If it is only one VCO, then things are different. It may be the cap
(C709), which I do not believe because it is a polypropylene type,
which have very good long term stability. I would go for the 3
transistors plus diode which form some kind of "unijunction" type
of behaviour: ICx11 together with Qx10 converts the CV to a constant
current, which in turn creates a voltage ramp through Cx9. When the
voltage across Cx9 reaches a certain value Qx2, Qx4, Qx6, Dx2 "fire"
and short Cx9, and you have a sawtooth wave. This is a very similar
design as in the MS series (MS-10, MS-20), even the values are similar.
First disconnect (remove) the flipflop (ICx10), which generates the
sub-octave waves and is not necessary for the 4' wave. Just in case ...
Then put the oscilloscope to the collector of Qx10 and watch the ramp.
(both good and bad voice). If the ramp is bad (not straight) then
probably Qx10 is faulty, but it could be ICx11 or Cx9. But i do not
think that this will be the case.
Good luck,
Johannes
gil_we wrote:
> You know what ?... now I relaize I replaced IC8 and ICx11 as well !
>
> What if the problem is related with only one of the VCOs...? lets say
> VCO1. What would you check/replace ?
>
>
> --- In PolySix@yahoogroups.com, Johannes Hausensteiner <johau@g...>
> wrote:
>
>>If you already replaced Rx39/Cx10 then the only thing in common
>>for both VCOs in one voice is ICx11 (TL072). THe value of Rx39
>>is not very critical unless it is very different to the nominal
>>value of 1k
>>
>> > By IC8 are you refering to ICx08 ? it's a 4558 opamp.
>>No, I mean IC8, which is a 4051. It is common to all voices; it
>>demultiplexes the CV for the VCOs
>>
>>Johannes
>>
>>
>>gil_we wrote:
>>
>>>Thanks !
>>>
>>>The Cx10 and Rx39 have been already replaced to no avail.
>>>
>>>By IC8 are you refering to ICx08 ? it's a 4558 opamp.
>>>
>>>btw, I now see that I had to replace Rx36 with a slightly
>
> different
>
>>>value because even after the machine is warm enough, with the
>>>original value the related VCO still cannot get tuned with the
>
> other
>
>>>oscillators, not matter how you set the VCO tuning trimmers.
>>>
>>>Thanks again.
>>>
>>>--- In PolySix@yahoogroups.com, Johannes Hausensteiner
>
> <johau@g...>
>
>>>wrote:
>>>
>>>
>>>>Hi,
>>>>
>>>>The caps are Cx10 ... where x is the voice number. On the
>
> schematics
>
>>>>only voice #7 is drawn in detail, so you can see C710. Maybe it
>
> is
>
>>>not
>>>
>>>
>>>>the cap but the accompanying resistor (R739), or the demux (IC8).
>>>>
>>>>This is a great machine. Two really independant VCOs!
>>>>
>>>>Johannes
>>>>
>>>>
>>>>gil_we wrote:
>>>>
>>>>
>>>>>Thanks very much !
>>>>>
>>>>>The tuning is bad on both VCO1 and VCO2 of that oscillator
>>>
>>>(although
>>>
>>>
>>>>>it seems more extreme on VCO2).
>>>>>
>>>>>I uploaded the schematics diagram for the VCO board to the files
>>>>>section - what would be the caps to check on that one ?
>>>>>
>>>>>Thanks again.
>>>>>
>>>>>
>>>>>--- In PolySix@yahoogroups.com, Johannes Hausensteiner
>>>
>>><johau@g...>
>>>
>>>
>>>>>wrote:
>>>>>
>>>>>
>>>>>
>>>>>>Hi,
>>>>>>
>>>>>>I only have the schematics for the MK I; which I do not know
>>>
>>>whether
>>>
>>>
>>>>>>it is similar to the MK II. Which VCO has the problem? Is it
>
> VCO-
>
>>>1 or
>>>
>>>
>>>>>>VCO-2 or both? If it is both then have a look at the hold
>>>
>>>capacitors
>>>
>>>
>>>>>>after the VCO CV demux IC15 (a 4051) on KLM-301 board. These
>
> are
>
>>>the
>>>
>>>
>>>>>>caps C25 - C32.
>>>>>>
>>>>>>Good luck,
>>>>>>
>>>>>>Johannes
>>>>>>
>>>>>>
>>>>>>gil_we wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>I've been recovering this one, found in the flea market.
>
> Overall
>
>>>it
>>>
>>>
>>>>>>>had about 8-9 different faults that I fixed, leaving two
>>>
>>>problems.
>>>
>>>
>>>>>>>The first is a bad SSM2056 chip which I need to find. The
>
> second
>
>>>i
>>>
>>>
>>>>>>>probably inherent in the discrete ciruit of the second
>>>
>>>oscillator.
>>>
>>>
>>>>>>>Upon powering it up, that particular oscillator will be out of
>>>
>>>tune,
>>>
>>>
>>>>>>>till it gets to its max. internal temperature where the tuning
>>>
>>>of
>>>
>>>
>>>>>>>that VCO is geting more or less - stable.
>>>>>>>
>>>>>>>All other oscillators are immediately in tune, no matter what
>
> is
>
>>>the
>>>
>>>
>>>>>>>temperatue inside the machine.
>>>>>>>
>>>>>>>I, of course, tried to figure out what the bad component is by
>>>>>>>blowing with a straw on each and each of the suspected
>>>
>>>compnents, but
>>>
>>>
>>>>>>>blowing on every component on the circuit of that oscialltor
>>>
>>>which
>>>
>>>
>>>>>>>seems to be pitch related, seems to effect the pitch.
>>>>>>>
>>>>>>>I also tried to switch transistors between the bad oscillator
>>>
>>>and
>>>
>>>
>>>>>>>another (working one) but the problem persisted.
>>>>>>>
>>>>>>>Does anyone have any other idea / direction ?
>>>>>>>
>>>>>>>Thanks
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>PolySix "Digiest" Page: http://www.acc.umu.se/~amber/Poly6
>>>Yahoo! Groups Links
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>
>
>
>
>
>
>
>
> PolySix "Digiest" Page: http://www.acc.umu.se/~amber/Poly6
> Yahoo! Groups Links
>
>
>
>
>
>
>