Korg Mono/Poly

Hi everyone,

I just noticed a problem on my Mono/Poly keyboard:

When I have the Transpose switch on the Up position the upper keys sound flat. Every note is fine until the higher E which goes flat by 20-30 cents. Eb sounds good, but from highest E to the highest C everything is flat.

The problem is only there when Transpose is in the Up position, it works fine when in Normal or Down.

Checked the PSU voltages and the Key Assigner Voltages shorting the Tune Point, and they all seem fine. As the synth was opened I calibrated the VCOs because they were a little out of tune when set to 2'. All good now.

Any thoughts will be much appreciated.

Thanks.

> Checked the PSU voltages and the Key Assigner Voltages shorting the
> Tune Point, and they all seem fine.

> Up to 5V all is good, but I can't get up to 6V. Highest C is 5,97V.
>
> I just noticed that the arpeggiator set to Full also shows the problem
> on the highest octave.
>

About those resistors you mention, as far as I see:

R19 is 14.3K. Are you sure the values written there in the schematics are not 28.9K and 14.3K? 

R21 is 14K. I think you misread 196 on what is actually 1% and the new value is written below, hardly legible, but it could be 14.0K.

You are right, the 196 is in fact 1%

Lets think about the complete circuit and the effect of the trimmers:
We have the 7Bit D/A around the 4096 with the resistor cascade. VR3 determines the impact of the most significant bit.
After IC8 we should be able to read the outcome of this D/A.
VR2 determines the current, which runs from the D/A to IC7.
VR1 determines an offset current, which runs also into IC7.

If everything is linear below the E then mostly the MSB line (which includes VR3) becomes suspicious. Why? From F (with transpose set to "down") to E (with transpose set to "up") there are 60 steps. We know from Tubbutec, that the firmware of the keyboard scanner is based on the Polysix (whose keyboard starts at C). From C to E we have 64 steps => the first 6Bits determine the notes between C and Eb. When the most significant bit (whichs output is controlled by VR3) goes high, then E and up are given.

So to me it looks like simply the description the first table in the adjustment instructions is wrong: VR3 is responsible for the high tuning. VR2 is for the V/Oct setting of the D/A and VR1 is for the offset (total tune).

In consequence I suggest to adjust the D/A measuring all voltages at the CV-Out socket like that:
1. set tune point to closed
2. set transpose to normal
3. adjust 1.000V per octave using VR2 while playing octaves jumps from lowest F to highest F. Do not obey the absolute voltage; only check, whether jumping one octave makes a voltage change of exactly 1.000V
4. when this is done, play always the highest E and toggle the transpose switch between normal and up; adjust VR3, that his octave jump reflects a change of 1.000V relatively (again: do not care for the absolute value)
5. when this is done, open "tune point", and then adjust VR1 to read straight 1.000V/ 2.000V/... at the C's,

If the adjustment of VR3 fails, then it might be necessary to decrease R52 to 1k.


What do you think about it?

Florian

> I'm not sure if I should remove it, mesure it and replace it with a
> resistor of the same value. It is working now, so why bother?

>
>
> I measured the trimpot and it is set at around 2.18K. I just noticed
> that the original R52 that I removed was indeed 10K 5%. Odd. I really
> don't understand what was happening when I replaced it with 1K, 3.92K
> and another 10K (!). But after 3 days trying to fix it, now that it is
> working correctly, I think I'll stop messing with it and actually
> spend some time playing it!
>
> Best regards.
>
>
>