Hi Malte,
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 didn¹t do a transplant!). From a practical point of view I
have constructed well over ten of these now and I only issue I have
encountered has been the clock instability, which, as we know, was rectified
by changing the above. Apart from that the board seems to offer a very
credible replacement to the original. 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. This might, if viewed by a non-technical
reader, also imply that the board isn¹t fit for purpose in some way and put
them off restoring their classic! (Which was the whole point in the first
place).
One thing I do hear from the people who have had a board constructed for
them is that there is something a little different about the tonal quality
of the clone board. Weird this as it does not actually sit anywhere directly
in the signal path. What they perceive as different¹ might just be cleaner
waveforms and control voltages acting on those components which do. Whether
this is a result of you observations or just what the instrument should have
sounded like when it was new remains to be seen.
If you want to pursue this line of enquiry then I guess the next stage is
cutting tracks and rewiring them taking different routes.
Just a thought!
Andy
On 12/11/2010 09:49, "Malte Rogacki" <gacki@gacki.sax.de> wrote:
>
> Hello everyone;
>
> since I wanted to get to the bottom of the capacitance problem with some
> KLM367A clones I got me a dedicated capacitance meter and did some
> measurements on various old and new boards. All measurements were done on
> populated boards with the processor removed. I measured:
>
> pin2 of IC22 socket to TP3 (that's essentially the capacitance across C22
> to ground)
> pin3 of IC22 socket to TP3 (that's essentially the capacitance across C23
> to ground)
> pin2 to pin3
>
> I realize the absolute values probably don't mean too much; however it
> should still be possible to compare the overall behaviour. The picture is
> quite clear IMO.
> All measurements in pF. The percent value is the difference between the
> first and second column.
>
> board pin2 pin3 pin2<->pin3 percent
>
> old1 40.7 46.7 92 14.7
>
> old2 32.2 36.1 89 18.3
>
> new1 23.1 28.7 81.3 24,2
>
> new2 24 33.5 56.6 39.6
>
> new2 (mod1) 19.0 18.4 53.0 -3.2
>
> new2 (mod2) 26.0 40.5 58.5 55.7
>
> board "new2 (mod1)" has two 10p caps installed, "new2 (mod2)" has two 33p
> caps.
>
> A couple of weeks my guess was that the difference between the two "clock
> legs" becomes too great for some boards to properly function. The above
> measurements seem to point into this direction as well. I have some more
> half-finished clones on which I will do additionla measurements once they I
> finished them.
>
> I have no idea why the overall capacitance for board "new2" was
> significantly lower than for board "new1"; I also don't know if this may
> affect operation.
>
> Ideas? Comments?
>
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