SV1 and SV2 assembly

[K O'Connor]

Hi Guys

If you are assembling SV1-Zh, SV1-Zp, SV2-Zh or SV2-Zp read this post for general guidelines, but see post-2 for specifics regarding the SVn-Z details.

The new kits are pretty tight, so the assembly order is important. The new boards are marked as SV1-D and SV2-D. The D refers to a much earlier circuit used in my amps, which I refer to as SD-1 (Super Design for fixed-bias). SD and SV are part of the Japanese-style naming

Overview
Cordwood construction is used to reduce the board size by about 30% from the previous layout. In this format, axial-lead components (mostly resistors) have one lead folded back to parallel the component body with both leads now pointing the same way. The silk-screening on the PCB shows how the formed part should be inserted to avoid having the folded leads touch each other.

The little transistors should be inserted by their SHAPE not by the written face. STM disregarded industry convention and labeled the BJTs on their backside (curved side). The middle-size transistor Q5 should be oriented by the side with writing on it facing into the board; ignore the embossed 'A' on the back.

Hopefully you have a 50-60W iron as a 25W type more easily leads to cold joints and overheated semiconductors. If you have a soldering station with variable temperature, select a hotter range 700-800F and use a conical tip. Also, use thin solder, say 1mm to 1.6mm, so solder connections can be made quickly. As always, use rosin-core solder meant for electronics, that is the 63-37 eutectic ratio, as this goes from solid to liquid without the intermediate plastic phase.

Assembly Details
When I assemble an SV1 or SV2, I install the big mosfets first so they can be lined up nicely. I use a roll of electrical tape to support the PCB when soldering. You can use other types of tape or anything that is a handy height that allows the mosfets to rest on the bench top. To install transistor packages, push the leads all the way through the holes then slightly splay the outside leads outward. Now push the device back up until the height is what you want and the device is snug in the holes. With the big mosfets you want to make sure the two devices are the same height and straight, then flip the board and solder each pin. Cut each pin individually.

For all the other transistors do the same. With the small devices (Q2,7,8,9), you have to hold the transistor at an angle to the board, insert the outside pin closest to the board, then gently push and angle the device so the middle pin goes into its hole, then angle and push to insert the third pin. With the small transistors on SV1 and SV2 we leave the leads full length, so just push through 1-2mm (1/16") then make sure it is straight. Q5 is mounted similarly.

Assembly Order
As I said, the big mosfets go in first. Be sure to ground yourself before opening the anti-static bag that holds the mosfets and thermopads.

Then I install all the diodes and the gate-stops (R9,13-1k) for the big mosfets to protect them from any static during the rest of the assembly.

Then all the small resistors that lie down go in.

Then I install the small transistors along the edge of the board (Q2,7,8,9) and then Q1. Q1 is usually shipped with formed leads and should grab onto the holes. Make sure it is straight.

Then R6 13k3 which is cordwood.

The rest of the resistors are cordwood, too, and I put them in and solder them one at a time.
R12 1k between the big mosfets.
R4, R21, R10, R22 150k-1Ws
R7, R5, R11, R14, R15 330k-1W working across the board.
Q6, then Q5, then C2 and C1.

C1 usually ships with formed leads, as well, as we want to avoid damaging the epoxy seal around the leads by simply splaying them.

Yes, you really must insert, solder and trim each of these parts one at a time to assure they are mounted straight and that the soldering is good.

Other Tips
When mounting radial-lead electrolytic caps, the leads can be inserted through the holes and the cap body pushed all the way to the board. Then either splay the leads outward or pinch them inward. The pinch makes the cap tight to board for soldering . In either case, after you trim the leads you may need to make a hot adjustment to make the cap perpendicular to the PCB.

It is rare that you ever have to trim a component's leads before insertion into a board, so rare that I cannot bring any situation to mind.

When you trim the leads on transistor packages it is convenient to cut all three leads at the same time using side-cutters. The cutter jaws squeeze the pin and create a small burr on the trimmed lead in the axis of the jaws. For a low-voltage circuit this reduction of space between the pins is of little concern. However, SV1 and SV2 may have over 400V between the collector lead and the adjacent lead. Therefore, it is preferred to snip each lead individually with the axis of the jaws perpendicular to the axis of the line of three leads. This creates the burrs on the face of the lead pointing forward and rearward instead of side-to-side.

The pots come with small PCBs as the pots have PC pins rather than solder lugs. It is the convention with all PCBs that the component mounts on the silk-screened side of the board. For the pot boards, this aligns the X and 0 with the correct CW and CCW rotation of the pot. If you have an automatic wire stripper, vise, or stiff pliers, you can insert the shaft of the pot into this tool with the pot pins pointing up. Drop the PCB over the pins with the silk-screen side down facing the pot. Solder the middle pin while propping the board up so it is perpendicular to the pot leads. Now solder the outside pins.

Installation
When installing the kit in the amp, dry fit the assembled unit where there is airflow over the outside of that chassis area. Mark and drill the mounting holes (3mm or 1/8") and deburr the holes on both the inside and outside of the chassis. Use a larger drill bit in hand to cut the burr and slightly chamfer the edges of the holes.

DO NOT FORGET THE THERMOPADS WHEN SECURING THE BOARD IN ITS FINAL POSITION.

It may be easiest to solder wires to the board while it is still free, but each installation is unique and the dexterity of the installer is a factor.

You can "fly" the board for testing without tubes, or for other trouble-shooting without tubes. Place a piece of cardboard or other insulating material under the wired kit so it does not short to anything else in the amp.

BE CAREFUL NOT TO TOUCH ANY OF THE MOSFET METAL BACKS OR BARE COMPONENT LEADS. It is easy to get a shock with a flying board that is live.

Have fun

[K O'Connor]

Hi Guys

This post refers to the Precision Power Scale kits using the Z-core.

As above, carefully sort the components but leave the mosfets in their anti-static bag until you are about to install them. The tips about mounting various component types all apply here

You could start along the back edge of the PCB with the large mosfets, QN3, QN4, R21, then work forward toward the front edge of the PCB. Where I used to begin by mounting the large mosfets first, with the Z-kits I tend to mount the small transistors first, but either way is fine provided you take your time and pay attention to the squareness to the board of each component as you mount it.

With the small transistors, there are three parallel groups of two in the middle of the PCB; QN1+QN2, QP2+QP3, QN5+QN6. Each pair can be mounted in one go, inserting the first BJT then the second, making sure they are square, trim the leads and solder them. Push the leads about half-way through. I used to solder then trim, but I rediscovered my flush-cut side cutters which leave a tiny bit of lead poking through on the solder side. This helps keep the connections small and neat, but use what you have and solder then trim as you go.

Then install the remaining small BJTs the same way.

There are a few lie-down resistors and two diodes. These can be inserted, trimmed and soldered in one go.

On SV2-Z insert D4 cord-wood style. The silk-screening on the PCB is not good for this part, so follow the kit note layout. The stripe on the diode points away from R30. Keep the diode body about 6mm above the PCB. You might wish to install C6 at this time. Push it firmly down the PCB, splay the leads, solder and check squareness, then trim. On the Zp boards, C1 is the same type as C6 on the SV2-Zn, so it should be mounted at this time. For the Zh versions mount C1 last, preforming its leads as the kit notes show if the leads are not preformed already.

On SV1-Z there are three identical lines of resistors as R5A/B/C/D, R11A/B/C/D and R23A/B/C/D, with three 82k5 and one 84k5 each. We have left these resistors on their cut tape to make sorting easier. SV2-Z just has the R5 and R11 groups. Begin with one group and pull the appropriate resistors off the tape, or pull all the resistors off the tape and keep the two groups separate and draw from each pile as needed.

Since the resistors within a group are in series it does not actually matter how they are ordered. Because of the tight spacing, you will fold over one lead on each resistor, insert one resistor at a time and solder one of its leads. Insert the adjacent resistor and solder their shared pads, and so on, until all four resistors of the group are fully installed. Then do the other groups.

Install the other small resistors around the PCB. Install D3. Install the trimpots. SV1-Zh has no trimpot; SV1-Zp and SV2-Zh each have one; SV2-Zp has two. The AT and VC trimpots have different pinouts, so they cannot be mixed up. The adjust knob should face the edge of the PCB.

There are three physical sizes of power resistor, so begin with the smaller ones. R21 1k is near the back edge by QN4. Bend one lead over as usual and insert both leads leaving about 6mm exposed beneath the body of the resistor. Splay the leads so it does not fall out when soldering. Check for squareness to the PCB and trim the leads. Repeat with R7,19 330k. Then with R17,18 75k. On SV2 there is also R30,31,32 75k.

Now install the large resistors, R4,8 150k, R13,14 120k and R22,29 330k. The latter have fairly short leads compared to the other and may have some adhesive on their leads from the cut tape they are shipped on. Clean this adhesive off prior to installation.

Now install QN3. Insert its leads so that about 2mm protrude on the old side. Solder one pin and make sure the part is square to the board, then solder the remaining leads. On SV1-Z install QN7 the same way, being careful to have the printed side facing into the board. Similarly, mount QP6 on SV2-Z.

When mounting M1,2, discharge your body of electric charge before removing them from the anti-static bag. Insert the leads all the way through the board; splay the two outside leads; push the leads back through the board until the body of the mosfet is about 12-15mm above the board. Solder one lead then check squareness, then solder the remaining leads. If you have inserted both mosfets at the same time and checked that they are about level with each other, alternate the lead soldering from one mosfet to the other, and back then forth to ease the heating of the devices.

Insert the electrolytic caps one at a time using the pinch method from post-1. Note that C5 is a smaller diameter than the other three caps.