Discussion about the Korg PolySix synthesizer

Andy,

retackling this board as well. I did order metal film resistors as the part list recommended and have installed most all as I am waiting on a few that are back ordered with Farnell. I have decided to swap out Q1 with a newly order stock from Farnell as you had suspected a short or the like at Q1. Also on back order are new LM3700s just in case they are an issue. So with the installation of a new Fet Q1 and the metal film resistors. I started the calibration over and with the MG on this board it is exhibiting the same behavior. I pulled IC 21 and one side of R18 as recommended. Should I resend data from those test points? What should I be looking for at this point?

-Daniel 


--- In PolySix@yahoogroups.com, Andrew Jury <andy@...> wrote:
>
> Hi Daniel,
> 
> Ha! Nice answer. I was asking for that really!
> 
> About the reading you took. There are some issues here definitely...
> 
> >> IC13 pin7 has a saw oscillation.
> 
> This is supposed to be the demuxed control voltage from the MG frequency
> knob. You should have a voltage there which swings between ­5 and +5v! 0v
> when the control is in the centre. Check the your layout and connect very
> carefully. Remove IC21 to prevent any damage and lift out one end of R18.
> This will disconnect the mux from the oscillator. Q3 shorted shorted emitter
> to base perhaps?
> 
> >> IC16 pin7 has a saw oscillation.
> 
> Looks right!
> 
> >> IC19 pin12 has a saw oscillation.
> 
> See the notes above! IC13 buffers the output of IC19!
> 
> Cheers,
> Andy
> 
> 
> 
> On 24/08/2010 02:34, "Daniel A" <apophis93@...> wrote:
> > 
> > Andy,
> > 
> >> > Now let me see... So you say you are getting just DC on TP5? Have you got
> >> the scope in set for AC input? ;-)
> > 
> > What!?! I shouldn't probe the outlet in my house as a 60Hz reference? ;)
> > 
> >> > First, you should note whether the MG LED is on, off or pulsing. However,
> >> in your case did we manage to get that blue wire back in place?
> > 
> > Yes the loose blue wire is now reconnected to the PS +5v rail as it was before
> > I found trace solder where it broke off. Light is currently holding steady ON.
> > Hopefully this changes when the MG Freq is back online.
> > 
> >> > Second we need to measure the a stable comprising IC16 and IC21 and see if
> >> it is in oscillation. You know about oscillators, right? You present the
> >> output as positive feedback to the input with a gain greater than 1 and the
> >> whole thing starts to resonate. Only this circuit is current controlled by
> >> pin 16 of IC21. The output of the mux (IC19), the buffer IC13 at pin 7 and
> >> the long tail pair transistor are responsible for altering the current and
> >> changing the speed of the MG. That is a very loose description of how it
> >> works but it should give you some pointers. You should hook up your scope to
> >> pin 12 of IC21, IC16 pin 7 and the drain of FET Q1 and see what you have. You
> >> should have some nice slow running triangle waves if running.
> >> > 
> > 
> > Here are my results I can give exact measurements if required:
> > 
> > IC13 pin7 has a saw oscillation.
> > IC16 pin7 has a saw oscillation.
> > IC19 pin12 has a saw oscillation.
> > IC21 no oscillation just DC shift
> > (T1) FET Q1 no oscillation just DC shift
> > 
> > - Daniel
> 
> 
> 
> [Non-text portions of this message have been removed]
>