Vintage Synth Repair

Hi guys

okay after three long years, I finally have had a chance to work on my 3200... it is working 100% except for one annoying problem.  I am hoping that someone out there might have an idea what is happening with this beast.. below is a description of the problem as well as a list of things we have tried to repair the problem.

the problem:  

the 3200 is working 100% except  there is an issue with the recall of memory.  the problem is centered only around the Signal Generator Waveforms ( most noticeably present in the pulse and square wave forms) after the patch is saved the recalled patch has a constant 5-7Hz square wave vibrato that is present on all notes unless the SG 1/2 waveform knobs are pulled (bypassing the memory).. we have been unable to find the source of the Vibrato .. it is not the Modulation generators or Ensemble oscillator

notes:

- keyboard pitch doesn't effect the vibrato rate
- DAC Gain Pot has no effect
- ADC Gain pot has no effect
- Memory Master Clock Freq/Duty cycle pots have no effect
- All Memory ICs were swapped and problem still is the same
- Changed all MUX and DMUX ICs and problem is still present
- PSU voltages all Ok and clean
- DAC IC changed and problem is still present

the last IC I have not changed is the teledyne 5000CJ ADC I have sourced the part but it costs over $150 with the shipping etc... this may be the problem but I am hoping to rule out anything else that we are not considering before I dish out the $$.

any one have any ideas?  any help is appreciated

thanks

-rick



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I've run across a problem in the past, with a piece of equipment that 
used a single DAC fed thru a demultiplexer.  Hopefully it is similar 
to your synth - I have not seen the schematic for the PS3200.

The output of the DAC is a sequence of voltages, generated from the 
stored memory data.  The demultiplexer is used to send each of the 
voltages to the circuit it controls, where it is stored on a 
capacitor.  Since the capacitor discharges over time, the DAC 
constantly cycles thru the sequence of voltages, recharging each 
capacitor before it can discharge enough to be noticeable.

Now for the problem.  If one of the capacitors is leaky, or the 
circuit it is connected to is discharging it, its voltage will drop 
significantly before the DAC gets back around to recharging it.  This 
will cause a noticeable modulation in the circuitry it controls.  The 
voltage across the capacitor will appear as a sawtooth waveform if 
you look at it on an oscilloscope.

HI

thanks for the tip.. this is the second reply I have received 
mentioning the caps.. I will go and check those the next time the 
korg is in front of me

thanks again for the ideas!!!
-rick



--- In vintagesynthrepair@y..., "jdb000001" <jdb000001@y...> 
wrote:
> I've run across a problem in the past, with a piece of 
equipment that 
> used a single DAC fed thru a demultiplexer.  Hopefully it is 
similar 
> to your synth - I have not seen the schematic for the PS3200.
> 
> The output of the DAC is a sequence of voltages, generated 
from the 
> stored memory data.  The demultiplexer is used to send each 
of the 
> voltages to the circuit it controls, where it is stored on a 
> capacitor.  Since the capacitor discharges over time, the DAC 
> constantly cycles thru the sequence of voltages, recharging 
each 
> capacitor before it can discharge enough to be noticeable.
> 
> Now for the problem.  If one of the capacitors is leaky, or the 
> circuit it is connected to is discharging it, its voltage will drop 
> significantly before the DAC gets back around to recharging it.  
This 
> will cause a noticeable modulation in the circuitry it controls.  
The 
> voltage across the capacitor will appear as a sawtooth 
waveform if 
> you look at it on an oscilloscope.