Install all bypass capacitors, ceramic capacitors and all resistors. Most of the bypass caps are formed on .300" forms; resistors are .400" forms. Trim excess lead wires close to the board. Install a bare wire jumper in place of resistor R23. Leave resistor R22 unpopulated.

Once all discrete components have been installed, begin installing the IC chips. Start off with ALU chips U5-U8 and solder. Next, install the 74LS193 counter chips U9-U12 and decoder chip U20. Solder. Install the row of 74LS157 Data Selectors U13-U16 and state machine counter chip U22 next and solder. Install the far-right column of chips U19, U21, U23 and U24 and solder. Then install the two 74LS374 octal latches at U3 and U4. Solder. Finish off by installing the MC1455 op-amp at U27.

Install a 20-pin low-profile socket in the place of U1, U2, U25 and U26. Install a 32-pin low-profile socket in place of U17.

Install the two 5K trim potentiometers at locations R15 and R16. Then install all upright electrolytic capacitors. Be sure to pay close attention to polarity! Install transistors Q1 and Q2. Observe polarity.

Install voltage regulator chip U28. Be sure to use a heatsink! This part will get quite warm during operation (normal) and without a heatsink it will burn up quickly. Use of silicone or Teflon heatsink compound is also recommended. Use a 4-40 x 1/2" screw and nut to attach the chip and heatsink to the PC board. Next, install the other three voltage regulators U29, U30 and U31. These devices mount directly to the PC board using a 4-40 x 1/2" screw and nut without a heatsink.

Install input/output connector block CN1. Solder. Install power connector J1. Solder.

Finish assembly by installing the four control potentiometers. Install a 5K pot for R17 and R18, a 100K pot for R19 and a 10K for R20. Install a SPDT toggle switch at locations S1 and S2. Use short pieces of scrap wire to hook the terminals of the switch with the corresponding pads on the PC board located directly beside the switch. Wire the pad straight to the lug. Next, install slide pot R24 and tuning coil L1 (see text below). Use a dab of hot-glue to help secure coil L1 in place if necessary.

Once the board has been completely assembled, check for soldering errors, shorts, opens and for missing parts. Then install A/D converter chips U1, U2 and U25, D/A converter chip U26 and RAM chip U17 in their respective sockets.

Winding the L1 Tuning Coil

But after doing some research I found that Radio Shack no longer sells this coil and nowadays it is very difficult to find. So I figured out the alternative "home brew" version. So if you cannot find one of these nifty tuning coils, you can try alternate coils or make your own. Here's how to make this coil:

You'll need:

First, dis-assemble the pen. Throw away the ink cartridge, cap and end cap at top of pen tube.

From the end cap end of the pen tube, measure and cut at 3 inches. Discard the tapered end of the pen tube.

Draw a straight line down the center of the pen tube from end to end. Use this line as a straight line reference. Starting at the cap end of the tube, measure off 1 3/4" and then 2 3/4". Drill a 3/16" hole completely through the pen tube at these measurements. Next, measure off 1" from the cap end and draw a line around the circumference of the tube. Drill a .040" hole just outside the 1" line (between the 1" and 1 3/4" line up against the 1" line) completely through the tube. Drill another .040" hole 1/16" from the cap end of the tube. Drill this hole only half-way through (only one side of the tube).

Thread approxamately 2" of wire through the cap end .040" hole starting from the outside (push wire from the outside of the tube through the hole to the inside of the tube, then pull the wire through the hole to the outside). Begin winding the coil around the tube by holding the pen tube in your left hand. Twist the tube to wrap the wire while using your right hand and fingers to guide the wire smoothly around the tube. It does not matter which direction you wind the coil. DO NOT WRAP THE WIRE AROUND THE TUBE -- USE THE TUBE TO "ROLL" THE WIRE INTO A COIL. Continue wrapping wire until you reach the 1" mark. At this mark, hold the wire firmly against the tube and wrap ONE SINGLE LAYER of masking tape around the coil.

Continue winding the coil by wrapping the second layer on top of the first layer. Repeat the same procedure as outlined above. When you reach the cap end of the tube, put another piece of tape around the coil and begin the 3rd layer. Repeat this process until you have used up all but about 3" of the wire. You should have about 7 layers of coil. If the final layer does not come out even, don't worry, just "stretch" the last layer out so that the windings are even from end to end and go on to the next step. When completed, thread the end of the coil wire through the .040" hole just outside the 1" mark. Once the wire is threaded through the hole, wrap the wire around the pen tube once more to secure the wire in place, then put one more layer of tape around the entire coil. Once the coil is secured, cut the two coil wires to 1" long, then using a piece of fine sandpaper or steel wool, polish off the enamel on the coil wires from 1/8" of the coil to the tips of the wires.

Tolerance is not critical when winding this coil. If you go over the 1" mark or put a few more turns per layer it is not going to matter. As long as you get it mostly consistent from layer to layer. If the wire breaks while winding, you can splice it as long as you make a clean tack-solder splice (no twisting!). When completed, the coil itself will be about 5/8" round. If the coil is thick at one end or fat in the middle, then rewind the coil. It will not work right if the coil is not evenly wound.

Finally, with both wires of the coil facing DOWNWARDS, install two #4-40 x 1" screws through the 3/16" holes from the top and secure with a nut on the same side of the tube as the wires. Tighten these nuts firmly but not so tight as to collapse the pen tube. Then thread an extra nut on each screw. Thread this nut down on the screw so that the bottom face of the nut is even with the bottom edge of the coil winding. The extra two nuts are used to mount the coil to the PC board.

Mount the coil to the PC board. If necessary, adjust the lower two nuts on the mounting screws so that the coil does not touch the PC board. First thread each coil wire through the two holes marked, "L1", then align the two mounting screws with the corresponding holes in the PC board. Fasten coil to the board using the remaining two #4-40 nuts and tighten firmly. Then solder the two coil wires to the pads on the PC board.

You can make the coil more permanent by coating the coil in epoxy. You can mix up a batch of 5-minute 2-tube epoxy and use a swab or cheap paint brush to quickly spread the epoxy over the entire coil body. You can also mix the epoxy in a "puddle" on a piece of scrap cardboard, then roll the coil in the epoxy. Once coated, allow the epoxy to competely cure before you mount the coil on the PC board.

Adjustment and Calibration

Before you can use the Digital Delay, you must calibrate the input buffer amp and set the reference voltage.

Hook up the delay to your stereo's tape loop. Put on some music and adjust for background listening level. Do NOT engage the TAPE MONITOR at this time! RAP or hip-hop music is good for this test because it has a lot of speech and smooth rythms but any form of music will do.

Power up the delay. Adjust both pots R15 and R16 to mid-rotation. Using a digital voltmeter, measure the voltage at U25, pin 12 and adjust pot R15 so that the voltage at pin 12 is 2.7V.

Set both of the DELAY TIME controls and the DECAY control to full counter-clockwise rotation. Adjust the DEPTH control to full clockwise rotation. Set the MIX switch to the OFF position.

Turn on the TAPE MON on your stereo. You should hear the music play through, but delayed from the original signal. Adjust pot R16 for the clearest, cleanest signal. Then slightly tweak R15 if necessary to clean up the signal more. Adjusting R15 will affect the overall volume of the output as it sets the A/D and D/A reference voltage; be sure not to set this pot too far to the far counter-clockwise rotation (low reference voltage) or the signal will overdrive when louder signal bursts are present. If you set this adjustment too high (high reference voltage) the signal will collapse at low levels and will appear to be "squelched". The near perfect adjustment is centered around 2.7V but may vary slightly due to tolerances and application.

It is normal for a delay line to restrict some high-end bandwidth. The amount of attenuation is mainly determined by the value of the capacitors used in the output filters. You can modify the output amp section to give more bandwidth by altering the values of the filter capacitors C21, C24 and C34. Modifying these capacitors for more bandwidth may introduce unwanted hiss or white noise so be careful!

Once the delay has been adjusted, rotate the DECAY pot to mid-rotation and the DEPTH pot to 3/4 rotation. Then slowly turn the DELAY TIME COARSE knob. You should hear the music with reverb, echoes and delay. Play with the DELAY TIME and DECAY pots to get the effects you desire.

Using the Digital Delay

In this section I will assume that you already know what a Digital Delay does, what it is used for and pretty much what you can do with one once you get one. So all you need is a quick review of the controls and their function.

But before I explain all of the controls, I must mention the following:

This unit is designed to work with LINE LEVEL signals. It is not intended to be used as a pedal capable of receiving microphone or pickup level (high impedance) signals. If you wish to use this device directly with a microphone, guitar or other "pickup"-type musical instrument, you will first need to send your instrument's signal through a pre-amplifier.

This unit will work directly with mixer boards, keyboards, powered microphones, audio systems such as CD-players, computer sound generators, synthesizers and any other instrument or music source that produces line-level signals (low impedance).

It is OK to "play" with the settings on the Digital Delay and figure out for yourself all of the effects you can do! There are many creative things that digital delays can offer. Grab the controls and twist away! It's all in your imagination!

Controls and Their Functions