Sequential prophet 5 - Section S; Service Notes; General.5-1; Pot Mux;Adc

Section 5
Service Notes
5-0 GENERAL •
It should be possible for the qualified technician to service the Prophet using the schematics and
standard troubleshooting techniques. The comments below suggest general approaches to correct
typical symptoms in each sub-circuit.
CAUTION: Observe the precautions in paragraph 1-1 of this manual.
5-1 POT MUX/ADC
Control knob malfunctions are most likely failures of U201-03. However, any data bus problem could
cause incorrect pot multiplexing. See Table 2-0. Dirty pots may "jump”. Clean them or replace with
10-100K linear taper. If several pots jump, check the ADC GAIN and U334 ADC itself. Observe VDAC
while turning knobs to check their effect on the staircase.
5-2 KEYBOARD (KBD)
Most keyboard problems are caused by dirty, bent, or broken J-wires.
Dead notes not caused by J-wires, usually occur in groups of eight, making it easy to isolate the prob¬
lem matrix row or column.
Also, check VV702 keyboard cable. Check for C$8 and -CSE.
5-3 SWITCH MATRIX
If a switch does not function its LED will also probably not light and the problem must be in the switch
matrix. If a switch LED Lights, but the function is not enabled, the problem must be in the correspond¬
ing output latch or analog circuitry.
Check operation of other switches to isolate malfunction to a single row or column.
Check for CS8 and -CSD.