday and ~£2 cheaper in total than Halfords.
Then I shall inspect the rest of the board, and CHECK CONTINUITY everywhere. Without the chip in place, I should get better access. Then, if all is well, I shall replace the chip and check the continuity from its legs, too.

Soldering/desoldering learning.
Following on from a link off the forum for Kapton tape supplied by DealExtreme in the States, and looking to see what else they had, I noted that they had desolder braid with lots of reviews, including one that said this particular desolder braid had flux in it, so you don't need to add any. Ah-ha, maybe that's why my desolder braid isn't very successful for me, as perhaps it doesn't contain flux, and I wasn't adding any.
Order from CPC (another part of the Farnell empire) a flux pen. Also a conductive pen as a temporary fix in case I blow any of my board tracks.
I shall try the fluxing/desolder braid to remove the remnants of the old chip legs. If I'm not getting anywhere I shall leave them in place rather than risk damaging the board. Then I'll clean the area with surgical spirit, in case there is contamination after the smokey smell.

Also, I have seen a video of soldering 1206 parts by hand with solder wire, and this requires flux to be put down first and touched with the soldering iron to "tack" the part in place, before laying solder wire partly along the end contact of a component, and just bringing the soldering iron (using a chisel tip, with the flat face downwards) down onto the edge of the pad, sliding it in to touch the part and sliding straight back away. This gives minimum iron-exposure to the 1206 component. I'm going to practise this technique when I have my flux pen. I suspect my 0.7 mm solder wire may be too thick, though; it needs more like 0.4 mm.

I have already practised using the chisel tip and soldering a 1206 component using the solder paste, which itself tacks the part down. This was pretty successful, too. Knowing I can use the soldering iron with the solder paste, when I replace my chip, I'm going to use the solder paste in a thin line across all the pads.

Farnell sub-total = £19.64 (agh, I forgot to order the headers!)
  • 350 W PSU, part number 1277264, £15.68
  • green LEDs, 1206, part number 122672, £0.061 each (x10)
  • red LEDs, 1206, part number 1318261, £0.079 each (x10)
No delivery charge, but total VAT of £2.56.

CPC sub-total = £25.14
  • no-clean flux pen, part number SA00859, £5.79
  • conductive pen, micro-tip, part number SA00462, £19.35
No delivery charge.

ScrewFix sub-total = £3.82
  • LR44 cells, pack of 2, part number 44122, £1.91 (x2)

Running total (excluding set of washers and screws from B&Q) = £432.63

So I STILL need 2 headers and another A3982 chip.

* On a complete and utter off-topic aside, I was thinking about how the past participle of the verb "to slide" is slid. That's (about?) the only one. For glide it is glided, pride it is prided, side it is sided, and for ride it is ridden.
Isn't English a wonderful and contrary language?!


  1. You might also need to add a dummy load resistor to the 5V rail to get the correct voltage on the 12V rail. IIRC, there is a site for one on the motherboard. Without it the 12V rail is likely to be only about 10V. High enough to test things but not good for performance.

    I wouldn't use desolder braid, it is very easy to lift tracks with it. Especially as I get the impression your iron struggles to maintain the high temperature needed for lead free.

    Simply crop the legs close to the chip being very careful not to stress the tracks. Then just use the iron to melt the solder and brush the legs off with it or use a solder sucker to suck away the solder and leg together. Wipe the pad with a flat bit to level any remaining solder. Don't heat it for long otherwise the glue melts and the pad will lift.

    For lead free work you should really use a temperature controlled iron. The solder melting point has gone from 173C to ~ 220 but the glue that holds the tracks to the board has not changed and the semiconductors have not changed. That means the margin before damaging things is now a lot smaller.

  2. Removing my chip in the way you suggested worked like a dream - I just had to brush them off with the end of the soldering iron and wipe the iron tip to remove the severed leg. Thank you very much for (all) your sage advice. Much appreciated.