Sequential prophet 5 - Dac. CV Omux. S; Audio Output.5-4; Vcos; Vcas.5-4

5-7 DAC, CV DMUX, S/Hs
You may find extra resistors in the DAC ladder. They may connect from a collector of Q301-07 to
U328-2 to "lower" the bit value, or to U328-6 to "raise" the bit. 10 megohms corrects about 6 mV
error. Don't change or remove these resistors.
Also, don’t substitute Q301-07,2N4250s.
VDAC itself is very informative. You will see the voltage levels strobed to the forty-one S/Hs in order.
Check suspected 4051s for S/H ABC and I addresses and VDAC at pin 3.
S/H output can be expected to "droop" up to 1/2-mV after 7 ms. Droop greater than this is usually
TL082 failure (input leakage).
Shorted or leaky capacitors will, of course, cause excessive droop. Open caps make the S/H output
pulse.
Both sides of a TL082 usually fail at the same time. Leakage causes the output to look like an inverted
sawtooth.
5-8 COMMON ANALOG
R3123 may be 20-22 kilohms to extend slowest glide rate.
The MSUM LM348s may be suspected for problems affecting all OSC A, OSC Bs or FILT FREQs.
5-9 AUDIO OUTPUT
The control path for U497-15 VOLUME VCA passes through J707 AMP CV IN jack. Check J707 (see
SD211, SD711) when troubleshooting for no audio output. (Also check quick-disconnects P402/J709
on J701.)
5-10 VCOs
WARNING: Switch power off while removing or installing PCB4. Otherwise, there is danger of
"blowing" devices through accidental grounding at a standoff.
In general, component failures can be quickly cured by isolating the affected voice and comparing
it — waveforms, voltage readings — to the other four voices. For example, to find a bad oscillator
match OSC A and B FREQs (OSC B KBD ON) and play in UNISON. Alternately turn down R416 VOICE
LEVEL trimmers to isolate the problem voice. Check the oscillator summers (e.g. OSC 1A, U455-1)
against one another. If they are basically the same the fault is probably the particular 2030 VCO itself.
'to
Obviously, suspect the COMMON ANALOG circuitry for problems affecting all oscillators, or just
the OSC As or Bs.
5-11 VC As
-The 2020 has a current output — there must be a load to measure any significant voltage.
CV input is actually a transistor base with a grounded emitter, so the voltage should not exceed
0.7V. Otherwise, the 2020 is bad.
5-12 ENV GENs
5-4