Many questions!

[Old Scotty]

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.

Geoff

[Old Scotty]

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.

G

[till]

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.

[luckynerd]

Figuring out the mods might help if we could see behind the back panel and under the modulation section.

[Old Scotty]

Ok thanks guys.

The story has changed a little in as much as I've been reading the manual and this has demystified a lot.

That said I'm still a little confused with some of the mods. I'll get some pics of the internals up as soon as I can.

In the meantime...thanks again.
G

[Old Scotty]

Figuring out the mods might help if we could see behind the back panel and under the modulation section.

Thanks Lucky

Ok I've attached two pics of the mod behind the modulation section. It looks like whoever did the mod used a knife and fork to do it!

I'm thinking of either removing it and re-fitting the jack sockets or making a nicer job of the mod on tag-strip for example.

Any thoughts?

G

[Old Scotty]

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.

Thanks till,
Excuse my ignorance, I'm completely new to this (I'm liking it) are these toggles factory mods or more likely done by the previous owner?

I've now tested it by reading through the manual and it seems to be working as it should although I think it may need some calibration.

Geoff

[Old Scotty]

Here are some shots from behind the front panel..(switches are indicated)

[till]

Excuse my ignorance, I'm completely new to this (I'm liking it) are these toggles factory mods or more likely done by the previous owner?

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.

[Old Scotty]

Excuse my ignorance, I'm completely new to this (I'm liking it) are these toggles factory mods or more likely done by the previous owner?

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.

I thought as much. The work looks nasty, I'm going to make the connections much better.

Thanks G

[Old Scotty]

Ok, I'm much more familiar with this Model D now.

Question:
The oscillators seem to fluctuate quite a lot. Any ideas? Capacitors perhaps?

Why is there a warm up time? I ask this because it is solid state after all.

Also I notice that the tuning adjusters have been modified. Were the originals regular potentiometers? (see pic)

Thanks for all your help so far.
Geoff

[MC]

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.

[luckynerd]

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.

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.

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.

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.

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!

[Old Scotty]

Hmmm interesting stuff.

I note that the main smoothing caps look a bit dodgy and thinking about it it's not a bad place to start.

I guess if the supply is flaky that might show down stream.(Ref osc varying)

Thanks guys
Geoff