Service Manual: Calibration and Checkout
All service centers are urged to obtain copies of the Chroma user manuals for performance, programming, sequencing and interfacing to have on hand as a reference source covering all aspects of operation.
A good starting point for calibration and checkout is the power supply. All three rectifier circuits are full wave, but an open diode or trace may result in half-wave operation. The regulators will still work but filtering becomes less effective, increasing noise on the corresponding supply line output. This may cause erratic operation, intermittent glitches or tuning problems. For this reason, it is desirable to check ripple frequency whenever a Chroma is in for service. Check the period of the ripple in all three circuits, on C3, C7 and C11. The period should be 8.3 millisec for 120Hz full-wave ripple In addition, measure the amplitude of the preregulator ripple. This should never exceed 0.5Vp-p. If higher than 0.5Vp-p tighten the mounting screws on C3 that hold the overvoltage protector. It is good practice to mount the wire terminals beneath the Overvoltage Protector Board for maximum conductivity. These connections are critical.
The preregulator DC voltage on C3 (in reference to ground) should be adjusted so the minimum dip in the ripple waveform is at a DC level of 6.2 volts. The peak should be about 6.6 volts. Minimum dip should not drop below 6.0VDC. Maximum peak should not rise above 7.0VDC. The adjustment is made by changing the bias on transistor Q3. Two version of the printed circuit fabrication exist for this supply. In one version a potentiometer (R46) is provided to correctly bias Q3. Simply adjust R46 for ripple dip of 6.2VDC on C3. In the other version, resistor R3 must be changed in value (usually by shunting it) to set the dip to 6.2VDC. We suggest using a decade box first, then soldering in the correct value. Use a DC scope to check the DC level of the dip and peak, a DVM will not suffice. Preregulator voltage that is too low will cause erratic operation.
The +5V analog supply provides the reference voltage for the D/A converters. Calculations indicate that the auto-tune functions operate closer to center range when the +5V analog supply is adjusted slightly on the high side. Empirical data from the field reinforces this condition. Set the +5V analog supply to +5.05VDC by adjustment potentiometer R22. Use a DVM to measure from the yellow wire at J1-4 on the supply to chassis ground. This supply voltage if set too low will adversely affect tuning and may cause the computer to fail Dual Channel Boards.
Although no adjustments are necessary for the +12V or -12V analog, check them anyway to ensure they are within 5% of their nominal voltage. The +5V digital should not dip below 4.9V and will usually be slightly high after setting the +5V analog to +5.05V, as this is where it takes its reference.
The next circuit needing calibration is on the I/O Board. The output buffer in the DAC circuit is a FET-input op-amp with an offset null trimmer. The offset voltage must be set to within 100uV of zero. Connect a DVM (3½ or 4 digit) between test point 0 and test point 1 on the I/O Board. Adjust potentiometer R1 for ±.0001VDC. This calibration must be correct to prevent tuning problems.
Prior to further calibration and checkout, it is necessary to set up a "scratch" patch. Activate both [EDIT A] and [EDIT B] functions, then while pressing and holding the [PARAM SELECT] switch, press each of the 50 numbered switches on the right panel. This disables all pitch and pulse width modulation. All parameters will now be set to their default "scratch" setting. Be sure the [NO LINK] LED is on and that all transpose LEDs are off. Playing a key using this unmodified "scratch" patch results in a raw sawtooth wave sound. The "scratch" patch will not affect any of the 50 programs stored in memory unless you use the [STORE] switch.
You may now check and calibrate the Dual Channel Boards following one of the procedures outlined below:
TRIM PROCEDURE: The only adjustment in the synthesizer circuit is the charge pump current, which is adjusted by R1 (R2). This adjustment affects the voltage level at the bottom of the sawtooth swing (the top is fixed at 5 volts). This in turn affects the pulse width, which suggests an easy way to do the trim. After setting up the "scratch" patch, modify parameter 3 (keyboard alg) to value 3 (all channel poly), this gives a common control voltage value to all 16 oscillators for ease of calibration. Set wave shape parameter (No. 33) to value 1 (pulse), and set pulse width parameter (No. 34) to 32 (50% duty cycle).
Connect your scope to TP1 on any channel board, set it for negative edge triggering, and adjust for a stable display. It is important that you trigger off the falling edge of the pulse that you see. Fine-adjust the horizontal sweep rate until the low portion of the oscillator pulse cycle lasts from exactly the first graticule division on the scope to the middle division. If you are looking at a perfect square wave, obviously the next falling edge of the cycle will appear perfectly lined up with the rightmost division on the scope face. Turning the trimmer R1 will not affect the low portion of the cycle, but will only shorten or lengthen the high portion of the cycle. Adjust this until you see the second falling edge perfectly lined up with the last graticule division. Repeating this test for each channel simply involves moving the probe to the next test point, readjusting the horizontal sweep speed so that the low part of the cycle takes exactly half the screen, and adjusting the trim so that the entire cycle takes up the full screen.
ALTERNATE TRIM PROCEDURE: The pulse width can be trimmed well enough without a scope by ear. This takes a bit longer, as one has to get the correct oscillator to sound before doing each adjustment. The easiest way to do this, is to set up a simple sound (one without pitch or pulse width modulation) using Patch 0, so that each note is played by a single oscillator. If the Wave Shape parameter (No. 33) is set to 0 and the Width parameter (No. 34) is set to 32, the fundamental frequency of each note should be suppressed, causing the notes to sound an octave higher. To understand this better, try varying the Width parameter on either side of setting 32. If an oscillator is incorrectly trimmed, the fundamental frequency will be nulled out at some setting other than 32. Once you have established that you are adjusting the oscillator you are listening to, you can easily perform the trim. Don't be thrown by the fact that the pitch is varied too. Just adjust to null out the fundamental. When you are through, you can do an auto-tune.
The easiest way to determine which channel you are playing is to reset the instrument (press [SET SPLIT] ). If you subsequently play sixteen different notes (a chromatic scale, for instance), the channels will always be heard in reverse order, starting with the B oscillator (R2 trim) on board 7. Board 7 is the board closest to the right rear corner of the instrument. The boards are ordered in a U-shaped sequence, so that board 0 is the board closest to the audio outputs. If you bump an extra note during this procedure, or get otherwise confused, just press [SET SPLIT]  again to reorder the channel assignment.
Once the Dual Channels have been "trimmed" you may want to check tuning. Use the "scratch" patch but change patch parameter  from value "0" to value "1." Instead of one oscillator per note the Chroma will assign two per note. Tuning is checked by listening to the oscillators beat together. Proceed by resetting the Chroma pressing [SET SPLIT] . Play the "C" one octave above middle "C" and count the beats over a 10 second period then divide by 10 to determine actual beats for one second. Repeat this going up-scale until eight notes have been played. Anything over two beats per second is unacceptable. An offending Dual Channel Board may be shut off by pressing [SET SPLIT] , (see the Diagnostic Chart). The data readout in the little display window will show an error followed by the board number that was shut off. Of course more than one board may be shut off using this feature. Example: The display [ERR 05] means that board "0" and board "5" are shut off.
If you find an unacceptable Dual Channel Board, swapping positions may help in analyzing the problem. Suppose the tuning is out of specification on the third note you play. Activating [SET SPLIT]  causes [ERR 2] to be displayed. Swap board "2" with board "1" (with power shut off) then try again. If the out of tune condition moves to position "1" [ERR 1] the problem is on that Dual Channel Board. If the out of tune condition still indicates [ERR 2], the problem is not on the Dual Channel, but likely in the strobing circuits on the Channel Mother Board. Random out of tune conditions are likely to be in the DAC circuits on the I/O Board, or problems in the +5V analog supply.
An alternate method to check tuning follows: Use the "scratch" patch but set Wave Shape parameter (No. 33) to value "1" and Width parameter (No. 34) to value "32." Press [SET SPLIT]  to auto-tune and order the channels. Using a tuning strobe or meter check each of the 16 oscillators starting with middle "C" and play chromatically up-scale. Do not stop at one octave, continue until 16 different notes have been played. Measure each note individually for number of hundreds of semitones off center frequency. Each note should be within 2.5 cents of its center frequency.
The remaining adjustments have to do with keyboard action. Refer to Figure 5-1. Measure the depth of keydown on a low, middle and high note. The depth should be 3/8 inch from keytop to keytop at the front edge of the key. Less than 3/8 inch key travel means the Damper Bar is too low. Figure 5-2 illustrates the hardware securing the Damper Bar. Loosen screws 1 and 2 and nuts 3, 4, 5 and 6. The Damper Bar should be positioned so that the lead weight just touches the felt with normal full keydown pressure. Check this on both ends and near the two center brackets (some early Chroma's have only one center bracket). After tightening all mounting hardware, check Damper Bar height. Press a key down with one hand and with the other hand grasp the wood key shank just forward of the lead weight. Pull against the Damper Bar felt. Proper adjustment results in restraint of further movement. If the Damper Bar is too high, the lead weighted end of the key will pull up higher against the felt even though you are holding the key full down with your other hand. As mentioned previously, adjusting the Damper Bar too low will restrict the keydown movement to less than 3/8 inch.
The left and right stack switch boards are mounted with three (3) screws going through elongated holes into "T" nuts (see Figure 5-3). Loosening these screws allows the stack switches to be moved up or down to their correct position. Set the stack switch boards so that the top (normally open) leaf travels upward about 1/32" after closing (as shown in Figure 5-4) before the key stops.