Moog Music Forum

Click

Chill Out Track done with my System 55 and the Formant Modular.

SPR wrote: Sun Sep 07, 2025 11:16 am A silent VCA out doesn't necessarily mean the VCA is dead.
...

Yes, so at the end you must do a kind of back tracing. Start at the VCA and "follow the rabbit" backwards to the saw core or whatever the first sound source is.

MIDI THRU normally is passive, meaning an incoming MIDI stream is just replicated, perhaps additionally buffered. I don't think that the MIDI stream is replicated by software. That would be possible, but is rather unusual. So you should see a bit stream at the output pin of the MIDI THRU socket with ...

The exponentiator section of a VCO is responsible for pitch drifts. The exponentiator converts a linear voltage (CV) into an exponential current within a VCO's core for human ears.

AFAIK the Minimoog used three different variants of exponentiators for the VCOs over the years: A discrete transistor ...

Congrats! YouTube videos of the Sound Studio 4 look and sound promising.

If you remove the regulator board that only the rectifier board is left, what are the outputs of the rectifier board? They should be about + / - 20V DC. Please check whether these voltages are achieved and check with a scope that these voltages are without any noise, especially the ground line.

I will never understand why the Moog engineers defined the PSU concept that way in that times. PSUs are that damned easy to construct, but not that way.

Enough of ranting. First of all you need clear precise voltages from the PSU. But this can be measured ONLY when really nothing else is connected ...

According to the video the noise vanishes when all oscillators are leveled down in the mixer. But there is a tone playing on any keystroke with a constant frequency independent from the key which is pressed. Where does it come from? Filter self oscillation? Or is that part of the problem?

The block schematic diagram of the documentation implies an analog technology behind. If that's true you will find an OTA like the LM13700 serving as VCA. Check the input and output of that OTA with a scope to see any signals reach and pass the VCA. If the output is dead, just replace the OTA.

...
What I did notice on the oscillator board, was after I put in the new chips, there was a TINY little solder bridge across 2 pins :oops: which I'm embarrassed to even admit because I'm super careful, but I guess it happens. Unfortunately, even swapping that chip out with a fresh one didn't ...

Check with a scope whether there is any signal or noise at the base of Q1204 when no external source is connected. That is the first tranistor of the overload lamp circuit. If there is no signal and the lamp flashes the lamp driver circuit is faulty.

If there is a signal without any external source ...

Important is that you test the PSU only, with no other board(s) connected, neither any of the VCO boards nor another. Then you can see whehter the PSU itself carries the error or any of the (other) synthesizer boards.

Not necessarily.

You should proceed systematically. Disconnect everything from the power supply, switch it on and check all supply voltages for correct voltages and for noise with a scope. Check ground for noise as well.

Connect a voice board and check whether waveforms appear on the scope and ...

The same from me. What a project. Perhaps I can help concerning circuit details.

Some years ago I was asked to repair an original Moog 904B high pass filter module. For me it was touching a piece of the Holy Grail.

Find the whole story here:

904B: Repair of a legend

I wish I had the chance to ...

I have recorded single notes from my Subsequent 25 and mixed them together to create triads, which takes ages :(


Aah, that must be annoying :x


Moog clearly has the knowledge and the skills but focuses on other things. Hopefully they will have time not too far from now to make the BEAST that ...