Ensoniq Dp4 Issue

hi to everyone,

need some help i own a dp4 and some problems,

channel 3 and 4 habe a lot of hum buzz sum, the sound is completly distorted. i made the audio test and i see on 3 is F for Failure, i test the dp 4 with the Sine generator from the effect processor and everything is fine on the outputs every channel so i think the failure it is between the input jack and the dsp (effect processing).

is there someone who had the same issue and find an solution ?

i need the schematic for board number 4001015001 i found alot on the internet but not in a good quality maybe someone can help

i recaped the most of the caps and check all the voltages

power supply voltages are completly okay and the check up on the 30pin ribbon cable for all 4 audio channels are also completly okay. and the sinewave from the low level audio Test is also fine on the scope on every audio channel

with these tests and knowlege i think i have a little grounding problem on the channel. between input 3 and 4 to the dsp

Do the leds on the front stay lit? mine does this and i was told that the 30 pin ribbon cable might be shorting and the voltage is leaking into the audio lines
sorry if this is not very helpful im not a tech just an enthusiast