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 current SV1 and SV2 Precision Power Scale kits using the Z-core.

This is a 2-hour build

Use flush-cut cutters.
Use thin solder and a conical tip iron.
Use a 50-60W iron.
Use a magnifier.
Trim solder points as you go.
Take your time.
Take breaks.

1 - Sort the components
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.

Note that some of the resistors are on tape; leave these as they are until you need them.

Note that there are two small bags labelled "QN" and "QP". leave these unopened until you need them.

2 - Trimpot(s) and poly cap(s)
Along the front edge of the PCB are a single trimpot for SV1 and two trimpots for SV2. The trimpots have different footprints so they cannot be mixed up.

Insert the trimpot and then splay its leads. Trim the leads and solder just one. Check that the pot is square to the board both vertically and horizontally, then solder the other two pins. Trim any solder points.

There is a small white poly cap on SV1 (C1) and a second on SV2 (C6). Insert the cap and splay its leads. Trim then solder and check squareness to the PCB. reheat a connection while adjusting position as necessary. Trim any solder points.

3 - Small BJTs QN first
Open the QN bag and sort them. There will be one small transistor that is different, marked as 2N3904, which goes in the QN4 position at the back edge of the board. Push the leads through about half way so the top of the transistor is taller than the trimpots by about 2mm or so. Flush cut then solder.

With the small transistors, there are three parallel groups of two in the middle of the PCB; QN1+QN2, QP2+QP3, QN5+QN6 and QN7+QN8. 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.

Install QN7+QN8 first. Insert, trim,solder.

Install QN1+QN2 second. Insert, trim,solder.

Install QN5+QN6 next. Insert, trim,solder.

All of these should be the same height so the PCB rests stably on the table while you solder.

4 - Small BJTs QP second
Open the QP bag and sort them. For SV1 these are the same, but for SV2 there is one different device BC560C which is QP4. Install as above.

Install QP2+QP3. Insert, trim, solder.

Install QP1. Insert, trim, solder.

Install QP5. Inert, trim, solder.

5 - Lie-down Rs and Ds
There are a few lie-down resistors and two diodes. These can be inserted, trimmed and soldered in one go, but it is more realistic to do just one or two at a time.

Insert the component and splay the leads. Flush cut on the axis of the component then solder.
R5,10,15,16 and D1,2 are common lie-down parts for both kits.
Sv1 also has R20, 24,25,26,27,28.

6 - Small cordwood Rs and Ds
The bulk of the cordwood components are small resistors and diodes. these are installed so that their height is the same as the small BJTs. For each resistor, we prefer to have the tolerance band at the PCB with the opposite lead bent around in parallel with the resistor body. This allows all the values to be read easily from one vantage point.

Begin with R4. Insert the leads; push the resistor down until it is about as tall as the transistors; then flip the PCB so the resistor can be adjusted slightly height-wise and trim its leads. Solder. R4 is oriented with its bent lead against the AT trimpot.

There are two series strings of four resistors on SV2, and three strings on SV1. Each string has three 82k5 and one 84k5. These are the resistors on the tape. At this time, pull the tape off the resistor ends and keep these two value groups separated.

Install R5A 84k5 with its bare lead towards the front edge of the board. Insert, trim solder.
Repeat with R5B, R5C and R5D 82k5 each.

Install R11A 82k5 with its body next to QN1. Solder only the body lead.

Insert R11B 82k5 with its bent lead next to R11A's bent lead. trim and then solder both bent leads.

Insert R11C 82k5 with its bent lead towards R11B's body. Trim then fully solder these adjacent pads and R11C.

Install R11D 84k5 with its body next to R11C's. Insert, trim, solder.

Install R19 with its body towards M1. trim, solder.

Install R1 with its body towards PS. Trim, solder only the body lead.
Insert R2 with its bent lead next to R1's bent lead. trim, solder fully.

Install R3 with its body towards R5. Trim, solder only the body lead.

Insert D3 with its anode lead bent parallel, bent lead beside R3's bent lead. trim, solder fully.

On SV1: Insert R23A 82k5 with its body towards R26. Solder only the body lead.
              Insert R23B 82k5 with its bent lead next to R23A's bent. Trim, solder fully.
              Insert R23C 82k5 with its body next to J1. Solder only the body lead.
              Insert R23D 84k5 with its bent lead next to R23C's bent lead. Trim, solder fully.

On SV2: Bend the node lead of D4 UF4007 to parallel the body. Insert with the cathode towards R34. trim, solder.
               Insert R33 332k with its bent lead towards the VC trimpot. Trim, solder.
               Insert R24 332k with its body next to the VC trimpot. trim, solder.
               Insert R34 with its bent lead next to the VC trimpot. trim, solder.
               Insert R23 332k with its body next to the VC trimpot. trim, solder.
               Insert R36 475 with its body towards the OC pad. trim, solder.
               Insert R37 1k with its body towards W2. trim, solder.
              Insert D5 with its body towards W2 and its anode lead bent parallel. trim, solder.

7 - Large mosfets
From here on, it is handy to have a roll of electrical tape or similar to lie flat on the table. We will use it to prop up the front edge of the PCB by resting the components already mounted on the tape roll.

When mounting M1,2, discharge your body of electric charge before removing them from the anti-static bag. For SV2, M3 is also in this 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 M1,2 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.

Now install QN3. Insert its leads so that about 1-2mm protrude on the solder side. Solder one pin and make sure the part is square to the board, then solder the remaining leads. On SV1 install QN7 the same way, being careful to have the printed side facing into the board.

For M3 on SV2, push the leads fully through the board, splay the outside leads the push the device back through so there is 1-2mm showing on the solder side. Solder the middle pin, check for squareness then solder the remaining pins.

8 - Large Rs
There are three physical sizes of power resistor, so begin with the smaller ones. All of these resistors have a circle indication on the PCB to show which lead is the body. Again, bend the lead that is at the beginning of the value indication.

Install R21 1k 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. For R22.29 leave about 1mm of the bent lead protruding through the board. Solder both leads then trim the body lead.

9 - Electrolytic caps
Insert the electrolytic caps one at a time using the pinch method from post-1. These are all the same value and voltage rating, so no sorting is required.

10 - Pots
The pots supplied have PC pins, but we also provide small PCBs for them to mount on. It is easiest if you have automatic wire strippers to use as a quick vise, or an actual vise. Lightly clamp the pot bushing with the pins up. Slip the PCB over the pins with the silk-screened side facing the pot. Solder the middle pin while holding the board up so it is perpendicular (right angle) to the pot body. now solder the outside pins.

[K O'Connor]

Hi Guys

With the very close spacing of the solder pads on many of the kit PCBs, it is almost a MUST that the assembler uses FLUSH CUT lead trimmers, as shown in the photo.

When you install a component, you should splay the leads then flush cut them, or slightly cant the cutter heads 1mm up from the board surface. Either way, when you make the cut the component should not fall out when you set down the board to solder. The actual flush cut leads may seem a bit scary to do, but you can make a nice solder connection.

With the 1mm canted cut there is a small protrusion of the leads and this will seem more "normal" to most hobbyists and techs.

In any case, the point is to have solder connections that do not have long points. You can alternatively do the shallow canted joint, or something more usual to your experience, then trim the points off. You may have to retouch a joint after trimming this way, and if you do, retouch only one connection at a time, wait a second or two before doing the second lead of the same component.

In the photo, there are three models of cutter shown. I use the YTH-23 which has the shallowest angle. You may prefer the more angled versions, and it certainly does not hurt to have all three in your tool drawer. These are from Aliexpress, but you can buy similar from domestic suppliers. try to buy something that looks as close to these as possible, as there are a lot of "flush" cutters that cannot make a flush cut, and are more generic wire cutters.

Having the right tool for the job makes life a lot easier 

Have fun