HAMPTONe2. Channel Card Assembly
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The HJFP channel card consists of two JFET (junction field effect transistor gain stage with buffers) configured as a microphone preamplifier. The input switching section allows 4 input choices: mic, mic -14dB, instrument with 2.2Mohm input impedance, and line with 160Kohm input impedance. The three controls from left to right are: +48 vdc, pad, and phase. The DI jack, power input (gnd, +24vdc, +48vdc) and gain control are wired via point-point, everything else is on the pcb. If you look at the front of the board, you will notice the ground plane is separate from the input to the output sides of the circuit, this is by design. It isolates the two and prevents the output from coupling into the input and causing hi frequency (RF) oscillations or hi frequency roll off (depending on the position of the pad). There is also a small protoboard section in the back that can be used for customizing the design.
The basic procedures is to populate the board starting with the lowest components (resistors) and working up to the tallest (transformers and caps). This makes assembly much easier
I recommend using solder that has water-soluble flux for easy cleanup. Also solder wick is needed in case a part needs to be removed. If so, be very careful not to over work the pads on the boards or they can be damaged. If you damage or lose parts, they are all available from Mouser.com, or Digikey.com where we buy them (except the audio transformers, pcbs, and chassis). We went to great lengths (every bag of parts you receive has been visually checked twice, and then weighed) to ensure the kits are complete. So make certain when you start opening bags of parts, that you do not lose anything, many of these parts are very small.
1. Wipe the board off with isopropyl alcohol and avoid touching any areas that you will be soldering.
2. Using an ohm meter, organize the audio channel resistors on the resistor worksheet. Following the parts list and layout, populate all the resistors, then hold them in place with a piece of duct tape or similar. All the resistors on the channel card should be snug (not tight). All the footprints (the physical arrangement of the leads) go either straight up and down, or from left to right, none go at an angle. If a part doesn’t seem to fit, or it crooked, you probably have it in the wrong hole.
3. Flip the board over, solder them in place, and then cut the leads. The leads should be cut at the top of the solder joint, not too close to the board, and not leaving any unsoldered lead. A good solder joint is concave down (like a volcano), and is continuous all the way around without any voids. You should use a small amount of solder on each connection, so there are no blobs, or balls of solder. NEVER move parts while the solder is cooling (bigger leads take longer to cool) or the joint will be compromised. Alternate from one part to another, don’t solder the two ends of the same part in sequence unless you wait for the first one to cool. It is easy to disturb the first connection while doing the second one.
4. Install the heatsinks on the 4 NPN transistors (ZTX 603 & ZTX653 two are darlington, a transistor that actually has two devices connected together to yield very a high beta or current gain). Make certain the heatsinks fit tight and cannot fall off. If needed, crimp the heatsinks slightly so they mount tightly. Form the leads to fit in the pcb "tripod" footprint. These leads break very easy, so be very careful not to bend them more than once. Press them into the appropriate places snug, but not tight, against the board. Make certain not to mix up the darlington NPNs (ZTX603) with the regular ones (ZTX653). The circuit works with them reversed but not correctly. Again hold them down with tape and solder them in place. Clip the leads. Install two 2N5457 JFETs per channels; these devices have all been tested and matched. If you lose or damage one, see the "biasing JFETs" page included in these instructions. Since they are only about $0.60, you can buy 10 and pick the best ones. Testing and selecting them only requires a DVM (Digital Volt Meter), even though we check many parameters. Be careful in future steps not to bend these. On the active device bag there are several resistor values, one will be circled. Write this value down on your parts list for future reference in case one is damaged.
5. Put the black caps on the three switches, and install them on the board. Pop the switches out so the spring has less tension, and solder them in place. Make certain they are tight against the board in the front and back, otherwise they will hit the front panel. DO NOT overheat these switches or you will damage them. These switches have about 3 times the operating life as a toggle, if they are installed properly.
6. Install the big caps C4 and C6, the main audio coupling caps. These are high-grade polypropelene with a very low dissipation factor (they are black and cylindrical in shape, with the leads coming out the ends). These have no polarity, though it is usually good practice to orient them so you can read the value on the part once its installed.
7. Next install the electrolytic caps (the tall cylindrical shaped caps with leads coming out one end). Its good practice to do one or two at a time to ensure they are tightly seated against the board. If they are not, the caps will stand up crooked, it will work fine but looks sloppy. All of the negative ends of the electrolytic caps connect to the ground plane (the large circuit board trace that extends around the perimeter of each channel, this is signal ground, and is labeled "G" in multiple places). The negative side of the electrolytic caps is indicated with a stripe, and has negative symbols printed in the stripe. If any are in backwards, they will be damaged when powered up. If you ever reverse polarize an electrolytic cap and apply power to it, replace it. They may still work but will more than likely fail prematurely, or work improperly.
8. Remove the inner plastic contacts of the XLR connectors. Screw them in place with the ¼" #4 self tapping screws (they look like small wood screws, they may be philips or regular) through the holes in the board. These screws are in the "AUDIO HARDWARE" bag. After the screws are tight, solder them in place making certain they are sitting flat on the board, and point straight ahead. Clip the leads off just like the other components, so they don’t stick down.
9. Using the point-point wire table in your instructions, cut all the point-point wire to the lengths given in the table. There is 12" extra of most colors. Be careful not to mix up the wire gauges, 22 gauge is the thinner of the two and is used in the audio channels, 18 gauge is the heavier of the two and is used for the AC power section.
10. DI jack point-point: Strip the ends of the single conductor hookup wire listed below (and on the point-point wire guide) on each end roughly 1/8". Lightly tin them with solder, but be very careful to keep the leads tightly twisted together, and use only the smallest amount of solder possible otherwise they will not fit into the holes on the pcb. Connect point-point wires to the corresponding holes on the pcb making sure all the wires go into the hole (frayed wires on the top side of the pcb can short between the wires causing problems) and leaving no exposed wire on the pcb side. The proper holes are located directly behind the female XLR (input) slightly off to the right. From left to right (looking at the board from the front) the colors go: yellow, red, green, and brown (the brown wire is back about 0.8"). The pads are labeled on the back of the board, do not use the 3 holes in back of the XLR slightly to the left, they are for the optional rear panel connection. All wires should be equal in length after installed. There is a spare "G" hole next to the "T/S" pad that is unused.
3" YELLOW: RING SHUNT "R/S"
3" RED:TIP "T"
3" GREEN: GROUND "G" or "S" (ground or shield)
3" BROWN: TIP SHUNT "T/S"
11. Twist them together and wire them to the corresponding terminals on the DI jacks. If you look closely at the back of the jack, the labels "R/S", "T/S", "R", and "T", are labeled on the jack. The "G" or ground terminal is the one coming out of the notched corner. Jumper the "R" to ground using a discarded piece of resistor lead you cut off earlier in the assembly. You can also use a short piece of buss or the spare hookup wire. Keep it short, but make certain when you plug in a ¼" jack, that it does not hit the wire or the DI input will not work.
12. Install the C3402-2 input transformers (the ones in the round mu metal cans) aligning the black dot on the label, to the square pad labeled "1" on the pcb. Solder all 8 leads to the board, making certain the transformer is snug against the board. Rev C (and higher) kits allow you to wire the input transformer in two ways, 4:1, or 8:1. Since the input transformer is a step up, the 4:1 will have ½ the gain (6dB less) than the 8:1, giving the preamp 6dB more headroom. If you are generally recording loud signals, use the 4:1. If you generally record softer acoustic material, or use ribbon mics, then you will probably want to wire them 8:1. If you are not sure, wire one each way and play with it to see which works better. If you look at the back of the audio pcb where the input transformer mounts, you will see 4 pads with the numbers 1 2 3 and 4 next to them. For 4:1, jumper 2-3. For 8:1, jumper 1-2 and 3-4. Use a small piece of discarded resistor lead for the jumper. DO NOT put these jumpers through the holes, just lay a small piece between the pads and solder it in place. If any wire lead comes through the board and touches the metal can (which is gnd), the transformer will not work right.
13. Remove the 4 small plastic tabs on the bottom of the C-2870 1:1 transformer being careful not to damage the metal pins. Install the C2870 output transformers. The output transformer pins are keyed so it only goes in one way (pins 1 and 2 are closer together than the rest of the leads). It can be a bit tricky getting all the pins in their holes simultaneously, DON’T FORCE IT! Again, make certain the parts are snug against the board. Solder all the pins, even the ones that are just pads.
14. Cut the wire listed in the point-point wire table for the volume control or "pot" (short for potentiometer) (2x7" red, 2x7"yellow, 4x7"green). Strip them back roughly 1/8" on each end inch using a wire stripper, and lightly tin. Twist them tightly into pairs: 2 x Red + Green, 2 x Yellow +Green. Each pot will get one RG and one YG pair.
15. For each pot (R22), solder the two green wires into the tab on the right if you are looking at the pot from the back (terminals pointing down). All of the wires should be attached so they come straight back off the pot, not down. Solder the red wire into the center terminal, this is the "RTN" or return. Solder the yellow wire into left terminal, this is the "SND" or send to the volume control. Measure DC resistance from the yellow wire to the green wire it is twisted with, it should read approximately 10k ohms (pots are +-10% in value), regardless of the position of the control. Measure DC resistance from the red wire to the green wire it is twisted with, it should read 0 (or very small) when the control is turned all the way down (CCW from the front view), roughly 1.7Khoms at mid position, and 10k when turned all the way up. It is also important not to stress the leads of the pot by applying pressure to them, they can break and cause the pot to be crackly or intermittent when operated. Snap the small tab off the pot, it is not needed.
16. Solder the other ends of the wire to the audio boards into the holes labeled "SND" (yellow) and "RTN" (red) toward the back of the pcb with the wires going into the top. Next to each hole is a pad labeled "GND", solder the green wires into these.
17. Clean the flux off the board being very careful not to get any water (in the case of water-soluble flux) on the component side of the board. DO NOT use paint thinner or any other strong solvent on the board, it will melt the solder mask, and possibly damage certain parts (trimpots, transformers…). Use isopropyl alcohol for acid core flux by applying a small amount to Q-tips and carefully scrubbing the boards. This can take quite a bit of work. For water soluble flux, dribble small amounts of water on the back of the board, while scrubbing with an old toothbrush. The flux will foam up and easily wash off. Thoroughly dry the board after cleaning. DIRTY CIRCUIT BOARDS OFTEN CAUSE PROBLEMS, DO NOT BLOW THIS OFF!
18. Check the 8 threaded XLR mounting holes on the front panel to make sure there is no buildup of paint inside them. If there is, run a bolt through the hole before installing the XLR connectors to clear the threads. Also check the LED holes (above the +48 switch), the LED should fit tight into the hole and be flush with the front panel (a small philips jewelers screwdriver works well for this). Clean out the 6 switch holes if necessary with a small file or an exacto of any paint buildup so the switches can operate smoothly. Install the outer shell to the 4 XLR connectors into the front panel using the 8 x 3/16" #4-40 flathead machine in the "Audio hardware" bag. If you want to be a super geek (like us), paint them satin black to match the XLR shells.
19. Inside the front of the each XLR is a locking mechanism that is accessed through the front with a regular jewelers screwdriver. CCW unlocks them, CW locks them. Turn these all CCW. Slide the electronics into the front panel; don’t force it. The connectors must be tightly pressed in from the back for the locking mechanisms to engage. Once the board is tightly in place with all 6 switches into their holes, secure it by locking the XLR's (turn the 4 internal locking screws CW). Check to make certain all the switches operate without rubbing against the front panel. If they do, they are not mounted flat against the board and need to be re-installed.
20. The phantom power LEDs attach to the top of the +48 switches. Cut/bend the LEDs so they match the template on the audio channel resistor worksheet. This will ensure they are cut to the correct length, and the slight outward bend of the leads holds them in place for soldering. Each LED has a flat side on the flange, this indicates the cathode or negative terminal. This side should wire to the front right terminal of the switch (looking at the board from the front). This is signal ground. The anode or positive side, should wire to the front left terminal. Before soldering, pop the switch out to relieve tension, and DO NOT overheat the switch. If your meter has a diode checker, you can check the polarity; if its properly polarized, LEDs will have a forward turn on voltage of roughly 1.5vdc (most meters will actually light up the LED).
21. Attach the potentiometers and DI jack to the front panel. The hole for the potentiometer has an anti rotation hole next to it for larger rotary switches (optional step attenuators we offer) if you choose to purchase them. The potentiometer does not need an anti rotation pin so remove it from the pot if you haven't already.
22. Attach the knobs to the pots, so that when they are all the way CCW, they point to about 7 o-clock, and fully CW at 5 o-clock.
23. Install the pilot light into the large hole in the center of the front panel, leaving it unwired for now. At this point the audio channels are complete other than the point-point wires for power.
Assembled Unit:
