[sdiy] little BIG help on KORG DELTA

Harry Bissell harrybissell at wowway.com
Tue Jun 28 15:58:54 CEST 2011

With R111 attached, I'd expect a range of maybe .1V to 10V drop across
R111. Measure across both sides of that resistor with a meter. I'm figuring that
the OTA probably can't use a value of less than 10uA, or more than 1mA.

(there are four OTA in parallel so the absolute maximums is 4mA for the LM13600.

With the 10K its not likely they can support much more than 1mA anyway
(you'd need 20V to get 2mA, not possible here...)

If you don't get this kind of range, the filter is probably not tuning...

H^) harry

----- Original Message -----
From: Antonio Tuzzi <antoniotuzzi at gmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: synth DIY <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 28 Jun 2011 09:24:27 -0400 (EDT)
Subject: Re: [sdiy] little BIG help on KORG DELTA

more info for you...

On 28/giu/2011, at 14.42, Harry Bissell wrote:

> 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.

that R71 is bad labeled in the schemes, should be R111 
anyway when detached, when I move cutoff I can see voltage going from .12 to .24

when i soldered to the circuit I'm getting around -13.99 volts almost fixed

that -13 V is going into pin 1 and 16 of both 13600

> 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

Harry Bissell & Nora Abdullah 4eva

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