Page 1 of 2

Minimoog Model D Osc 3 Control switch defies expectations

Posted: Wed Apr 20, 2016 1:59 am
by spiders
Hello again friends.
I scored a minimoog. I've been looking at these things since the 90's. This is a big deal to me.

It was in pieces in a basement. I was buying other things from the owner of said mini, he didn't even really mention the thing. I couldn't stop thinking about the mini, and called the owner to work out a deal. He gave me an extremely reasonable offer, but still more than I could afford. The next day, boom, like a bolt of money lightning from the sky, I get the word that a wurlitzer 140b I rebuilt on spec some time ago was to be bought by a big studio here in town. Half the money appeared. Not 12 hours after this first windfall a friend bought a keyboard from me I'd been trying to sell. There was the rest of the money. I ran screaming (internally anyway) to guys house and made the deal.

Huzzah

It is a 1974 that was sent back to the factory for the new osc board and octave buffer. At some point hobbyist mcghee did a pretty bad electrolytic cap job on it, but they work. I have worked out all the issues with it with the exception of the Osc 3 control switch. It boosts the frequency of osc 3 to a huge degree when in the off position. This is obviously the opposite of the expected result. When you have the range set to LO the triangle waveform looks like it is at about the '8 setting. I think it may be the osc summer, but not totally sure.

The edge cards are clean and make good contact. Switching the osc 3 control flips one voltage between -10 and -5, and connects the 7.5v line. Seems to be wired up properly. Does anybody have any suggestions. Once again, this is the new oscillator board.


Thanks !!!!!

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Wed Apr 20, 2016 8:17 pm
by Stevie Ray
Model Ds are now so ancient and generally hacked around by clueless DIYers that anything is possible. The only way you'll get to the bottom of this is with a lot of patience and a good meter and 'scope. A while ago I did a Model D restoration for a former band-mate (he's a great player but completely clueless about electronics). The thing was a wreck. Hundreds of hours of my life I'll never get back. I even sanded down and re-varnished the woodwork - it looked mint. The worst thing was he sold in on almost immediately and got a Voyager with the proceeds.

Make sure you replace all the electrolytic caps even if you think they are working. Check the voltages at points 9A, 10A, 11A and 15A on the osc board. Check that transistors Q3 and Q19 are functioning properly. Good luck!

http://www.synthfool.com/docs/Moog/mini ... Manual.pdf

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 5:29 am
by noddyspuncture
Stevie Ray wrote:Make sure you replace all the electrolytic caps even if you think they are working.


Doing this might be regretted later... I did just that on one of my Mini's and it never did sound the same again.
Always replace the electrolytics in the power supply, yes... but all the others I would leave... unless they are obviously leaking. It can really make a 'Great' sounding synth sound just 'OK'.

Cheers,
Tom

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 7:09 am
by megavoice
Stevie Ray wrote:..........The worst thing was he sold in on almost immediately and got a Voyager with the proceeds........


OH MY GOOOOOOOOODDDD !!!!!!!!!!!!!!!!

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 10:57 am
by spiders
Hey there, Thanks for the replies! I sorted it out. My problem was twofold

Fold the first ) The guy I bought it from said that this was an issue, and I believed him. I am a trusting person by nature. This preconception of malfunction followed me to....

Fold the second) I've never messed around with a real mini before. My weed ravaged brain read the service manual incorrectly and I thought flipping the osc control should lower the frequency of the oscillator. What it in fact does is expand the range of osc 3 frequency control, amongst other things. So I was expecting a drop in frequency when I hit the switch, but was met with a jump due to the change in behavior of the osc 3 control AND the fact that the mini applies it's own CV to the osc when control is off. So I convinced myself I had a problem where there was none, and set fire to several hours. Oh well. I learned a lot about how the oscillators work during the process, and that is valuable knowledge. I also learned for the millionth time to pay closer attention to technical documents.


other than osc 2 making occasional coughing sounds the mini is done. I put it in its case last night. Here's a pic.

IMG_3774.JPG


for some reason this pic is upside down, if you click on it everything works out ok, though.

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 3:39 pm
by Stevie Ray
noddyspuncture wrote:
Stevie Ray wrote:Make sure you replace all the electrolytic caps even if you think they are working.


Doing this might be regretted later... I did just that on one of my Mini's and it never did sound the same again.
Always replace the electrolytics in the power supply, yes... but all the others I would leave... unless they are obviously leaking. It can really make a 'Great' sounding synth sound just 'OK'.

Cheers,
Tom


Good point Tom. As they age electrolytics can add noise which can add character to the sound. Personally I would replace them but your point is a very valid one. The one I restored sounded awful so there wasn't much sonic character to lose.

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 3:43 pm
by Stevie Ray
megavoice wrote:
Stevie Ray wrote:..........The worst thing was he sold in on almost immediately and got a Voyager with the proceeds........


OH MY GOOOOOOOOODDDD !!!!!!!!!!!!!!!!


Yeah. My reaction exactly - worse he didn't let me know he'd sold it until showed up at a rehearsal with a spanky new Voyager. I would have bought it off him. He did pay me a good amount for the restoration work so I guess it was his business. Even so :cry:

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 3:56 pm
by Stevie Ray
spiders wrote:Hey there, Thanks for the replies! I sorted it out. My problem was twofold

Fold the first ) The guy I bought it from said that this was an issue, and I believed him. I am a trusting person by nature. This preconception of malfunction followed me to....

Fold the second) I've never messed around with a real mini before. My weed ravaged brain read the service manual incorrectly and I thought flipping the osc control should lower the frequency of the oscillator. What it in fact does is expand the range of osc 3 frequency control, amongst other things. So I was expecting a drop in frequency when I hit the switch, but was met with a jump due to the change in behavior of the osc 3 control AND the fact that the mini applies it's own CV to the osc when control is off. So I convinced myself I had a problem where there was none, and set fire to several hours. Oh well. I learned a lot about how the oscillators work during the process, and that is valuable knowledge. I also learned for the millionth time to pay closer attention to technical documents.


other than osc 2 making occasional coughing sounds the mini is done. I put it in its case last night. Here's a pic.

IMG_3774.JPG


for some reason this pic is upside down, if you click on it everything works out ok, though.


Looks great!

The osc3 control switch disconnects the control voltage from the keyboard so it can be used as a modulation source.

No idea what the coughing sounds could be. Is it a burst of noise or what?

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Thu Apr 21, 2016 6:37 pm
by spiders
Hey Stevie. Yes, I was aware that the osc control switch decouples the keyboard cv from the osc, I just was just surprised by the behavior of this action. Also, as I mentioned before, I misread the portion of the service manual pertaining to the control, so I was expecting it to do something that it does not. You can wait on that sort of thing all day.

The coughing noises were one half of a 393 op amp going a little funny in osc two. The cough only happened on the pulse waves, I isolated it with a scope and changed it out. It was fun seeing the wave freak out on the scope. I figured that was the problem from the first time I looked at the schematic, so I ordered the parts from mouser last week. At the time the coughing was the least major problem with the mini. So, as of this afternoon, this is a 100% working mini, resurrected from from the grave. #4714 is back in the game!!!!

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Fri Apr 22, 2016 6:11 pm
by Stevie Ray
Good to hear your Mini is fully functional. Great word Spiders!

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Mon Apr 25, 2016 7:18 am
by megavoice
noddyspuncture wrote:Doing this might be regretted later... I did just that on one of my Mini's and it never did sound the same again.
Always replace the electrolytics in the power supply, yes... but all the others I would leave... unless they are obviously leaking. It can really make a 'Great' sounding synth sound just 'OK'.

Cheers,
Tom


All what I know is VICE VERSA. I`ve always heared through the years the caps in the PSU should NEVER be changed, as they`d have a influence to the sound (???????????)

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Mon Apr 25, 2016 12:38 pm
by noddyspuncture
megavoice wrote:
noddyspuncture wrote:Doing this might be regretted later... I did just that on one of my Mini's and it never did sound the same again.
Always replace the electrolytics in the power supply, yes... but all the others I would leave... unless they are obviously leaking. It can really make a 'Great' sounding synth sound just 'OK'.

Cheers,
Tom


All what I know is VICE VERSA. I`ve always heared through the years the caps in the PSU should NEVER be changed, as they`d have a influence to the sound (???????????)


Not at all... the electrolytics everywhere else in the synth would be more likely to "change the sound"...!
Granted, really badly performing, faulty or out of spec power supply caps can change everything - analogue synths absolutely rely on "perfect voltages" - but to fix that you would change them..!

Cheers,
Tom

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Mon Apr 25, 2016 2:19 pm
by MC
noddyspuncture wrote:
megavoice wrote:
noddyspuncture wrote:Doing this might be regretted later... I did just that on one of my Mini's and it never did sound the same again.
Always replace the electrolytics in the power supply, yes... but all the others I would leave... unless they are obviously leaking. It can really make a 'Great' sounding synth sound just 'OK'.

Cheers,
Tom


All what I know is VICE VERSA. I`ve always heared through the years the caps in the PSU should NEVER be changed, as they`d have a influence to the sound (???????????)


Not at all... the electrolytics everywhere else in the synth would be more likely to "change the sound"...!
Granted, really badly performing, faulty or out of spec power supply caps can change everything - analogue synths absolutely rely on "perfect voltages" - but to fix that you would change them..!

Cheers,
Tom


^ I'm an EE and I approve of this message :wink:

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Tue Apr 26, 2016 6:59 am
by megavoice
Well YOU`re the experts, of course this seems to be right.........

Re: Minimoog Model D Osc 3 Control switch defies expectation

Posted: Tue Apr 26, 2016 7:27 pm
by Stevie Ray
Megavoice -- replacing ageing electrolytic caps in the power supply can only be a good thing. As others have noted above, replacing caps in the audio signal path can affect the the sound and character of the instrument so that's up to you. Personally, I replace ALL electrolytic caps and I've only ever noticed an improvement in audio quality and noise levels after doing this. Replacing the electrolytic caps in an ageing PSU is the first thing you should do in any restoration project IMO.