Erica Synths EDU DIY Dual VCA Eurorack Module Kit User Manual

Page 51

Next, insert the first DIP socket, hold it

in place and solder one of the pins. Con-

tinue with the next DIP socket. Make sure

the DIP sockets are oriented correctly –

the notch on the socket should match the

notch on the PCB’s silkscreen. Now, turn

the PCB around and solder all remaining

pins of the DIP sockets. Then proceed

with the ceramic capacitors. Place the

PCB in your PCB holder or on spacers,

insert the capacitors and solder them like

you did with the resistors & diodes before.

Now, your PCB should look like this.

Now take a close look at the part place-

ment diagram. Some resistors are marked

with an M next to their value. These

should be matched in pairs with as sim-

ilar resistances as possible. Normally,

you’d need a bunch of, say, 20 kOhm

resistors for this. You would check each

one with your multimeter and select pairs

with matching resistances. They don’t

have to all be 20 kOhm exactly – it’s just

important that the individual values are

matching. Two resistors with 19.98 kOhm

resistance, for example, would be perfect-

ly okay.

But as this is a pre-assembled kit and we

didn’t want to just drop a big pile of resis-

tors on your head, we pack it with ±0.1%

tolerance resistors where needed.

Which means that there’s no real need for

matching – they’re going to be close

enough straight out of the box. You can

identify these ±0.1% tolerance resistors

by their light blue bodies – use them

wherever you see an M on the part place-

ment diagram. If you’re unsure about a

resistor’s value, use your multimeter to

double-check.

Next, we have two 10 Ohm resistors (R11 and R12) on the + and – 12 V rails, with decou-

pling (or bypass-) capacitors C1 – C4. These capacitors serve as energy reservoirs that

keep the module’s internal supply voltages stable in case there are any fluctuations in the

power supply of the entire modular system. In combination with R11 and R12, the large

47 microfarads pair (C1 and C2) compensates for low frequency fluctuations, while C3 and

C4 filter out radio frequencies, high frequency spikes from switching power supplies and

quick spikes created by other modules. Often another component – a ferrite bead – is

used instead of a 10 Ohm resistor, and there’s no clear consensus among electronic

designers which works best, but generally, for analogue modules that work mostly in the

audio frequency range (as opposed to digital ones that use microcontrollers running at 8

MHz frequencies and above), resistors are considered to be superior.

Another advantage of 10 Ohm resistors is that they will act like slow “fuses” in case

there’s an accidental short circuit somewhere on the PCB, or an integrated circuit (IC) is

inserted backwards into a DIP socket. The resistor will get hot, smoke, and finally break

the connection. Even though they aren't really fuses, just having them there as fuse substi-

tutes is pretty useful - you’d rather lose a cent on a destroyed resistor than a few euros

on destroyed ICs.

Capacitors C5 – C8 are additional decoupling capacitors. If you inspect the PCB, you’ll

see that these are placed as close to the power supply pins of the ICs as possible. For

well-designed larger PCBs you will find decoupling capacitors next to each IC. Like the

others, their job is to simply compensate for any unwanted noise in the supply rails. If the

input voltage drops, then these capacitors will be able to bridge the gap to keep the volt-

age at the IC stable. And vice-versa - if the voltage increases, then they’ll be able to

absorb the excess energy trying to flow through to the IC, which again keeps the voltage

stable. Typically, 0.1 uF capacitors are used for this purpose.

Before you start soldering, we highly recommend printing out these part placement

diagrams with designators and values. Because some of our PCBs are rather densely pop-

ulated, this will help you to avoid mistakes in the build process.

Place the dual VCA PCB in a PCB holder

for soldering or simply on top of some

spacers (I use two empty solder wire coils

here).

I usually start populating PCBs with lower,

horizontally placed components. In this

case, these are R11, R12 and the power

protection diodes. Bend the resistor

leads and insert them in the relevant

places according to the part placement

diagram above. All components on the

PCB have both their value and denomina-

tion printed onto the silkscreen. If you are

not sure about a resistor’s value, use a

multimeter to double check. Next, insert

the diodes. Remember – when inserting

the diodes, orientation is critical! A thick

white stripe on the PCB indicates the cath-

ode of a diode – match it with a stripe on

the component. Flip the PCB over and

solder all four components. Then, use

pliers to cut off the excess leads.

In order to save space on the PCB, some of our projects, including the dual VCA, have ver-

tically placed resistors. So the next step is to place & solder those. Bend a resistor’s legs

so that its body is aligned with both legs and insert it in its designated spot. Then solder

the longer lead from the top side of the PCB to secure it in place, turn the PCB around and

solder the other lead from the bottom. You can insert several resistors at once. Once done

with soldering, use pliers to cut off excess leads.

Once you are done with soldering all resis-

tors, your PCB should look like this.

Next up: inserting & soldering the tran-

sistors. Make sure you place the transis-

tors in their designated spots. Also, they

need to be properly aligned with the

marked outline on the silkscreen – orienta-

tion is critically important here.

Next, insert & solder the electrolytic

capacitors. Electrolytic capacitors are

bipolar, and you need to mind their orien-

tation. The positive lead of each electrolyt-

ic capacitor is longer, and there’s a minus

stripe on the side of the capacitor’s body

to indicate the negative lead. On our

PCBs, the positive pad for a capacitor has

a square shape, and the negative lead

should go into the pad next to the notch

on the silkscreen.

Now your PCB should look like this.

Next, solder the precision trimmer and

the power supply socket. Make sure the

orientation of the socket is as shown in the

picture below – the arrow pointing to the

first pin is aligned with a notch on the silk-

screen. The key on the socket will be

facing inwards to the PCB.

Insert the potentiometers, but don’t

solder them yet! Fit the front panel and

make sure that the potentiometers’ shafts

are aligned with the holes in the panel –

and that they’re able to rotate freely. Now,

go ahead and solder the potentiometers.

Install the front panel and fix it in place

with the 6 hex nuts on the jack sockets.

We are almost done!

Now, insert the ICs into their respective

DIP sockets. Mind the orientation of the

ICs – match the notch on each IC with the

one on its socket.

Congratulations! You have completed the assembly of the mki x es.edu dual VCA

module! Now connect it to your eurorack power supply and switch it on. If there’s no

“magic smoke“, it’s a good sign that your build was successful. Connect a VCO’s SAW

output to VCA IN1, and VCA OUT1 to a mixer (connected to monitors or headphones).

Rotate the OFFSET1 knob all the way clockwise and check if you hear the oscillator. Then,

connect OUT2 to the mixer, turn OFFSET2 all way clockwise and again check if you hear

the oscillator. If you do in both cases, you can proceed with calibrating the VCA as

described in the ELIMINATING TRANSISTOR MISMATCH-section above.

Now, turn the PCB around and inspect

your solder joints. Make sure all compo-

nents are soldered properly and there

are no cold solder joints or accidental

shorts. Clean the PCB to remove extra

flux, if necessary.

Insert the jack sockets and solder them.