MICrODEC Assembly

0.  Your workspace

Find a good place to work – good lighting, clean space, and no ESD buildup. Don’t walk across a shag carpet in the middle of winter and pick up your PCB.  Always touch something that is grounded before touching your board for the first time.

1. Check the PCB

Inspect your PCB.   Make sure all the surface mount components are placed, and that the PCB itself has no burred edges.  If you purchased a case, be sure it fits into the PCB slots smoothly.  It will be much easier to make modifications to the PCB before you place more parts.

Inspect your parts. Make sure your kit came with all the parts you ordered, and that they aren’t damaged.  Notify Open Music Labs immediately if something seems amiss.  For our first run, the 2pin dip header and jumper are pre-placed on the PCB, so make sure they are there.

2. Solder in the 14-pin DIPs

In these instructions we will place all of the parts for the MICrODEC in order of height (shortest to tallest).  This is generally a good approach, as you can flip the board over, and have the weight of the board press down on the parts you are soldering, helping them stay in place.

Place the DIP sockets.  Be sure to place the notch on the socket in line with the notch on the PCB drawing (see above), this will let you know which way to put your opamps in later.

You can bend the corner pins over on the underside of the board to help hold the socket in place .

Solder opposing corners first, and then make sure that the socket is firmly seated against the board.   If it’s not, it’s much easier to reheat only two joints to adjust the placement now rather than spotting this after all the pins are soldered.  After you are satisfied that the socket is seated properly, finish off the rest of the pins.

3. Check your solder joints

Check your solder joints.  You can compare them to the picture above (click for a larger image).  They should fill in the pad, but not be large blobs (they should have a convex surface).  If your joints don’t seem satisfactory, practice soldering on some old PCBs just to get the hang of it.

Some soldering tips:

  1. 1.  Make sure your equipment is decent.  Your iron’s tip should be tinned, and should accept solder when placed against it – The solder should not ball up and roll off.
  2. 2.  Keep the temperature between 660F and 700F.  Usually for this larger work, I keep it on the higher end, but it varies from iron to iron.
  3. 3.  Place your iron to the juncture you wish to solder – wait 3 seconds for the juncture to heat up – apply solder until the juncture is adequately filled  – wait 3 more seconds for the heat to spread – remove the iron and let the joint cool.
  4. 4.  It is also a good idea to wear protective gear (respirator, gloves), or to use a fan to remove fumes.  At the bare minimum, always blow on the joint while soldering, as this ensures you are not inhaling, and blows the fumes away from you.

4. Place the headers

Now place the 6pin header(s).  If you dont intend to use the FTDI 6-pin header (1×6 with red plastic), and you are putting your MICrODEC in a case, you can leave this part off.  If you are going to use it, you might want to trim the pins down a bit, as they come very close to touching the case.  This will be fixed in the next revision.

Even if you dont intend to use the ISP 6-pin header (2×3), you should place it anyways, as it provides stray capacitance to the SPI lines and helps keep them from glitching.  Again, tack one pin in place to start with, then make sure the part is flush to the board before soldering the rest down.

5. Install the DC power jack

Place the power jack.  It is very important that this jack is flush to the board, and that its square with the silkscreen outline.  If you are putting your MICrODEC into a case, the jack will not fit if its not in the right place.

The pads on the jack are much larger than the pins we have been soldering thus far, so they will take a bit longer to heat up, and require a lot more solder. Be sure to fill in the holes completely, because the solder joint acts as a mechanical connection as well.  Since you will be putting force on this socket when plugging and unplugging power cables, you want it to be as strong as possible.

6. The electrolytic capacitor

Place the electrolytic capacitor.

Be sure the black negative stripe is facing the back of the board.  If you place an electrolytic capacitor in backwards, it will explode when you power up your board.  You can bend the leads to hold the part in place, and then clip the leads when you’re done.

7. The 1/4 inch jacks

Place the 1/4″ jacks.  These may take a bit of pressure to snap into place.  Again, be sure they are flush to the board so your case will fit.

You will notice that some of the pins take more time to solder than others (the ones connected to ground).  This is because the ground acts as a heat sink, and absorbs heat while the pins are getting hot enough to stick to the solder.

8. Placing the pots and rotary encoder

Now we move on to placing the pots and rotary encoder.  Pay special attention in order to get the right pots in the right places.  If you look at the PCB, you will see that the pot outlines are labeled with a value (50kA, 10kB, 50kB, 10kA).

Look at the top of the pots (see photo above).  You will see a value written on top that matches one of these values.  Match up the pot value with the silkscreen value, and put it on the board.  Make sure all pots are placed and you’re certain of their placement before continuing.

The pots and rotary encoder need to be flush to fit in the faceplate.  The dual pots take a fair bit of force to snap in, so be sure they are fully seated and not sticking out a bit.  The pot on the right in the photo below is NOT seated correctly, whereas the left one is.  Be sure the legs are flush to the board.

Finish placing all the pots and the rotary encoder before soldering any of them.  We will discuss soldering them in the next step.

9. Soldering the pots and rotary encoder

The pots used here have a plastic conductor, which is sensitive to heat, so it is important not to damage them while soldering.  The trick is to solder one pin quickly and properly, and let the device cool before soldering other pins, so the heat does not build up.  But, it’s more important that you get a good joint on the first try, so that you don’t spend time rehitting the same joint.  So take your time and make sure each pin is well soldered in one go.  I usually solder one pot pin at a time, and then solder a pin on the next pot, and so on.  This gives time for the pot to cool down before you come back and solder the next pin on that pot.  We shot a brief video to give you an idea of how to do this.

Once all the pins are soldered, you can solder the legs down.  These take a lot of heat, and a lot of solder.  If you have a variable temperature iron, turn it up to 700F.  Heat the leg and pad with the side of the iron, this will give the most surface area in contact.  Begin filling in solder as soon as the iron will melt it, and continue until the hole is filled.  You may have to move the iron to the other side of the hole.  After the hole is filled, make sure the solder is completely melted (it will go from slightly concave to slightly convex, and look wet and shiny), and then quickly hit the second leg of the pot.  It’s good to do both legs in sequence, as the other leg will already be hot.

10. The rotary switch

Place the rotary switch.  I’m sure you’re sick of hearing it by now, but make sure it’s fully snapped in place.  Solder the pins first, and then solder the legs.  Follow the same procedure for the legs as you did for the pots, but this time use even more solder.  If you place the iron inside the curve of the leg clip, you can get good heat conduction.  These legs provide mechanical connections for all the knob twiddling you will be doing, so make them good.

11. Double-check your solder joints

Now is a good time to check over your solder joints while your pots cool off. Bad solder joints happen even to the best of us, and are particularly notorious on pins that connect to the ground plane. Bad joints can cause intermittent problems in electronic systems and can drive you crazy when you need to track them down in a hurry right before a performance.

12. The op amps

Place the op amps.  The pins on most DIP packages are a little too wide to fit into sockets or PCB footprints, so I usually bend them a bit before putting them in.  The easiest way to do this, is to place the pins down on the table, and gently bend the op amp up 15 degrees or so.  This bends all the pins at once, and makes sure they stay in a line.  Flip the chip over and repeat the process.  You can then put them in the sockets, and press them down until they are seated.  Make sure the notch on the opamp matches the notch on the socket (the opamps point opposite directions).

And now, if you didn’t order a case, you’re pretty much done.  Just install your LEDs in the method best suited to your needs, then plug in power and play!

13. Joining your pcb and faceplate

If you do have a case, now is a good time to install the faceplate (it will help you installing the LEDs later).  The faceplate should fit on easily, with a little bit of room to wiggle.  If it does not, check that all your pots are flush to the board.

You will note that the rotary switch does not screw to the board.  It is recessed so all the knobs line up.  Tighten its nut down (or remove it) before placing the faceplate, that way it won’t rattle around later.

Place the 5 washers and 5 nuts on the pots.  Do NOT tighten the nuts at this time, you want to make sure the faceplate has a little slip to help with alignment in the next steps.

14. Decide which screws you want to use

The kits usually come with 2 types of screws (this is provided by the case manufacturer, and is not up to us).  The thread forming screws (the rear screw in the photo above) have a torx head, which is a less common form of screwdrive.  In addition, the thread forming screws bind easier than the thread cutting screws (the front screw in the photo).

The thread cutting screws have philips heads, a more common driver to have around the house, but these screws produce metal shavings when inserted, so if you use them you will need to protect your PCB from these metal shavings.  To do this, you can place tape over the channels that the screws go into.

This will catch the shavings as you install the screws.  (Note that in the pictures above the case is sitting upside down)

However, the thread forming screws don’t produce much in the way of metal shavings, and probably dont need the tape applied (although it wouldn’t hurt).

15. – Attach the faceplate

Place the PCB in the case.  the PCB goes in the third slot from the bottom, and there should be a mounting hole in the center on the top.  Place the backplate on the MICrODEC at this time as well.

Make sure all the holes line up, and put the 10 screws a few turns in.  This will ensure the endplates are aligned.

Now tighten the 5 pot nuts, to lock down the position. Do not overtighten these nuts, they only need to be snugged down.

After the faceplate is secured to the PCB, remove the PCB and faceplate from the case for the LED installation.

16. Placing the LEDs

Find the shorter leg of your LED, this is the negative end, and should be placed in the hole which has the flat side on the LED image on the PCB.

You will need to bend the LEDs to go into the PCB and faceplate.

Make sure the shorter leg is on the right when looking at the LED from behind.

Place the LEDs in the faceplate holes, with the green LED on the right (when looking at the back of the faceplate).

Now solder the legs in, and clip off any excess.  There won’t be very much excess, especially from the short leg.  You can hotglue the LEDs in place on the back of the faceplate, or leave them as they are.  Hotglue will ensure they don’t move around, but will also make it difficult to mod in the future.

17. Putting it all together

Reassemble your MICrODEC.  Place the PCB back in the case, and put the backplate on.  Tighten up the screws, but do not overtighten.  Just snug them up so there is no gap between the case and the endplates.  The screws are not very strong, and can snap.  If you are using the threadforming screws, and they are binding, back them out and try again.  Do not force them, or you will break off a screw in the channel.

Place the 1/4″ jack nuts on.  The washers which come with them are not of much use, so feel free to not use them.  Do not over tighten the 1/4″ jack nuts, as this well put undue stress on the 1/4″ jacks (they do not rest on the backplate – you can only have one alignment feature in an assembly, and in this case its the frontplate and pots).

You don’t need to screw your MICrODEC together if you are using rubber bumpers, and intend to open it up frequently.  The 1/4″ jack nuts will hold it together pretty well, and the rubber bumpers will make sure everything is aligned.

18. Knobs

Place on the knobs.  It takes a bit of pressure to get them on, so be careful not to bend back the rotary switch or rotary encoder while doing this, as they are not secured to the faceplate.  If they are particularly difficult to get on, try adding a drop of oil to the pot shaft.

19. Bumpers

Place on the bumpers.  The bumpers have a semi-circle to cover the top-middle screw, so just make sure its covered, and you’re all set.

- instructions by guest