Service Manual ProTone
Page 7 of 20
© 1996-2001 Spectral Audio
3 History of Series
There where 3 different series made:
Serie # 1 : 960000 .. 960020 with golden front panel
Serie # 2 : 960021 .. 960161 with red front panel, C66 and C67 added
Serie # 3 : 960162 .. 960470 with red front panel, now with switch and trafo for 115V / 230V; R173,
R174, C63-C65 inbuilt; R174 and R175 (NTCs) added (by hand); GND Connection to case
This manual describe the last serie.
This manual contains no the hole schematics. Spectral Audio may send it to you for important
reasons, please write to
.
4 Improvements and explanations
4.1 Sense of R174 and R175
These two resistors helps to increase the lower limit of the cutoff frequency. Otherwise a ploop-
noise may occur when reaching 0 Hz cutoff frequency with a high resonance. You may reach the
0Hz using the LFO.
They are in serie connected from U20 pin 16 to GND.
4.2 Sense of C63 – C65
With time, the pots may become noisy. You may hear this effect especially at the Cutoff pot. Using
the C63 – C65 solve this problem.
They are connected from the middle pin (+) of the pot to GND (-). Resonance has negative voltage,
so middle pin is (-) and GND is (+):
