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The first thing to check when you power up is the expected behavior of the VCO cores. Any problems here
need to be worked out before proceeding to other checkouts.
Set each VCO to base operating mode for calibration:
All modulation sources minimized
VCO in KBD mode
0V or nothing connected to the KBD CV bus
Coarse Frequency control minimized
Fine Frequency control centered
Observe the sawtooth wave output on the scope and frequency counter. Then, sweep the Freq Cal
trimmer. At the minimum setting on the trimmer, the VCO will be typically be stalled. Maximize the
trimmer. You should see on the scope a rising ramp wave, 10V peak-to-peak, with +5V DC offset. On the
frequency counter you want to see at least 40hz, and no more than 90hz.
If the shape looks good but you cannot get the frequency into correct range, there are suggestions in the
Tips section, below.
Its best to calibrate the internal clock and VCO2 triangle and sine shapes first, since these trimmers are on
the back of the panel. The
test points for these are clearly marked. Connect your scope’s ground
reference to the GND pin of the TRIG header in the EG section. For VCO2 shapes, first set offset and
symmetry while observing the triangle output. Then, set purity and gain while observing the sine output.
You will not get a pure sine wave, but you should be something close and it should be 10V P-P, with no
offset.
Don’t forget to check out the 4072 VCF core and ensure it calibrates. Start with it’s on
-board (Piher)
trimmer centered. If you have difficulty getting init frequency and 1V/Oct trims for at least three octave
range, work with the Piher trimmer.
Only work with the reverb wiring while power is off. The return section has such high gain that just
touching the center connector with your finger will destroy the LM301 in the return amp, if it is running.
The relevant checkout sections to run through in the ARP 2600 Service Manual…
https://drive.google.com/open?id=1TUjsTKBTVcuaf_m3qcXhh3aoX6WBkeX2
… are:
2.3.3 Preamplifier
2.4.3 Envelope Follower
2.5.3 Ring Modulator
2.6.3 VCO 1 (bypass the Freq Cal and Volts per Octave steps if using the Fuzzbass calibration)
2.7.3 VCO 2 (bypass the Freq Cal and Volts per Octave steps if using the Fuzzbass calibration)
2.8.3 VCO 3 (bypass the Freq Cal and Volts per Octave steps if using the Fuzzbass calibration)
2.9.3 Voltage Controlled Filter