DV Maxikorg Tuning drifts across octaves

[Musicwerks]

Hi all,

I recently acquired a 3rd hand Univox Maxikorg DV800 (I paid a bomb for it ).

The lower deck works fine. But the upper deck, no matter how I tune the left master and fine tune buttons, the notes go out of tune as I scale down the octaves.

In the upper deck, the 2nd C is out of tune with the middle C...

a) Is it a case of leaking capacitor? In need of re-cap? if so, how do I do it?

b) Or is there a master tuning-pot (what it that? something I read that needs a screwdriver to turn inside the synth, I know my newer synths had a tuning behind the synth) in the synth which I have to re-calibrate?

Be gentle with me, I am a newbie, everyone starts somewhere...and I am sincere to learn with a genuine question here to ask.

Many thanks.

[Solderman]

If it uses the same circuit boards as the Minikorg, there should be two trimpots on each VCO board inside the machine, for upper and lower calibration of pitch scaling. I couldn't find a proper Maxikorg service manual online though, so you may want to hold out for someone who has actually done the procedure.

[Jabberwalky]

Solderman is onto it. I've never seen a Maxikorg, but I wouldn't be surprised if they just dropped in 2 minikorg boards.

[mmp]

Schematic here:

MaxiKorg Schematic

There are two pots used for calibration.

[Musicwerks]

Hi guys,

Thanks for telling me about the trim pots. So, one trim pot is for each deck I supposed.

The previous owner told me old synths need to be warmed up to to sound in tune? It did happen, the tuning of the octaves improves after warming up about 30mins. Is that a case of leaky capacitor which cannot retain the electric current?

My minikorg doesn't have this problem, it is rock steady and stays in tune even when just switched on...

Cheers

[mmp]

I am sorry my memory failed me. If you look inside the Korg, there are a row of pots at the top of the two oscillator boards. There is one for trimming the tri wave, then offset, pitch hi, pitch low, then two for tuning the bender pitch hi and pitch low. it is the 2, 3, & 4 trims that will tune it for you. The procedure is a bit difficult to explain and I haven't done it for a while, but basically you set the pitch knobs on the front panel to the 12 o'clock position. Then use the offset trim while hitting middle c to get that in tune, then hit the top c and bring that in tune by using the pitch hi trim, then hit the low c and trim the low pitch trim pot....repeat & repeat until the instrument is in tune. It sounds a bit simpler than it is, because as you adjust each trim pot, everything slides a bit that you did before...but, after a while you hone in on it.

[Solderman]

So, one trim pot is for each deck I supposed.

No, for each VCO there is an upper and a lower trimpot for calibration of pitch scaling. Going off mmp's description, it should be trimpots 3 and 4.

The previous owner told me old synths need to be warmed up to to sound in tune? It did happen, the tuning of the octaves improves after warming up about 30mins. Is that a case of leaky capacitor which cannot retain the electric current?

This is normal behavior for most vintage design of oscillators. That said, I seem to recall the oscillators in the Minikorg did not need much warm up time, so it's possible there's some kind of problem, although I doubt it's due to old electrolytic capacitors. Tempco circuits for VCO's don't work off of a charge, they work off principles of changes in heat proportional to opposing changes in resistance.

[Musicwerks]

Hi Solderman and MMP,

Thanks for the lesson on VCO and trimpots...

Thanks mmp for directing me to the principles of trimpot tuning


"Tempco circuits for VCO's don't work off of a charge, they work off principles of changes in heat proportional to opposing changes in resistance".

Ehrmm, so what do you suppose might be the problem of the DV800?

Thanks guys for the quick lesson here.

Cheers

[Solderman]

I unfortunately do not know enough about the design to say for sure, you'd have to ask someone with experience on maintenance and repair of the Maxikorg. I suggest letting it warm up for an hour or so, then if the tuning procedure still needs to be done, do it and see what difference it makes for next time.

[Musicwerks]

Hi Solderman and MMP,

Thanks for the great advice given to me...

I took out the panel of the MaxiKorg and tuned the Pots and managed to get the synth more in tune across the octaves. Now it just takes 5-10 mins of playing for the Synth to get in tune.

Grateful for the help you guys have given to me!! Now my Maxikorg is certainly a lot more in tune and the $$$ I spent is well now considered well-spent

Up next is doing up the cosmetics of the Maxi Korg! I have airbrushed the panels and now I am left with repairing the chipped wooden panels with wood putty and painting it with Ronseal.

I am now a happy synth guy

Cheers

[mmp]

I am glad to have helped. I think the Maxikorg is a great synth. Have a blast!

[mmp]

I will add that I think there is probably something failing in the synth if it takes that long to stabilize. While that is a common problem with many early synths, the Maxi is not usually afflicted that way. I think 5-10 minutes isn't probably a huge problem, but it does sound like there is an issue & the symptoms point to leaky capacitors to me, maybe in the power supply if it seems to affect both oscillators.

[Musicwerks]

Hi MMP,

It only affects the upper deck of sounds. The lower decks stays in tune all the while...

So leaky capacitors in the upper deck?

[mmp]

Probably so.

[Musicwerks]

Okay....cool...

So does anyone knows how to recap a synth without blowing it up?

Anyone I can refer to?

Thanks!