Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

tasters · Post by **tasters** » Sat Apr 25, 2009 5:14 pm

hello guys amd thanks again for the great community!
i'm the proud owner of a 1974 minimoog 6447 wich i'm restoring and now is in near perfect working order but remain an huge unique problem to fix:

The OSC 1 and OSC 3 works very good,
they tuned perfectly and make their work, but

if i move the RANGE SWITCH of the OSC 2 it affect the RANGE of the other two oscillators (the OSC 1 and the OSC 3).

This happen with or without the OSC 2 sqitch ON in the mixer section.

the only way to mantain the range of the OSC 1 and the OSC 3 right is to set the OSC 2 range to LO.
in this way the OSC 1 and the 3 works good but the OSC 2 in LO mode is unusable.

Musically speaking if i press A on the keyboard with the two working oscillators ON and i move the OSC 2 RANGE switch from LO i obtain:

OSC 2 in LO range = the other two oscillators at unison in 16' range works good and maintain the A note.
OSC 2 in 32' range = the other two oscillators at unison in 16' range sound the F# note.
OSC 2 in 16' range = the other two oscillators at unison in 16' range sound the G# note.
OSC 2 in 8' range = the other two oscillators at unison in 16' range sound the E note.
OSC 2 in 4' range = the other two oscillators at unison in 16' range sound the C note.
OSC 2 in 2' range = the other two oscillators at unison in 16' range works good and maintain the A note as in LO range.

i hope i was as clear as possible
any help to solve this problem?

thanks to all
ciccio

Steve Jones · Post by **Steve Jones** » Sun Apr 26, 2009 12:50 am

Faulty LM741 (IC2) on the octave buffer board?

tasters · Post by **tasters** » Sun Apr 26, 2009 10:19 am

i try to check the little buffer board today and we see

thanks

tasters · Post by **tasters** » Sun Apr 26, 2009 2:58 pm

here it is a small video i've made to show the problem

also i've see at the same RANGE settings the OSC 3 is an octave lower than OSC 1...

tasters · Post by **tasters** » Mon Apr 27, 2009 5:13 pm

hi guys
today finally i've solved the problems! A special thanks go to Steve Jones, as he said, was a faulty LM741 (IC2) on the octave buffer board, now the oscillators sings perfectly!

thanks again!

Steve Jones · Post by **Steve Jones** » Mon Apr 27, 2009 10:46 pm

Congratulations, well done! The IC had probably developed a short internally and was biasing the other two chips as it would have been drawing excessive current.

sam · Post by **sam** » Tue Apr 28, 2009 9:37 am

Good fix guys....

iProg · Post by **iProg** » Tue Apr 28, 2009 9:45 am

Wow, Steve, cool that you're able to recognize the issue so fast and knowing exactly how to fix it...impressive!

I also have a great tech around...he never lets me down and it's not expensive. He's also well equipped with original spare parts!

tasters · Post by **tasters** » Tue Apr 28, 2009 1:45 pm

yes, the skills are great, steve have many many years of experience as i've read in his site!
also, here in Sicily, i don't have any good technician for repairs!

thanks!

CPP_programmer · Post by **CPP_programmer** » Thu Jan 05, 2017 3:06 pm

I would like to discuss a related problem. My Mini has non-working OSC 2 completely.

It is definitively NOT an issue of VCO board (Mid - 3046/741 based model), because I have an original one (SN9xxx) and another one with SN19xx, both behave the same. With both boards it has exactly the same symptoms - OSC 1 is flat by about fourth, OSC 3 is in tune. OSC 2 doesn´t work completely, it doesn´t oscillate at all (checked by oscilloscope in saw core). By moving OSC 2 Range switch, OSC 1 goes little sharp or flat by a few cents. I can hear a very very slight (one should be very carefull to hear it but it is there) OSC 1 pitch change by moving OSC 2 Freq knob from lowest to highest position and vice versa.

I already knew about things discussed here, so I checked buffer board as one of the first places. There were some unquality sockets there (strange, originally there are none), so I replaced them by modern precious ones and I used new 741s from my spares, because original ones didn´look good. I measured all voltages on CO1A socket and checked their changes, measured various voltages on PCB, everything looks good.

The strangest thing is, that it even worked for a minute in one evening when I was debugging it. I had one key secured in pressed state and manipulated both OSC 2 Range switch and Freq knob while I measured voltages on osc board and it started work. It was unstable at first, but after a few seconds it was ok. There was no change in pitch of OSC 1 by moving OSC 2 Range switch too. But then it stop to work again. I never manage to do it again from then.

My thought is there is a bug in Range switch of OSC 2 itself. But from a schematics it is not clear for me, how it could affect that, because from buffer board goes to OSC 2 a correct voltage (same as those to 1 and 3 in the same range settings). Do you have any advice? Thank you in advance.

Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1 & 3

Re: Minimoog Model D problems: OSC 2 range: it control OSC 1