Detailed Assembly of 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. The tops of the transistors should be about 3-4mm above the top of the trimpots.
4 – Small BJTs QP second
Open the QP bag and sort them. For SV1 these are all 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.
To achieve an even height with the small BJTs, insert the resistor according to the orientations listed below, flip the board and push the resistor/diode down until it rests on the table, check that it is vertical, trim the leads and solder per the individual directions below.
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, but leave it for now. 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.
An easy way to assure the alignment of M1,2 is to use a piece of flat material that has a square edge. Draw a pencil line about 3cm away from the edge and parallel to it. With the mosfets inserted on the PCB, push the edge of the PCB against the edge of the flat material and then gently adjust each mosfet until the top edge of the metal tab aligns with the pencil line. make sure both devices are square.
Now install QN3. Insert its leads so that about 1-2mm protrude on the solder side. The printed side faces into the board. 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 others 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.
Once Assembled…
Follow the kit notes carefully, regarding installation and testing. if you find that the screen voltage at the tube socket does not go all the way to zero, short out D3,R3 at the front left corner of the board using a piece of cut-off resistor lead.
