problems with tuning osc on Korg Mono/Poly

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moldysoul
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problems with tuning osc on Korg Mono/Poly

Post by moldysoul » Mon Aug 06, 2007 4:53 am

delicately reworded repost, now in the correct forum, my apologies:

well, I followed the instructions on the Zen Mono/Poly webpage on how to tune the oscillators, but one of them, osc 3, is being very unruly. it's wavering in and out of tune so badly at times it almost sounds like a random sample and hold. I thought opening it up and just turning the pot would fix it but lo and behold, it's gotten worse. I really want to try to fix this myself if at all possible. Any ideas?

Jared

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Post by Maschinengeist » Mon Aug 06, 2007 5:56 am

Zen Monopoly faq is not the best, especially about the tuning procedure (known as "Ghetto Tune") - the best way to f**k the oscillators. Another reason why I hate those websites giving away half-a*s "technical" advices.

Use this method, directly from Korg:

Image

Hope that helps. Keep us posted.
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Post by moldysoul » Mon Aug 06, 2007 4:28 pm

thank you very much... I will keep you posted. i appreciate it.

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Post by prophei » Mon Aug 06, 2007 6:27 pm

weird, my osc3 is doing the same kind of thing. very wavery, and all scaled wrong.
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Post by plus321 » Fri Sep 07, 2007 4:08 am

I don't really understand HOW to scale the oscillators. I understand that I need to check the value with a voltmeter, but after I find out it is wrong, then how do I correct this? I assume a trimpot, but I don't know where it is.

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Post by Maschinengeist » Fri Sep 07, 2007 4:21 am

To adjust, you need to refer to the chart above and set the proper VR (Variable Resistor or TrimPot). The number of each TrimPot is printed directly to the PCB (board).

To adjust the TrimPot, use a jeweler's screwdriver. And do it gently. This is not an operation to be done in 5 minutes.
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Post by plus321 » Sat Sep 08, 2007 12:38 am

I've figured out that the reason I seem to be having a hard time figuring out what everyone is talking about with respect to the chart is that my pcb is not labeled. The trimpots are there and I can see the holes, but there is no indication of what they are. I am also unsure of how to test the offset voltage. Any help would be appreciated.

Thanks.

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Post by plus321 » Sat Sep 08, 2007 3:24 am

After literally hours of trying to do this, I've come to the conclusion that I can proceed no further without some sort of help/advice.

I have been following this procedure and have had NO luck scaling the oscillator.

First I tune A#4(Highest A#) with VR 102 at 16' to be exact (0 cents off).

Then I tune A#1 (Lowest A#) with VR 103 at 16' to be -5~-10 cents off.

Then I tune A#1 with VR 101 at 2' to be exact (0 cents off).

Then I am left with tuning A#4 at 2' with VR104 which simply does not work, because, at this point, it is impossible to tune to A# at the extreme of the trimpot.

While writing this I thought I may have reversed the tunings for VR 104 and VR 101 so I switched the which trimpot I tuned with and it allowed me to tune all of the keys to A# as desired.

However, the keyboard is still badly out of scale. The first F is very (+20 cents) sharp and it sort of flat on almost every note until a certain note at which it goes sharp again.

I believe this problem to be with the trimpot VR1 and to be an effect of the offset voltage not being +-0.1mV. However, I'm not at all sure how to check this. I went to radioshack and bought a digital multimeter which was a bad idea in that its lowest readable value is 0.1mv. Even so, I don't know where or how to verify whether the offset voltage for the VCOs is 0.1mv (or even close).

Thanks in advance.

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Post by plus321 » Sun Sep 09, 2007 4:08 pm

Even if someone could please confirm that I am following the correct tuning procedure I would be very grateful.

I also need to know how to "test the lead wire from KLM 398" to test the offest voltage, but I will take any and all advice.

Thanks.

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Post by plus321 » Fri Sep 14, 2007 1:39 am

I've spent more time at this to no success. Everything seems to function, with respect to the keyboard, I just can't scale oscillator 1. In fact the other oscillators are perfectly scaled, but this is pissing me off because this really shouldn't be an issue.

I can't figure out how to "test the offset voltage" of VCO1. I have the tools, but don't know where to put the probes since my KLM 354 PCB is completely unlabeled.

Even after I measure the offset voltage of the VCO, then how do I correct it to +-0.1 mV? Which VR or trimpot do you change to change the offset voltage of the oscillators?

Thanks in advance.

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Post by Maschinengeist » Fri Sep 14, 2007 2:59 am

I haven't been much around (due to business), asap I'll try to clear things up.
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Post by plus321 » Tue Sep 18, 2007 4:19 pm

I've even tried to find a place (locally) that will service it, but have had no luck. Even if I found a place I'm afraid that I will be charged too much for them to do what I am already doing (turning trimpots).

I'm willing to paypal someone $25 for information that helps me scale the oscillator. I would just like to fix this so that I can actually use the mono/poly I got over a month ago.

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Post by Maschinengeist » Wed Sep 19, 2007 4:31 am

Drop me a mail at: tech@muzikelektronix.com. I'll help you out.

I don't have a mono/poly under hand right now but I'll prepare stuff for you. And I need the money too :D
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Post by psygoat » Fri Sep 28, 2007 4:42 pm

If you can't scale it by the method posted then the oscillator is faulty and no amount of tweaking is going to sort it out.

It's quite a straightforward procedure. A bit fiddly, but OK.

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Post by plus321 » Fri Sep 28, 2007 5:14 pm

No, the power supply needed calibrated and then when I went to tune it finally, VR104 shorted out and it needs to be replaced. I suspect that this was the problem all along. It could have been the power supply though, but after a high pitched squeal was made by the oscillator while turning it (gently), the trimpot no longer works.

Either way it is going to a professional now simply because I'm done messing with it.

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