[sdiy] Korg KR-55 drum machine query
Tom Wiltshire
tom at electricdruid.net
Mon Nov 13 14:44:37 CET 2023
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 <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 <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
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Electric Druid
Synth & Stompbox DIY
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