Minimoog possible bleed?

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1

Hi, I’ve just bought myself a late serial (124**) Minimoog and am over the moon with it…apart from a fault that’s suddenly appeared following a 4 hr car journey back from the seller’s house (where it was playing beautifully!).

Basically with all Oscs switched off pressing any key on the keyboard will generate clicking along with occasional bursts of something else, static-like in sound. If you switch in an Osc on sawtooth or square the signal-to-noise ratio means you can’t hear the noise, but on something like a triangle wave you can clearly hear the interference.

I am a learning novice when it comes to electronics, I initially guessed at some kind of LFO bleed (from Osc 3?) but changing the Octave knob doesn’t affect the interference being produced at all.

Does anyone have any idea what the problem might be, or where to start looking? I’ve opened the back up and it looks very clean to me, all boards look securely seated etc. but of course I could be missing something equally obvious…

I’ve attached a small audio example so you can hopefully hear what’s going on:

https://www.dropbox.com/s/c5uq5n38f8x2u7a/Minimoog%20Noise.m4a?dl=0

I start with all Oscs off, and the keyboard pressed down, changing the cutoff. Then bring in Osc1 on sawtooth, fade it down so you can hear the interference still present, then with a triangle wave where the signal to noise ratio is much worse.

I would really appreciate any help at all! I’m hoping it’s something very simple as it was working fine when I demoed it at the seller’s house (I checked everything and it was all perfect).

Thanks in advance for any advice!

Steve

2

Hi Steve, welcome to this amazing forum on all that is Moog!
Simply put three obvious things could have changed between when you first checked out the Minimoog and after you took it home:

1 - An electrical connection could have shaken loose inside it on its journey home (like part of the wiring harness under the mod wheels).
2 - The playback chain is now different (including patch cords).
3 - The electrical outlet and local interference from household devices, radio stations and other EMI have changed.

Your solution is in pursuing these three potential issues starting with 2 & 3.

Your issue is not a major one. Vintage synths actually have some noise present when the filter is open and the oscillators are off and not masking the noise, but it shouldn’t be noticeable in most situations.
I wasn’t able to listen to your sound sample yet but i hope I have given you some useful guidance here about resolving the significant increase in noise that became apparent after you took your Minimoog home.