[sdiy] Korg KR-55 drum machine query

[René Schmitz]

Hi all,

Well 8.1n is not a E12/24 value at all, 8.2nF would be.

Not consistent to use 8.1nF for 8529Hz, and in the next closest 
frequency 1.5nF (9073Hz)

Its plausible that this is a typo, and it's really 1.8nF.


Best,

  René


Am 13.11.2023 um 14:44 schrieb Tom Wiltshire:
> Hi All,
>
> I've been studying old drum machines again. This time, I've been 
> looking at the Korg KR-55.
>
> I've come up with one quirk or schematic error, and I'd like you to 
> take a look and see if you agree with me.
>
> The machine uses eight simple squarewave oscillators to generate two 
> "metallic noise" signals for cymbal synthesis. These are called Ring1 
> and Ring2, because they're generated using pairs of oscillators "ring 
> modulated" together by 4070 XOR gates. The schematic looks like this:
>
> https://electricdruid.net/wp-content/uploads/2023/11/RingGenerator.jpg
>
> The whole thing is here:
>
> https://www.synthxl.com/wp-content/uploads/2021/10/korg-kr-55-Service-Schematic.pdf
>
> The schematic gives us frequencies for the eight squarewaves. We also 
> know that the general pattern for such oscillators is Freq = k/RC, so 
> it's just a question of working out what k is in this case (using the 
> LM339 on a 5V supply).
> It occurred to me that we can assume the designers wanted the tuning 
> pots roughly in the centre, so if we feed in the required Frequency, 
> with the midpoint R value and the given capacitor, we can work out k. 
> That gives us this:
>
> Required Freq 	R 	C 	K
> 272 Hz 	125 KΩ 	22 nF 	748000
> 8529 Hz 	49 KΩ 	8.1 nF 	3385160.1
> 633 Hz 	125 KΩ 	9.1 nF 	720037.5
> 9073 Hz 	49 KΩ 	1.5 nF 	666865.5
> 1458 Hz 	125 KΩ 	3.3 nF 	601425
> 13500 Hz 	49 KΩ 	1 nF 	661500
> 731 Hz 	125 KΩ 	7.5 nF 	685312.5
> 3946 Hz 	125 KΩ 	1.5 nF 	739875
>
>
> This is all pretty reasonable, with values between 600,000 and 
> 750,000, which is not much of a spread. However, that second row is an 
> exception - it's way out. Working out the average value, excluding 
> 8529Hz, gives us k = 689,002. We can then put that k value back in and 
> see where the tuning pots need to be in theory:
>
> Required Freq 	Required R 	Midpoint R
> 272 Hz 	115.1K 	125 K
> 8529 Hz 	10.0K 	49 K
> 633 Hz 	119.6K 	125 K
> 9073 Hz 	50.6K 	49 K
> 1458 Hz 	143.2K 	125 K
> 13500 Hz 	51.0K 	49 K
> 731 Hz 	125.7K 	125 K
> 3946 Hz 	116.4K 	125 K
>
>
> As you can see, all the "Required R" values are pretty close to the 
> midpoint R values. The fifth line, 1458Hz, is pushing it a bit - we 
> have 150K as the maximum value from the resistor+trimmer, so that's a 
> bit close to the end, but it's still within, just about.
>
> The BIG EXCEPTION is that second line, which needs an R value five 
> times smaller than the available 39K+20K trimmer. What's going on? Is 
> this just a mistake on the schematic? Is the frequency wrong, or is it 
> the cap value that is wrong?
>
> Any thoughts much appreciated.
>
> Thanks,
> Tom
>
> ==================
>        Electric Druid
> Synth & Stompbox DIY
> ==================
>
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