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16-Channel g Desk, Pt.4 In this final article on our new 16-Channel Mixing Desk, we present the test procedure for the equaliser/VU boards and the wiring details for the whole mixer. By JOHN CLARKE & LEO SIMPSON As we noted last month, this mixer is a very big unit to assemble and so it is wise to proceed slowly and carefully. By rushing in and assembling all the boards at once, you could make a lot of mistakes and have a lot of work to do to fix them. By far the best procedure is to assemble and test one of each of the boards first, before building the others. So far then, we have given the assembly procedure for all the boards and the test procedure for all of the boards except this last one, the equaliser/VU board. There ,"';""·•,'7:'" ""t> ' ' are four of these and they each carry a master output level control. The effects board is the biggest used in the mixer and it carries the most circuitry. To check it, you will need to refer to the circuit diagram on page 67 of the March 1990 issue and the wiring diagram on page 73 of the April 1990 issue. You will also need to wire a 1okn slider (VR13) to the board. Now connect the + 15V, -15V and 0V power inputs for the equaliser part of the PCB and the + 15V, - 15V, + 5V and 0V for the display section of the PCB. Apply ~"->-!=l~~.'"::' ' This rear panel view shows the mounting locations for the six 6.5mm insulated mono panel sockets and for the four 3-pin XLR male plugs. The 16 XLR 3-pin sockets are mounted to the right of the XLR plugs. 48 SILICON CHIP power and check for + 15V on pin 8 of IC9, IC10, IC11 and IC20, pin 7 of IC13, IC16, IC17, IC18 and IC19, pin 4 of IC12 and IC20, pin 3 of IC14 and IC15, and pins 2 and 14 of IC21. For the - 15V rail, check pin 4 of IC9, IC10, IC11, IC16, IC17, IC18 and IC19, and pin 11 of IC12. Apply a 10mV signal to the channel input (marked CH1/CH3 on the layout diagram) and measure the signal at pin 1 of IC9. It should be 340mV RMS. Theil with the master fader VR13 at maximum setting, a 1.39V RMS signal should be present at pin 7 of IC9b. Wind the slider up and down and observe that the signal can be varied over the full range down to zero. With VR13 at maximum setting, check for a 1.39V signal again at pin 7 of IC10. The operation of the 5-band equaliser is checked by setting your audio oscillator to 60Hz and measuring at pin 7 of IC10. With the 60Hz pot at full clockwise rotation, the signal should be boosted by about 12dB (4 times the signal) to 5.5V. Check that the cut is - 12dB (1/4 the signal) to 350mV at full anticlockwise rotation of the 60Hz pot. In a similar manner, check the 240Hz, lkHz, 3.5kHz and 10kHz equaliser bands. · Now with all equaliser pots recentred and a lkHz signal, check for a 1.39V signal at pins 1 and 7 of IC11. The signal at pin 1 should be inverted with respect to the signal at pin 7. You can check this with an oscilloscope or by connecting your multimeter across pins 2 and 3, the balanced line outputs of the board. The AC reading should be 2.78V RMS; ie, the outputs add. Without adjusting the signal levels, you should also find 1.39V AC present at the monitor output; ie, the headphone output. The effects, equaliser/VU & preamplifier boards are all secured to the front panel via their rotary pots. While you have been doing the above checks with signal present, the level meter should be operating. The bar section of the display should rise with increasing signal level and when the signal is reduced in level via the fader, the peak hold LED should drop back at a slower rate than the average bar signal. The TRIM3 and TRIM4 trimpots can be adjusted by first removing signal by shutting off the fader control. Adjust both TRIM3 and TRIM4 so that the lower LED for the ba'r and dot display just extinguishes. TRIM2 allows the setting of the display to read 0dB for a 1 V RMS signal at the balanced line output. This can be done now. This completes the testing procedure for the boards. Now to repeat what we said before: only one board should be assembled and then tested, for each of the mixer boards. When the tests on these initial boards are complete, then and only then should you assemble the other boards. Otherwise, if you There are 20 slider pots on the front panel & these are all mounted using 15mm spacers, grub screws and black dress screws (see text). assemble all boards first , you may find yourself undoing a lot of mistakes. Now we can proceed to the mechanical assembly of the mixer, starting with the main panel. Front panel assembly Before any wiring can be done, all the hardware has to be assemblMA Y 1990 49 Adjusting the rotary switches Single-core shielded cable is used for the connections between the 6.5mm jack sockets and the two effects hoards, while twin-core shielded cable is used between the 3-pin XLR plugs & the equaliser/VU boards. All 16 rotary switches are specified as 3-pole 3-position types. What you will be supplied with is 4-pole 3-position types which can be adjusted to give only three positions. To do this, you just remove the switch mounting nut and lift off the washer/stop. You then put it back so that the stop lug is back one setting. Screw the mounting nut back and rotate the switch shaft to check that the switch now has three operating positions. Install each rotary switch using a lockwasher underneath the securing nut. Orient the switch so that when set to the middle position, the flat of the shaft is parallel to the side of the panel and on the left hand side. The switch knobs (grey insert} can then be fitted. Mounting the slider pots This interior view shows most of the 16 identical preamplifier boards while Fig.6 in the April issue shows the wiring details. Install the leads in the order shown in the wiring table. ed onto the mixer panel. At all times you need to be careful not to scratch the finish of the panel. We suggest you lay an old towel or blanket over your workbench or table while this task is in progress. Before the printed boards are mounted, all the switches, sockets and slider pots need to be installed. As with the pots, the shafts of the 50 SILICON CHIP rotary switches need to be cut to 10mm in length. They can be cut with a hacksaw and cleaned up with a file so that the knobs go on easily. While you are working on these switches, nip off the plastic locating pin with a pair of side cutters. This must be done since there are no holes in the mixer panel for the locating pins. Now for the 20 slider pots: these are fitted with 15mm tapped spacers at each end and then mounted on the front panel. Supplied are two spacers, two grub screws and two black dress screws for each slider. The grub screw is screwed halfway into the slider mounting hole and then the tapped spacer is screwed down onto the the grub screw. The slider is then secured to the front panel with the black dress screws, with pins 1 and 2 oriented towards the bottom of the panel. The slider knob can then be fitted. The headphone sockets, in/out switches, 2/4 channel switch and overload LED bezels can now be installed. Note that the 16 toggle switches must be oriented so that the toggle moves sideways to agree with the front panel artwork. The PCBs can now all be installed. To begin with, just secure each board with a washer and nut on the top pot and one on the bottom pot. This is important since some of the boards may have to be removed to correct unforseen faults. For the same reason, do not install the knobs on the pots at this stage. The equaliser PCBs are mounted so that the LED displays fit flush with the surface of the front panel. If necessary, use more WOOD FOR CHIPS .. . WOOD FOR CHIPS ... WOOD FOR CHIPS ... WOOD FOR CHIPS .. WOOD FOR CHIPS .. WOOD FOR CHIPS ... WOOD FOR C Tool Box This month we're concentrating on tools. We get a Jot of requests for quality tools from professional . users. Well feast your eyes on this lot! ~ :i: <..J cc C) "Cl C) ~ ~ ::r: <..J cc C) "Cl C) ~ Bonus FREE Weller Pyro-pen With Giant Xcelite Tool Kit this month! The serviceman's dream is here in this huge assortment of Xcelite hand tools. Case is covered in posh tan leather grain vinyl and has removable tool pallets in lid and bottom plus solid brass hardware. 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'~-~j<:·,.:. ~~:;1;::::: ~':=:# ~ "'"ROSE ~ TO Z GlAOESVIUE BRIDGE . § .,, Tax exemption certificates accepted if line value exceeds $10.00. BANKCARD, MAS TE RCA RO, VISA , CHEQUES OR CASH CHEERFULLY ACCEPTED ~ a C) a "" :i:, r, ::r: j °t Jo 0 ~ss.1.11,:.---- -.. _ _ _ J........... _ I J....... L2... Here are the actual size artworks for the PC boards used in the 16-Channel Mixing Desk. They are as follows: SC01102901 (effects board - 2 required); SC01102902 (equaliser/VU board - 4 required); SC01102903 (preamplifier board - 16 required); and SC01102904 (power supply board - 1 required). 52 S ILICON CHIP 0 1 T'" 0 (.) ~ ~ 1--o 1 ;,_i O 0 ~ -- J sc~ 01102901 o-0 I MAY 1990 53 washers on the pots to allow this. Chassis assembly & wiring The chassis can now be assembled by installing all the sockets and plugs on the rear panel. The rear panel label for the effects and foldback sockets should also be affixed at this stage. In addition, the wooden side panels can be installed. The assembled front panel can now be laid face down on your workbench (still covered with a towel to prevent the panel being scratched), ready for wiring. The wmng is divided into three separate sections: the top and the lower sections of the panel, and the wiring between the control panel and the chassis components. We suggest that you begin by wiring the lower part of the panel; ie, the sliders, in/out switches and headphone sockets. To aid in the wiring process, we have included a chart which lists the colour of wire, the number of wires and the length required. This should be read in conjunction with the wiring diagram for each printed board. We recommend that constructors follow the wiring table in the order described for ease of construction. When wiring between the PCBs and the chassis, it is easier to keep the top panel lying flat on a table and the chassis lying on its back rather than the chassis standing on its feet and the panel upright. Mixer testing Once wiring is complete, go over your work carefully to check on any faults. These should be corrected before switching on the power. Keep the mixer in its wiring position with the front panel lying flat on the bench and the chassis upright for initial testing with the power switched on. When power is first switched on, immediately connect up a multimeter to check on the voltages applied to each PCB. Check that the power supply is delivering the ±15V and ±5V rails. If not, switch off immediately and check your connections again. Once the power supply rails are up and running, you can check the 54 SILICON CHIP Wiring Table For The 16-Channel Mixer LOWER PANEL WIRING Input Preamplifier PCB BLU 50mm x 1 4 between poles of in/out switches - foldback bus GRN 50mm x 1 4 between poles of in/out switches - effects bus RED 50mm x 14 between poles of in/out switches - CH1 /CH3 bus YEL 50mm x 14 between poles of in/out switches - CH2/CH4 bus Note: keep 1-8 and 9-1 6 buses separate RED 90mm x 1 6 headphone monitor socket signal to PCB GRN 90mm x 1 6 headphone monitor socket ground to PCB Note: readers may wish to connect the socket for mono operation with stereo headphones . BLK 11 0mm x 32 faders pins 2 and 3 to PCB BLK 180mm x 16 faders pin 1 to PCB BLU 80mm x 16 wiper of in/out switch to foldback out on PCB GRN 80mm x 16 wiper of in/out switch to effects out on PCB RED 80mm x 16 wiper of in/out switch to CH 1/CH3 out on PCB YEL 80mm x 16 wiper of in/out switch to CH2 /CH4 out on PCB BLU 70mm x 14 -1 5V power bus between PCBs RED 70mm x 14 + 1 5V power bus between PCBs GRN 70mm x 1 4 OV power bus between PCBs -1 5V power bus between 1 -8 input and 9-1 6 input BLU 100mm x RED 100mm x 1 + 1 5V power bus between 1 -8 input and 9-1 6 input GRN 1 OOm117 X 1 OV power bus between 1-8 and 9-16 input PCBs Equaliser/VU PCB BLK 11 0mm x 8 faders pins 2 and 3 to PCB BLK 1 80mm x 4 faders pin 1 to PCB RED 450mm x 1 CH2 bus from in/out switch to CH2 PCB YEL 450mm x 1 CH1 bus from in/out switch to CH1 PCB RED 1 90mm x CH4 bus from in/out switch to CH4 PCB YEL 190mm x 1 CH3 bus from in/out switch to CH3 PCB GRY 290mm x 1 CH1 headphone signal lead to CH1 and CH2 headphone socket GRY 290mm x 1 CH2 headphone signal lead to CH1 and CH2 headphone socket GRN 290mm x 1 headphone ground lead to CH1 and CH2 headphone socket GRY 150mm x 1 CH3 headphone signal lead to CH3 and CH4 headphone socket GRY 1 50mm x 1 CH4 headphone signal lead to CH3 and CH4 headphone socket GRN 1 50mm x 1 headphone ground lead to CH3 and CH4 headphone socket BLU 70mm x 3 + 1 5V supply bus between PCBs RED 70mm x 3 -1 5V supply bus between PCBs GRN 70mm x 3 DV supply bus between PCBs Effects PCB BLU 600mm GRN 600mm RED 220mm YEL 220mm BLU 300mm x 1 1-8 foldback bus to 1 -8 PCB x 1 -8 effects bus to 1 -8 PCB x 1 -8 CH2 bus at preamplifier PCB to CH2 PCB input x 1 1-8 CH1 bus at preamplifier PCB to CH1 PCB input x 1 9-1 6 foldback bus to 9-1 6 PCB GRN RED YEL BLU RED GRN BLU RED GRN BLU 300mm 170mm 170mm 100mm 100mm 100mm 70mm 70mm 70mm 110mm switch GRN 11 0mm switch RED 110mm YEL 11 0mm x 9-1 6 effects bus to 9-1 6 PCB 9-16 CH4 bus at preamp PCB to CH4 PCB input x 9-16 CH4 bus at preamp PCB to CH3 PCB input x x -1 5V supply bus on PCB to equaliser PCB x + 1 5V supply bus on PCB to equaliser PCB x 0V supply bus on PCB to equaliser PCB x 1 -1 5V supply bus between effects PCBs x 1 + 15V supply bus between effects PCBs x 0V supply bus between effects PCBs x 2 foldback bus from each effects board to 2/4CH x 2 effects bus from each effects board to 2/4CH x 2 CH1 and CH3 to 2/4CH switch x 2 CH2 and CH4 to 2/4CH switch TOP PANEL WIRIN G Input Preamp PCB Wiring of switch S1: single core shielded cable 70mm length x 6 per switch x 1 6 switches Equaliser PCB Bargraph Display Power Bus BRN GRN RED BLU 45mm 45mm 45mm 45mm x x x x 3 3 3 3 +5V bus between PCBs 0V bus between PCBs + 1 5V bus between PCBs -15V bus between PCBs CHASSIS TO FRONT PANEL WIRING Input & Output Wiring Figure-8 shielded cable 280mm x 1 6 from preamplifier input plug to PCB (note numbering of pins on rear of plug) Figure-8 shielded cable 51 0mm x 4 from equaliser output socket to PCB (note reversal of numbering on socket pins) Single shielded 400mm x 2 effects return input socket to 1 -8 and 9-1 6 PCBs Single shielded 4 1 0mm x 2 effects send output socket to 1-8 and 9-1 6 PCBs Single shielded 420mm x 2 foldback output socket to 1 -8 and 9-1 6 PCBs Power Supply wiring RED BLU GRN RED BLU GRN RED BLU GRN BRN GRN Twin 460mm + 1 5V preamplifier power supply bus to power supply 460mm -1 5V preamplifier power supply bus to power supply 460mm 0V preamplifier power supply bus to power supply 460mm + 1 5V equaliser power supply bus to power supply 460mm -1 5V equaliser power supply bus to power supply 460mm 0V equaliser power supply bus to power supply 230mm + 1 5V bargraph display supply bus to power supply 230mm -1 5V bargraph display supply bus to power supply 230mm 0V bargraph display supply bus to power supply 230mm + 5V bargraph display supply bus to power supply 460mm 0V on power supply to earth lug on chassis core polarised 480mm lead to power LED mixer operation with signal applied. Before you do this, switch off the power and attach the front panel to the chassis . Mixer testing is made easier if you have an XLR line plug for applying signal to the inputs, and an XLR line socket for measuring the output signal. Also, two mono 6.5mm line plugs are useful for measuring the Effects and Foldback outputs and applying signal to the Effects return input. Set the 2 Channel/4 Channel switch to 2 channel operation and set a channel input for "Line Unbal" mode. Apply a 100mV 1kHz signal to pin 2 or pin 3 of the XLR input socket input and check for signal at the CH1 output. With the input 1 PAN control set centrally, the CH1 and CH2 bargraph display should indicate the same signal level, provided the main faders are set equally. Adjust the PAN control to see its effect on the CH1 and CH2 outputs . This change will be seen on the LED VU displays. Rotate the input (pre-fade) attenuator (fully anticlockwise) and set the channel fader fully up . The overload LED should now be alight. Now reduce the signal with the attenuator until the overload LED is extinguished. Check also that the in/ out switch works and that you can monitor the channel signal output with headphones . Now test the Effects and Foldback outputs at the 6.5mm socket outputs. The signal levels at these outputs are adjustable with the Effects and Foldback Sends controls for each of the input channels and with the Effects and Foldback output controls . A signal can now be applied to the CH1/CH2 Effects return input to check that it is applied to the CH1 and CH2 bus . The CH1 and CH2 display should show the signal level and this is adjustable with the Effects return control. The pan control adjusts level between CH1 and CH2 . The remammg 2-8 channel inputs can be tested similarly and the entire 9-16 inputs can be tested with the signal measured at the CH3 and CH4 outputs . Finally, the Mixer can be tested for 2 channel operation. ~ MAY 1990 55