Mini Moog D OSC gone Super High

[grayedout]

Hi All, just found my OSC 1 has gone super High (like a dog whistle ) just audible .. wonder if it a simple fix know problem ?


Andy

[analogmonster]

One of my Prodigy oscillators had the same problem, caused by a dead transistor array. AFAIK the Mini has SG 3821 arrays inside, perhaps one of them went to the happy silicon hunting ground. If they are in sockets you might do a "quick" check by exchanging two of them (OSC2 <=> OSC1 perhaps) to see whether the problem changes to OSC 2.

[grayedout]

One of my Prodigy oscillators had the same problem, caused by a dead transistor array. AFAIK the Mini has SG 3821 arrays inside, perhaps one of them went to the happy silicon hunting ground. If they are in sockets you might do a "quick" check by exchanging two of them (OSC2 <=> OSC1 perhaps) to see whether the problem changes to OSC 2.

that could be it, do you have any pointer to were they are on the board or what they look like / cheers!

[Kevin Lightner]

While you might locate the problem by switching the tranny arrays, please understand that the Minimoog uses only 2 arrays for 3 oscillators.
They're shared.
So the problem might move if the chip is bad, but it's hard to say.
I'd guess more that an opamp has died and is putting out full voltage to that oscillator. (10 volts, ie: 10 octaves of pitch.)

If you don't know a local tech that you trust, I can accept individual Mini boards sent in for repair.
It's a whole lot cheaper than sending an entire Minimoog anyway and the repair would be given a new limited warranty.
Either way, I wish you luck with the instrument.

[grayedout]

While you might locate the problem by switching the tranny arrays, please understand that the Minimoog uses only 2 arrays for 3 oscillators.
They're shared.
So the problem might move if the chip is bad, but it's hard to say.
I'd guess more that an opamp has died and is putting out full voltage to that oscillator. (10 volts, ie: 10 octaves of pitch.)

If you don't know a local tech that you trust, I can accept individual Mini boards sent in for repair.
It's a whole lot cheaper than sending an entire Minimoog anyway and the repair would be given a new limited warranty.
Either way, I wish you luck with the instrument.

My Problem is I live on the isle of wight. I have a very good friend thats a tech but we cant get the parts local so it takes for ever finding the fault the ordering parts.. would I just send you the OSC board ?

[grayedout]

also spotted this cap was not connect on one leg , is this correct ?

[grayedout]

One of my Prodigy oscillators had the same problem, caused by a dead transistor array. AFAIK the Mini has SG 3821 arrays inside, perhaps one of them went to the happy silicon hunting ground. If they are in sockets you might do a "quick" check by exchanging two of them (OSC2 <=> OSC1 perhaps) to see whether the problem changes to OSC 2.

I just swapped them over but no luck..

[thealien666]

also spotted this cap was not connect on one leg , is this correct ?

Certainly not.

I have answered in your other thread about this. It is C2 which is a 100 pico Farad ceramic capacitor, directly connected to IC 2 which has the current mirror for VCO 1 pitch control (as well as VCO 2).

Re-solder or replace the capacitor.

[grayedout]

Thanks for this. I did re solder it. but it didn't fix the OSC problem.. very odd at it looked like it had never been solder i the first place

can't find a simple patch to test for bad cap in OSC 1 look a schematics but can work it out..

Andy

[thealien666]

Looking at the photo of your oscillator board that you provided, in another thread, it dates back to 1972.
http://forum.moogmusic.com/viewtopic.php?f=1&t=19666

I would suggest replacing both SG3821N with more recent CA3046. I suspect that one of the transistors inside IC2 might be defective. Or, as Kevin has suggested earlier, one 741 op amp might have failed.
Replacing all aging op amps (741) would be good.

[MC]

On the 3046 VCO board, *one* of the 741s must remain. I don't remember which, but it is not any of the summing circuits. The 741-centric circuit doesn't work with any modern opamp like LF351, TL081. I suspect that the circuit was designed around the input or output stage of the 741, never did get around to any analysis on it. That style of design prevailed during the design era of the minimoog.

CA3046 and SG3821 are compatible. SG3821 was original equipment because the matching was much better. The CA3045J is actually a closer substitute to the SG3821.

[grayedout]

OK that sounds like a good place to start .. will that change the sound in anyway

Does anyone know best place to buy CA3046 or 741 Op Amps..?

[grayedout]

would some on ebay . will these work?

http://www.ebay.co.uk/itm/CA3046-Genera ... 2326e7006c


http://www.ebay.co.uk/itm/741-OPERATION ... 45f13f050f

[thealien666]

Yes.

will that change the sound in anyway ?

It should not, since the op amps will be the same. The transistor arrays (SG3821) are only for pitch control and not for waveform creation.
You will probably have to recalibrate the oscillators once chips have been replaced, however, which is normal.
I would also suggest installing quality chip sockets for the op amps (for easy removal/exchange next time one fails). Choose sockets with round legs (not the cheap flat ones).

[MC]

OK that sounds like a good place to start .. will that change the sound in anyway

Does anyone know best place to buy CA3046 or 741 Op Amps..?

Try Mouser.com or Digikey.com

LF351 or TL071 best substitutes for 741 and it will improve the tuning stability.

CA3046 have been out of production, only direct substitution is NTE912 from Mouser.

Part substitutions:

http://home1.gte.net/res0658s/electroni ... _subs.html