30
changing out the transistor pair (2N3904/2N3906) on the 4027-1. When you have a good match, the Freq
Adjust trimmer should provide a range of 0 - ~90hz (with 0hz to 40hz ideal), while the VCO is in baseline
state. Indication of a bad match for the transistor pair is not being able to reach 32.7hz, or only reaching
it at the maximum setting on the trimmer. Another indication of a bad match is if the range of the
Frequency Adj trimmer is greater than 0hz
–
90hz.
If you have no output, it may be the VCO core. If you cannot trim the correct wave shapes, it may be the
shaper sections, or the VCO core. The sawtooth wave output is taken directly from the VCO core, so if
that looks wrong on the scope, there is probably a problem in the 4027-1 core.
When testing the Sample and Hold circuit, use it’s CV out to drive the pitch of a VCO, and set the clock to
a very low speed. This will allow you to hear whether the circuit is really holding, or drifting. Drifting or
sagging is a common build problem with the S&H circuit. If you hear it drifting, the problem will most
likely be one of the JFETs in the circuit being bad or damaged, and not pinching off completely. The
memory element is the 22n capacitor. On the schematic you will see there are JFETs on both sides of it (a
single and a dual JFET). If either of these is leaky between clock cycles, the capacitor will discharge and
the CV will drift. The most likely one to be defective in a drift situation is the 2N4392.
Flip the board face up to perform all the rest of the checks and calibrations. Be sure to only connect the
reverb unit while power is off. One little mistake here will destroy the LM301 in the return amp.
If your EG outputs don’t rise to the full +10V, and you di
d not install a Gate Booster, that is probably why.
The LEDs in the sliders are grouped into six serial groups. Like old Christmas tree lights, if one LED is not
functioning, all the LEDs in that group will be out. There are brightness settings on the LED trimmer that
may result in a fully dimmed clock rate indication before the other LEDs are extinguished. If you don’t see
your clock rate flashing it may be the LED dimmer, or it may be calibration of the Internal Clock.
FUZZBASS TTSH VCO/VCF PITCH CALIBRATION ROUTINE
The point of this alternate calibration routine is to allow the player to set the VCO controls visually to
baseline
, play A4 (MIDI note 69), to produce 440hz out of the VCO or VCF. It does not preclude tuning up,
but it gets you close to concert pitch quickly. The secondary point is to allow the builder to position the
base frequency of the VCOs and VCF at a point most usable for the CV sources at hand.
The ARP 2600 service manual describes the standard calibration routine appropriate for use with a 3600
series keyboard. Below is an alternative method. This only varies from the standard routine in the value
selected for base frequency.
In all cases, ignore the frequency markings on the VCOs and VCF Coarse Frequency controls printed
on the panel.
•
Initial frequency calibration
sets the operating frequency for the VCOs and VCF at CV of 0.000v.
•
1V per Octave tracking calibration
adjusts how the VCO and VCF frequency tracks pitch CV across
the scale.
The Freq Adj and 1V/Oct trims interact with each other. When adjusting one, you usually have to adjust
the other.