ADSR modulation depth. Problems can first be traced to the LM324 U6,
but proper functioning of this IC is assured if the output follows the input.
The input levels on the resistor network associated with this summing
amp should be checked for conformity with a good amp. Problems with
the ADSR can be traced to the output buffers/demux circuit formed by
U17 (TL064) and U4 (CD4051). The buffer outputs should have a DC
level of about 7V, which is the ADSR sustain level. This will vary if the
ADSR mod depth is changed on EDIT PAGE 2. Again, note that the F
c
modulation outputs should all be 0V.
4.7.4 Step 11-27: VCO Volume Test
These steps check for proper operation of the SSM 2024 Quad VCA ICs
U22 and U23. If problems occur, first try a new IC, then check the VCO
volume control voltages generated by the buffers in U15 (TL064) which
should be about 7-10V when the particular VCO is supposed to be on
and 0V for VCO off.
4.7.5 Step 28-31: F
c
& Q Modulation
These steps check for proper operation of the F
c
and Q modulation
demultiplexer formed by CD4052 U1 and the associated TL068 buffers.
The outputs of all the TL068s should have a sweeping LFO sawtooth if
they’re functioning properly. If not, check if the CD4052 inputs (pins 3 &
13) have sawtooths. If they do, check the buffers and scan the clocks on
pins 6, 9 & 10 on the CD4052.
4.7.6 Step 32-35: VCO1 & 2 Modulation
This is the same as Step 28-31, except it refers to U2 and has an LFO
square wave output instead of sawtooth.
4.7.7 Step 36-39: Noise Audio Path
This checks if the noise (generated on the MIXER) makes it through all
four VCFs. If not, check the resistor pack inputs for noise. If it’s not there,
check for shorts or see if the MIXER actually has noise coming out of it.
4.7.8 Step 40-43: VCO Bypass VCF
The functioning of the MC14551 quad switch U14 is tested here. If the
board passed the VCO volume tests and fails this test, then U14 is
probably defective. Check, however, that the control pin 9 is actually
working (this comes from the DAC) or else the switching won’t occur.
You can do this by switching between STEP 40 and 39 to see if the
voltage pin 9 toggles between 0V and 8V.
4.7.9 Step 43: TL068 Drift Test
The TL068s serve as buffers for the modulation demultiplexers U1 and
U2. If they are excessively noisy or have a high drift (which can be
