Hi all, I’m brand new to this forum and also to the world of the Minimoog.
Ok here goes!..I have taken possession of a model D. It was last used about 30 or so years back, I’m in the process of renovating it and have a few questions. Rather than ask too many straight off the bat I’d like some info regarding a couple of mods it’s had.
It appears to me that it has gained 2 or 3 toggle switches (see pics). I wonder if anybody’s seen these mods before?
Where there should be 2 jack sockets on the left controller module there are 2 rotary controls! Any ideas?
Also, while I do have sound it is constant IE. play a note and it plays constantly until another key is played and then that tone/note plays continuously. I’m confused..
I’ve attached some pics of the areas. Any help or advice would be gratefully received.
Update: The continuous tone is a function of the toggle switch above the oscillator modulation switch. So that’s that one solved.
I now have this odd problem that osc 3 stays on on frequency IE: whatever key is pressed first dictates the tone regardless of what key is pressed next but this is only on osc 3. The other two behave correctly.
The toggle switch next to the external audio in gain might use the well known, normally external cabled, self feedback track.
Switching the audio output to the external in and the activated external input volume used to overdrive the mixer and filter.
As these mods were normally done only on a very few or single instruments, they do vary in layout of the controls and the features.
This looks not like a factory modification.
Most modifications that involved additional switches and knobs are very likely done by some technician or a musician. And not by Moog or a Moog technician.
And some of the pictured modifications appear not being done well.
Trimpots have a limited life. As they age they can cause VCOs to drift. The ones in your unit are not original.
Don’t replace any caps if they are original. The original polystyrene caps were designed to compensate for temperature changes and should remain in place.
Warm up time is required because the linear to exponential converter in the Minimoog VCOs contains a term in its transfer function that is impacted by temperature (read: the circuit is sensitive to temperature). The components warm up and eventually reach a steady operating point which the circuit functions at its optimal condition - accurate pitch tracking.
The other reason why Minimoog VCOs drift is the design of the system not just the VCOs. It takes more than replacing trimpots and caps. The power distribution must be corrected, reference biases must be redesigned, components with poor tempco specs replaced with modern ones, control sources must be redesigned.
The original trimmers were wirewound and aren’t made anymore. I used to sell them, but I sold off all my parts nearly 3 years ago. The mod done here is one approach to solving the problem of obsolete parts I’ve never seen before.
The electrolytic ones do need to be replaced. One of the ones in that mod in the first set of pictures is blue and looks blown already.
There’s also 6 sets of 3 matched resistors on the right side of the oscillator board that benefit from being replaced. I’m in the process of making these kits again. I’m being gifted a good meter, which will speed up the process tremendously!
