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SV1 and SV2 assembly
#1
Hi Guys

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 Smile

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.

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 type 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 taxis 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
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