Discussion about the Korg PolySix synthesizer

Hi Malte,

Interesting that you should mention the IC socket. I generally fit those
which have contacts made of Beryllium, but sometimes tin, gold and other
mixtures of the elements. I wonder if the type of metal affects the
capacitance? I guess it must have a certain value if it conducts. Would
explain the reason why some worked and others failed.

Cheers,
Andy


On 12/11/2010 11:53, "Malte Rogacki" <gacki@gacki.sax.de> wrote:

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> Hi Andrew, let me address a few points:
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>> > Since there were no circuit modifications other than C23 being lowered to
>> > 10pF then we will have to suspect the layout, density and shape of the
>> > copper on the new board (or even the properties of the new ceramic
>> > resonator, if you didn't do a transplant!).
> 
> I actually did transplants for the two new boards. It would also be
> possible that even the type of chip socket for IC22 plays a role here.
> After all, we're talking about very low capacitance values here.
> Interestingly both the Polysix and the Poly-61 all use the same values (10p
> and 22p) in the clock circuit for their 8048 and 8049 chips.
> 
>> > As interesting as circuit analysis is
>> > I think there is a risk of getting to bogged down in theory and looking for
>> > problems where there are none.
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> As I see it we have a problem (clock instability on some boards), we have a
> solution (lowering C23) which points at the general source of the problem
> (capacitance mismatch). I think it is a good idea to pinpoint this as far
> as possible. This in turn should give us the chance to simply measure this
> part of the circuit on newly constructed boards and see if we are within
> certain specs (and then possibly change the values accordingly). It may
> also lead to new values for C22 and C23 that fit all boards.
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>> > This might, if viewed by a non-technical
>> > reader, also imply that the board isn't fit for purpose in some way and put
>> > them off restoring their classic! (Which was the whole point in the first
>> > place).
> 
> Perhaps, but they would be wrong about this.
> 
> I'll work on this a bit more when I have time. A deeper search led to an
> Intel Application note (AP-155) that discusses the various oscillator types
> from roughly that era (and how to troubleshoot them!). Interesting read, to
> say the least.
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