Leach Amp Circuit Board
Alright, here comes the bulk of the work. This covers the process of assembling the circuit board. This involves a lot soldering (about 200 joints) so I will include a brief refresher on how to properly solder. This is not intended to teach people who have never soldered how to solder, if you have no soldering experience I highly recommend practicing this technique on something other than your Leach Amp circuit boards =).
I used a Weller WLC100 soldering station with some silver-bearing solder from Radio Shack to make all of the solder joints in this project. The tip the WLC100 came with was too large for PCB work (it was a good size for crossovers though) so I replaced it with this tip. I chose the silver-bearing solder from Radio Shack because it was thin, available locally, and cheap. The fact that it also had some silver content was just a bonus. I highly recommend the WLC100 station, but if you don't want to spend that much money you could do just as well with one of these and this. For the majority of the solder joints I had my WLC100 set to around "4" out of "5" which I estimate to be about 30 watt.
The very first step that I recommend is to separate out all of your components. The easiest way to do this would be to get about 6 small bowls and place all the resistors in one bowl, all the capacitors in another, and everything else in a third. Now you can go through the assembly process adding all of a particular type of component at once and placing the extras in one of the other three bowls. Since there are so many components (literally over 100 individual components per board) this will help you stay organized. Below are two pictures (made by Dr. Leach) of the circuit board layout, they are labeled for version 4.4 and we are building version 4.5, but both of these versions have the same layout. The first shows all of the components (and their labels) as well as all of the circuit board traces, the second just shows the components. Note that both of these show the layout from the component side (that's the side without all the little silver circles on it). You should print both of these out as well as a copy of my parts list spreadsheet and keep them on the table as a reference while you are assembling this board.
Before you start soldering you should plug in and turn on your iron and let it warm up, while you're waiting you can go wet your sponge (a folded and damp paper towel works just as well). Once warm you should melt some solder on the tip and allow it to flow until the tip is covered with solder (this is known as "tinning" the tip). Now wipe the tip off on your sponge, some of the excess solder will come off and your tip your be left with a thin coat of solder. Pick out the component you plan to add to from the spreadsheet, find it in one of your bowls, and find it's label on the layout. Now bend the leads so that they slip through their designated holes in the PCB. Turn the PCB over allowing it to rest on the component you are about to solder, this should push the leads all the way through the board and allow the component to be flush against the surface of the PCB. Press the tip of the iron lightly against the PCB so that it is touching both the solder pad (little silver circle) and the component lead at the same time. After waiting a few seconds (how many will depend on how high you have the iron turned up) start slowly feeding the solder onto the intersection of the iron tip, the component lead, and the solder pad. You should see the solder start to "flow" around the solder pad, you shouldn't need to move the tip of the iron or the solder you're feeding in for it to flow, it should flow around the pad on it's own. Less is definitely more here, so as soon as the entire solder pad is covered with solder withdraw the solder and then the tip of the iron. What should be left is a small shiny conical joint between the component lead and the solder pad. Now take a small pair of scissors or wire cutters and snip the component lead right above the solder joint, be careful not to cut into the solder joint. Again, soldering is sort of a skill/art, practice definitely helps.
I recommend soldering just one component at a time for the first few components and as you become more comfortable with the process you can start doing many at one time. One thing that will make your process move more quickly (especially with the resistors) is to place all of one type of a component one the board at once (say, all the 300ohm resistors) and then flip the board and solder them all at one time. Slow and steady will always win the race though, so don't trade off speed for accuracy, you'll be hating yourself if you accidentally solder in a 1.1K ohm resistor in the place of a 11K ohm resistor, or a PNP transistor in place on an NPN transistor. Just to reiterate the process should be to find the component you plan to add on the spreadsheet, then find it in one of your bowls, and make an note of it's label. Take a look at the PCB layout and find that label on the layout. Then add the component to the board. I cannot stress enough how important it is to PAY ATTENTION and make sure you are placing the correct components in the correct places.
There is an order in which I recommend you add the components to the board. They are ordered this way so that the smallest components go on the board first and you work your way up in order of size. It gets more difficult to add smaller components to the board once the larger components are in place. I will note the components that need to be orientated a particular way by placing an asterisk beside them, if they don't have an asterisk beside them it doesn't matter how they are oriented. You should notice the component "J" on the list, although it doesn't appear on the spreadsheet. "J" is simply a straight piece of wire, I recommend using one of the snipped leads from the diodes and using it as "J" (the "J" stands for jumper, if you were wondering). Here is the order:
You may now be wondering what the correct orientation for those asterisked items would be. The diodes are marked with a black stripe on one side of them, this black stripe must be in the same position as it is shown in the layout.
For the potentiometer the "dot" must "point" toward the signal input section of the board.
For Q12 - Q15, the small metal flap must be oriented as it is shown on the layout.
For Q1 - Q11 the curved side of the transistor must be oriented as it is shown in the layout, you will have to bend the middle legs of each of these transistors forward slightly to allow it to fit in the holes. As you insert the transistor gently press down on it so that the middle leg continues to bend as it slides into it's hole. These components are not intended to be mounted flush against the board, so as soon as you feel the resistance start to increase as you are pressing them into place, stop.
For C4, C5, C13, C14, C21, and C22 the "dash" shown on the layout corresponds to the gold color "stripe" on these capacitors. Both the dash in the layout and the dash looking figures in the gold stripe are intended to represent a "negative sign", but in my opinion neither resemble one.
For Q16 and Q17 there are a few things that need to be done. First, the heatsinks have to be placed on them. When you look at one of the heatsinks you'll notice that it's screw hole is not centered. You want to insert the head of each transistor into the side furthest from the hole and press it in firmly until the hole in the transistor lines up with the hole in the heatsink. Next you'll want to bend the inner lead at a 90 degree angle to the body of the transistor and the two outer leads at the same 90 degree angle, but slightly closer to the body. This will allow the transistor leads to fit into their designated holes. Once the transistor is in place solder it as normal. Once each transistor is soldered into place you should use a #4-40 1/2" machine screw and bolt (I purchased them from Home Depot) to affix it to the board.
R49/L1 is a component that needs some assembly. It's actually the R49 resistor with some 22 gauge solid wire wrapped around it 11 times. The easiest way to do this is to take your length of wire, strip one end, and solder it to one side of the resistor. Now wrap the wire tightly around the resistor 11 times (it will fit!) and mark the wire where it meets the other end of the resistor. Unwind it, cut the wire just past that mark, and strip it. Now re-wind it and solder it into place.
You may notice that D1, D2, D3, D4, Q18, Q19, Q20, Q21, R50, and C25 were not mentioned in my ordered list. That is because all of these components are not mounted on the PCB and their placement will be covered in the Assembly section.
