Recalibrating an old Minimoog

[SynthWizard]

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Recalibrating an old Minimoog

Hi guys,

I have a mid-1970s era Mini - older oscillator board. The tuning isn't quite right (oscillator 3 is offset by a full semitone from the other two oscillators at the zero point) so I figured it's time to tune it up properly. I've had the Mini about 15 years, I'd always intended to get around to doing this but have never had the time, as per usual. Before I got distracted, I straightened out the keyboard and replaced all the keyboard bushings, had the cabinet restored, and fitted it with all-new knob caps from the dotcom crowd, so it looks the part. The only minor niggle is that the woodworker sanded the edges of the two badges a little, rather than taking them off and then gluing them back.

Inside, the boards seem to be original as they have been marked with the serial number in pencil in the factory. I can see that someone in the past replaced all the trimmers on the oscillator board with modern-looking sealed plastic ones with a little brass adjustment screw. The rest of the trimmers throughout are the open type. The new trimmers are stuck to the board and have little pieces of wire routing back to the original pinholes.

There is no sign of blown electrolytic caps that I can see. The synth operates more or less as you would expect apart from the tuning issue and one or two pots being a little scratchy. I understand that many of the pots cannot be easily replaced which is a real shame. There has always been crosstalk from the oscillators ie if you turn the output up high enough you can hear the oscs even though the VCAs are not operating. I've always assumed this is just the Mini being itself.

For obvious reasons I went to check the power supply rails first and located appropriate test points on the oscillator board. I found that the +ve is at 10.04V, the -ve at 9.85V. But I've found that these voltages correspond to the furthest position on the trim pots, any adjustment only moves the voltage further away (the pot seems to have a range of about 0.5V ?). The fact that the pots were set to the furthest position when I found them suggests it's been like this for a while. Is this an indication that something on the power supply board has drifted out of spec ?

For the power itself, I'm in the UK so I've been using a 240v->110v step down transformer. However it looks like the Mini has a 220/110 switch on its PSU. Is it just a matter of flicking the switch and changing the plug ?

submitted by /u/belfastard
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