Unbalancer 9-pin.pdf - Tube CAD Journal

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14Unbalancer 9-Pin PCBAssembly & TestingAssembly Cleanliness is essential. Before soldering, be sure to clean both sides thePCB with 90% to 99% isopropyl alcohol. Do not use dull-looking solder; soldershould shine. If it doesn’t, first clean away the outer oxidation with some steel woolor a copper scouring pad. If the resistor leads look in the least gray, clean away theoxidation with either steel wool or a wire sniper’s sharp edges. Admittedly, with newresistors and a fresh PCB, such metal dulling is rare; but if the parts have sat in yourcloset for a year or two, then expect a good amount of oxidation to have developed.First, solder all the small diodes in place, and then solder the resistors, rectifiers,capacitors, and heatsinks. Be consistent in orienting the resistors; keep all thetolerance bands on the resistor’s body at the right side as you face the resistorstraight on. This will pay dividends later, if you need to locate a soldered a resistor inthe wrong location. Because the board is double sided, with traces and pads on eachside, it is easier to solder the resistors from their top side. It is often easier to attachthe LD1085 (heater regulator) to its heatsink first (using the heatsink hardware kit)and then to solder both the heatsink and regulator to the PCB at once. As the PCB isso overbuilt, it is extremely difficult to remove an incorrectly placed part. Be sure toconfirm all the electrolytic capacitor orientations, as a reversed polarized capacitorcan easily vent (or even explode) when presented with high-voltage. Confirm twice,solder once.Testing Before testing, visually inspect the PCB for breaks in symmetry between leftand right sides. Wear safety eye goggles, which is not as pantywaist a counsel as itsounds, as a venting power-supply capacitor will spray hot caustic chemicals. Make ahabit of using only one hand, with the other hand behind your back, while attachingprobes or handling high-voltage gear, as a current flow across your chest can result indeath. In addition, wear rubber-soled shoes and work in dry environment.Remember, safety first, second, and last.1. Attach only the heater power supply’s transformer winding, leaving thehigh-voltage transformer leads unattached and electrical tape shrouded, withno tubes in their sockets.2. Use a variac and slowly bring up the AC voltage, while looking for smokeor part discoloration or bulging.3. Measure the heater regulator’s output voltage without and with a load. Ifthe heater regulator fails to regulate, try either lowering the heater voltage atad, for example 12V instead of 12.6V, as the 0.6V difference might beenough to bring the regulator back into regulation.4. Next, power down the heater regulator and attach the high-voltage windingsand insert the tubes in their sockets.5. Attach the transformer to a variac and slowly bring up the AC voltage.6. Measure the voltage across ground and B-plus pads in the center of thePCB; then measure the voltage across capacitors, C2 & C4, at the bottom ofR12. If the two channels differ by more than 10Vdc, try switching tubesfrom one channel to the other. If the imbalance does not follow the tubes,there is a problem, probably a misplaced part.Only after you are sure that both heater and B-plus power supplies are working well,should you attach the line-stage amplifier to a power amplifier.
GlassWare Audio Design 15Double-Sided Board Because the board is double sided, with traces and platedthroughpads on each side, it is often easier to solder the resistors from the top side.In addition, it is often easier to attach the LD1085 (heater regulator) to its heatsinkfirst (using heatsink hardware kit) and then to solder both heatsink and device to thePCB at once. You may solder the tube sockets on the top of the PCB and solder allthe remaining parts on the bottom, which allows attaching the PCB to the chassis’stop panel, with the tubes protruding through holes, which can easily be done with theUnbalancer PCB, as the solid-state regulator and LM334 can be soldered on thebottom without having to twist their leads to conform to the inverted pad positions,as the heater regulator gets a redundant set of solder pads on the PCB's bottom side.Just be sure to attach the regulator the other side of the heatsink.Capacitors Because the PCB is so overbuilt, it is extremely difficult to remove anincorrectly placed part. Thus, be sure to confirm all the electrolytic capacitororientations, as a reversed polarized capacitor can easily vent (or even explode) whenpresented with high-voltage. Confirm twice, solder once. In addition, large couplingand bypass capacitors should be adhered to the.Fusing Usually a fuse is placed in series with the power transformer’s primary, butboth the high voltage and the heater secondaries can receive fuses as well. Use a slowblow fuse for the heater and quick blow fuse with the B+ output.Standoffs Tubes are microphonic by nature, being mechanical structures. Thus, itpays to prevent vibrations from reaching the tubes. One easy technique is to use smallrubber O-rings below and above the PCB where the screws enter the PCB’s mountingholes. The screws should be just tightened enough to keep the PCB from ringing orrattling when tapped.RC Power-Supply FilterResistors R17 and R12 are the voltage-dropping resistors in the two RC powersupply filters used in the Unbalancer. Resistor R17 is used in the first RC filter. Itsvalue can be anything between 1 to 1kohms. This resistor will see the entire highvoltagecurrent draw of the Unbalancer line-stage amplifier. (This resistor can bereplaced by a low-DCR choke that mounts on the chassis.)The CCA kit supplies five pairs 3W resistors for R12 use: 1.6k, 2k, 3k, 3.9k, and10k. Page 9 holds a chart that shows the voltage drop across the R12 versus thecurrent flow. Remember each channel gets it own R12 resistor. For example, aUnbalancer line-stage amplifier might run a total of 20mA of idle current perchannel, for a total of 40mA for the entire line-stage amplifier. So by looking up the20mA column, we can see the resulting voltage drops. Thus, a 2k resistor will drop40V, so a 300Vdc raw DC power supply will deliver 260Vdc to the input and outputtubes. An * denotes excessive current or voltage, so that resistor value cannot be usedwithout risking damaging the at least one of the resistors.from C7R17R12to tubesC8C5C4
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Page 23: Top Side PCB Mechanical Layout6.0 i

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