I’m new in this forum, my name is Patrick and I live in France. I’m trying to restore some synth for myself and friends, I’ve gain some experience with Polymoog, ARP, Korg and else.
Today I’m trying to tune two Mini model D, one with oldest oscillator board (s/n 3351), one with latest oscillator revision (s/n 13034).
I could tune succesfully the oldest oscillator board, following the manual I tuned the 3 VCOs to get A440 on lowest A in 2’ range.
When I tried to tune the revised oscillator board, all the VCOs were adjusted surprisingly one octave too high (A440 was on lowest A in 4’ range).
I could readjust succesfully VCO #1 (A440 on lower A in 2’ range), but it’s impossible to adjust VCO 2 and VCO 3 - shift trimmer is always at the end, impossible to get correct adjustment.
I guess there could be a fault which was compensated by adjusting the VCOs one octave higher, I think UA726 could be faulty…
I need some advice before buying those ICs (huge price… ), is it a fault, or any mistake from my side?
I’ve send a similar question on PM before seing my post appeared in the forum, sorry for that, do not take it into account…
I’ve checked the resistor values, and found that the resistor between the osc frequency pot on the front panel and ground was 3.3k instead of 5.1k as on the schematics. I will replace them and retry.
This problem sound more like one is installing a VCO board that’s been modded with a resistor to compensate for a pitchwheel modification that’s no longer present.
Check the pitchwheel and if it has added resistors and a diode, it’s been modified for a dead band.
Some photos of your VCO board and pitchwheel wiring would help for us to know.
I’ve just find the fault right now.
You are right, it was located at the pitch wheel. Pitch wheel pot has been replaced by a previous owner and incorrectly installed. It was just at the limit to tune osc 1, but out to tune osc 2&3.
Now I could retune all perfectly.
There is no dead band mod on this Mini, is it a mandatory mod, or just to add some playing comfort?
Once again, thanks to you and your very helpful documentation.
The dead band mod is a very personal choice. Some people find the pitch bend wheel too sensitive near the center notch.
I rather use that to my advantage by allowing me to create manual vibrato, by very sligthly moving it in its notch, without having to sacrifice osc 3 as an LFO assigned to pitch to do that with the mod wheel instead.
But I agree that it’s not always easy to put it back perfectly in pitch sometimes. I had to re- adjust the tension on the spring for the center notch though, in order to prevent the wheel from moving too easily…
It’s entirely up to you to decide if you mod it, or not… And I wonder how “wide” is the dead band after modding it ?
The original non-deadband pitch wheel was also a source of drift. There is no guarantee that the voltage at center detent will remain exactly the same, hence the tuning will drift.
The diodes create a deadband. (no change in voltage from the pot within a short range of travel.)
A diode generally has about .6 volts of voltage drop when placed inline with a voltage.
To create the deadband of the pitch pot, one physically locates the exact center of the pot (normally a virtual ground.)
Then they place two diodes back to back inline with the pot voltage.
That means the pot must travel a certain distance before any voltage is output.
In other words, it KEEPS things in tune. ’
The tradeoff is that one must move the wheel a little farther in either direction before it sends out voltage.
If this mod is not present, a wheel assembly must have a very good and reliable physical center detent AND a clean, noise-free pitch pot.
Yes indeed! Luckily, the Allen Bradley sealed, carbon wiper, conductive plastic surface, potentiometer type, helps a lot at being reliable. Although as sensitive as an electric guitar’s whammy bar (vibrato arm).
), is it a fault, or any mistake from my side?
That could really throw something out of tune!