[sdiy] little BIG help on KORG DELTA

Harry Bissell harrybissell at wowway.com
Tue Jun 28 14:42:48 CEST 2011


I'd check like this..

Look at the voltage across R71 (R73 maybe)... coming from the collector of Q12.
As you tune the filter you should see a change in voltage and you can calculate the
current into the OTAs. If it isn't moving you might have the filter tuned to cutoff
all the time. That would kill the audio.

The 10M resistor is supposed to stop the cutoff from going too low. without that the
OTAs can shut completely down

signal sneaking dorm the BPF resonance pot might mean that IC9 is bad (I'd expect
the feedback around IC9 to remove most of the
signal... and I'd also check pins 7 and 8 of IC11... the low impedance there should overwhelm the
audio bleed through the resonance pot...

Detaching point 3 is not a good test, the OTAs will be (almost) completely cut off...

Try looking for broken foil traces as well, the problem seems like it might be an open circuit.
Ohmmeter from the IC pins to the resistors (don't push too hard or you might make the connection
good while you measure...

Let us know your progress...

H^) harry


----- Original Message -----
From: Antonio Tuzzi <antoniotuzzi at gmail.com>
To: synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 28 Jun 2011 05:14:28 -0400 (EDT)
Subject: [sdiy] little BIG help on KORG DELTA


Hello friends,

I have korg delta that is giving me headache...


the string parts works correctly
but the synth part is buggy in the filter area:


http://img692.imageshack.us/img692/196/deltasynthpcb.png

in this schema you get some info on what is working:

Audio signal is present at point  1
if I connect to point 2 (out of the filter) directlyI'm getting the signal in final VCA

if the filter is in BP mode (point 1, down) the signal leaks in the Resonance pot (point 4), when is high and I can hear in the final VCA

the SA798 is working and I changed it too (point 3)
if I detach (desolder) the point 3, still the signal is missing

I changed 2 sets of LM13600 (I have sockets )
and I changed the caps of the filters (c21 22 23 24)

(the ic9 is changed too, but in any case is involved only when in BP mode)

I really don't know the audio is completely lost in there...


If you have any suggestion on how to continue debugging... 

thank you
antonio



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Harry Bissell & Nora Abdullah 4eva



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