Discussion about the Korg PolySix synthesizer

Andrew,

Yes, the voltage must swing between -5V and +5V. It is not so easy
to tell whether it happens on the input side (KLM-369/370) or the
output side (KLM-367). Do the following:
* connect CH2 of your scope to testpoint "TP4 CLOCK" on KLM-367.
   It is right beneath the 8048 CPU, IC22. Set the trigger to CH2
   and adjust the trigger level to see a narrow pulse every 20msec.
* connect CH1 of scope to test point "TP2", which is right beneath
   the "RESET ADJ" poti, VR1.
* select MANUAL on the Polysix front panel and turn all knobs
   fully left. The scope reading should be mainly -5V. There may be
   some garbage at the end of the cycle.
* now turn each knob fully up and down again. You should see a
   small "bar" going from -5V to +5V. Depending on the knob the
   position within the cycle is different. The order is according
   to the outputs of 4051 multiplexers IC18, IC19.
   What you see is the analog CV /output/
* Move the CH1 probe to CN11/pin2, labeled "DATA". Here you will
   see a similar thing; the bars will be different in length, but
   the principle is the same. Again turn each knob up and down.
   Observe that each knob reaches the full span of -5 .. +5V.
   This is the /input/ side.
Now you can judge if the fault is a 4051 on KLM-369/370 or KLM-367.
If you have problems to interpret the scope screen take pictures and
email them. I will try to figure out what's wrong.

Note: I had it once or twice that the leaked acid from the NiCd
battery crawled via the GND connection (thick black wire) from
KLM-367 to KLM-370 and did damage there. Check for this!

Good luck !

Johannes


Andrew Jury wrote:
> Johannes,
> 
> After a major fault finding session I would like to report back the 
> following:
> 
> The first thing I did was be rather lazy and swap the transconductanc 
> op-amp IC21 to see if it made any difference; no! I then checked the 
> FET Q1 s-g, g-d and g-d. Similarly I checked the matched pair of bi-
> polars at Q3 c-b, b-e and c-e. No problems. I then very carefully 
> checked all the resistors in circuit, including VR5, which all 
> checked OK. I tested C19 which 0.339uF. Not bad for an electrolytic 
> which is over 25 years old! The MG was still ocillating flat out at 
> 3.4hz even with VR5 at it uppermost setting. I then checked the dc 
> voltage present at pin 7 of IC13. Changing the MG frequency control 
> from 0 (+ 4.98V) to 10 (-0.17v). But wait? Can this be right? I 
> checked pin 12 of IC19 which is the mux output X3; same voltage 
> reading. Working back to IC2 on the KLM-370 board I checked the mux 
> input X2 at pin 15. The voltage swings accurately around the 
> frequency pot wiper from +5 to -5v. However, taking a reading at the 
> muxed data output on pin 3 of IC2 the voltage will only swing as 
> mentioned before from +4.98 to -0.17v. Now forgive me if I am wrong 
> but I understood the HA14051 worked like this. You appliy a voltage 
> to one of the 8 mux inputs marked X0-7. The voltage change is 
> detected and a bitmap is set on data address lines A, B and C 
> according to which pin was strobed. At the other end this bitmap is 
> decoded the data read on the receiving mux and the appropriate signal 
> (in this case a voltage) will appear at the correct pin as determind 
> by the bitmap. The fact that the voltage doesn't swing correctly 
> between the two poles of the pot would suggest a fault with the 
> transmitting HA14051? Am I right in assuming this? If the address 
> lines were cut/missing then some of the muxed output would be never 
> selected and hence always at 0 volts. This might also account for the 
> other problem in that the ADS bit of the EG works (sort of!) but the 
> release pot does nothing! (Again the voltage swings about the same as 
> the MG frequency control)
> 
> Your expert opinion on this matter is greatly anticipated!
> 
> Best regards,
> Andy
> 
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@...> 
> wrote:
>> The oscillator design is of the multivibrator type. The not so 
> common
>> thing is that it is voltage controlled.
>> The control voltage is buffered by IC13. it is converted to current
>> by R16, R69, TH1 combination (temperature compensated by use of 
> TH1).
>> This current is mirrored by dual-transistor (for good match) Q3. The
>> mirrored current is fed into control input of transconductance 
> amplifier
>> 1/2 of IC21. Download the data sheet of this IC to find out how it
>> works. LM13700, NE5517, CA3080 are similar devices. It is an 
> amplifier
>> with voltage input (high impedance) and current output (high 
> impedance),
>> hence the name transconductance (current divided by voltage is 
> called
>> conductance, the reciproke of resistance). So you input a voltage 
> and
>> get a current on the output. That is the reason why timing capacitor
>> C19 does not have any resistor associated with. The transconductance
>> (= similar to gain of opamps) is controlled by the current which is
>> drawn out of the control pin (pin16).
>> One half of IC16 is the other vital part of the multivibrator 
> circuit,
>> it acts as the schmitt trigger (positive feedback).
>> How does it work?
>> Lets assume any voltage level on IC16/pin6 after power on. It will 
> be
>> more or less than the voltage on IC16/pin5. Depending whether it is
>> more or less the output of IC16 will be positive or negative 
> saturated,
>> which is about +Vcc -1.5V or -Vcc +1.5V (+/-13.5V). This voltage is
>> divided by resistor divider R53 - R52 and fed into the 
> transconductance
>> amp. Depending on the control current the output of IC21 delivers a
>> proportional current. This current charges the timing capacitor C19
>> developing a linear ramp voltage. The high impedance voltage is 
> buffered
>> by JFET Q1 and fed back to IC16 input. When it reaches the same 
> level
>> as voltage on IC16/pin5 the output switches to other saturation 
> level
>> and C19 is being discharged until IC16/pin6 trips pin5 and the 
> output
>> reverses polarity again (multivibrator principle). Since the output
>> current of IC21 is controlled by the control current, which in turn 
> is
>> determined by the "MP SPEED" control voltage, the oscillation 
> frequency
>> is controlled (larger current charges/discharges the timing 
> capacitor
>> faster). Is it clear?
>> What can go wrong? - Anything 8-/.
>> I assume you did the adjustment procedure for MG speed (VR5). The
>> service manual says, "If these values (50Hz) cannot be obtained 
> after
>> replacing IC LM13600, adjust R78 and R52 respectively."
>> First check the control voltage (IC13/pin7). It should reach -5 .. 
> +5V.
>> Next check the output swing of IC16 (pin7) it should be at least +/-
> 13V.
>> Next compare voltages before and after the buffer Q1; should be 
> (nearly)
>> equal.
>> More measurements are not so easy because it is all current driven; 
> you
>> cannot simply measure voltages. Best thing to try is replacing 
> components
>> (C19, IC21, Q3, maybe TH1). If you dont find a dual transistor you 
> can
>> use two discretes of the same type (would be good of the same lot).
>>
>> Good luck!
>>
>> Johannes
>>
>>
>> Andrew Jury wrote:
>>> Hi!
>>>
>>> I have just released with my MG (LFO) seems to be running very 
> slowly 
>>> with the frequency knob at 10. Before I dip in with my soldering 
> iron 
>>> does anyone know how the MG oscillator works? I have studied the 
>>> schematic and there is nothing in the design which looks remotely 
> like 
>>> one! I believe the MG should run at 50Hz flat out. My 'scope says 
> just 
>>> over 3Hz when I measure at the VCA MOD test point. There is an RC 
>>> network connected to the gate of Q1 on the KLM-367 which possibly 
>>> triggers a voltage at the drain over a time period. The RC nework 
>>> values (R27, C19) 10K, 0.33uF equate to a frequency of just over 
> 48Hz, 
>>> which looks promising. Can anyone shed any light on how this bit 
> of the 
>>> P6 works?
>>>
>>> Thanks,
>>> (A)
>>>
> 
> 
> 
> ------------------------------------
> 
> PolySix "Digiest" Page: http://www.acc.umu.se/~amber/Poly6Yahoo! Groups Links
> 
> 
>