[sdiy] little BIG help on KORG DELTA

Antonio Tuzzi antoniotuzzi at gmail.com
Tue Jun 28 15:24:27 CEST 2011


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




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