PCM70 Delays works but chords and reverbs don't...

[aku]

Hi!
I' m trying to repair my pcm70 since few month...reading the service manual to understand the heart of this babe but now I need help...
here's the problem:
some algos don't works, and others works...really strange for me.
the delay bank seem ok, but no rezo chords and...no reverbs...
with rezo chord, no sound at all and with reverbs it's seem the first reflexions works but no diffusion...no tail

I work with a friend and we have 3 units...1 working, mine broken and an other one for spare parts.
I tried to swap all the circuits on support with those of the one that works
swapped the mcu, aru, dram between units and they works
tried with a new Z80...same problem

I tried to change some flip flop but I need a better approach...!

I have several questions in mind:
how the headroom circuit interact with the audio processing?
the dsp don't receives some parameters?
the dsp don't receive the algo from the eprom correctly?

if you have already encountered this problem or a lead to follow...

 

[JohnRoberts]

a faulty memory chip with random bad sectors could work or not depending on memory needs of the specific algorithm.

JR

 

[aku]

Thanks for your reply!
But if i swap the dram chip the problem must move, no?

I tried to swap the memory...between the pcm and inside the same...same problem.
Tried to start with only some of the chips...the less number of chip...the worth is the sound...but in all situation, loading a preset from chord bank result in no sound and loading concert hall or other reverbs...no tail

 

[LiveWireRemote]

Do you still have the PCM70 parts only machine? I'm looking for U18, the CMU. It's a Lexicon Custom 40 pin IC made by Harris Part # 330-03768. Doug McClement [email protected]. It's U20 on a PCM60.