Author Archives: MacroDX

CS15D channel 1 modification mainboard

This is a follow-up to the general modification article.

In an effort to come up with a more detailed documentation for the mod I made an annotated picture of the mainboard followed by some explanations:

Yamaha CS15D DXmod mainboard annotated

Yamaha CS15D DXmod mainboard annotated

Explanations of annotations

Power supply

In the upper left corner, in blue, you see the power supply subcircuit for the board (see also The Tr7 Problem in this article).
The connector goes to connector C7 on the SSK board, which is fed with +/-15V directly from the PSU board. The various OpAmps on the board connect directly to it. Also the +15V line feeds the μA 7810 voltage regulator, which feeds all the potentiometers.

Control CV subcircuits

CS15D manual control subcircuit

CS15D manual control subcircuit

The control CV subcircuits are built around the three LM324 OpAmps in the board center.
The inputs (From Control Pot) are connected to the two brown on-board connectors which connect to the potentiometers and switches on the control board.
The outputs go to the two original connectors PM1-C2 and PM1-C4.

PM1 C2/C4

These two connectors are the original connectors that are normally connected to the PM1 preset board.


When installing the modification, not only is the preset board for channel one removed, but also the presets for channel two.
Therefor we need to implement the functionality of the channel 2 Manual switch. This is done by simply shortening pins 1 (+10) and 3 (MS2) of connector PSW-C1.
PSW-C2 is connected because we need to feed the 10V CV voltage to the control section of channel two. This is done via pin 9 (MS1) of PSW-C2.

External CV

I have installed three external control volatge inputs for pulse width, filter cutoff and resonance amount. They are simply added the respective control pot inputs of the OpAmps.
The external inputs are laied out for input signals in the 0-5V range. To assure the input rating each input is equipped with a clamping circuit which I took from Doepfer´s DIY page.
Since the CS15 works with CVs of +10V internally, the voltages are amplified by 2:1 non-inverting amplifier circuits around the LM324 on the lower right of the board.


That is basically the whole shebang of the mod. I will try to come up with  proper schematics of the board eventually. I have already started it.
But actually, if you are to eager to do the mod yourself and don´t want to wait, you should have sufficient information now to go ahead. You would of course have to find the correct values for the control pots in ther sevice maual, but if I could do it you can do it.

For discussions please refer to the muffwiggler thread.
So long…

Behringer Neutron calibration

Do you also have the problem that sometimes the an oscillator of your Behringer Neutron is out of tune when you bypass its tune pot?
I had that a couple of times now and didn´t know what to do about it until today.
I assumed being driven by 3340 ICs the Neutron would not need any warm up time but maybe I am wrong. However during boot the Neutron calibrates the OSCs internally. I found that when you lock a Tune pot and the corresponding oscillator is out of tune thereafter, you should switch power off and back on so to force re-calibration. Mine was perfectly in tune after having done so.

Behringer VP1 LFO rate mod

Following a topic on a forum I modded my VP1 for slower sweeps of over one minute. Someone came up with the instruction where you put a large resistor between the center tap of the rate potentiometer and ground.
I did however not obtain a satisfactory result with the resistor values suggested by the other poesters. I found that for my purposes a value of 680k was best. So maybe you´ll have to do some experimenting as well.
Here´s what my installment looks like:

Installment of LFO rate mod in the VP1

VP1 LFO rate mod

Midiverb II repair

I´ve had a broken Alesis Midiverb II lying around for quite some time now. Just recently I remembered that I´ve always liked Patch #28, even before I dove into ambient and drone. So I thought now was finally the time to open it up and give it a go, especially since not to long ago I aquired a Midiverb 4 from an old friend and thus had a working 9V AC/AC adaptor again which the Midiverb II also requires.


You´d plug in the power adaptor and nothing would happen, neither visually nor sonically. The unit was basically dead, or so it seemed …

Fault trackdown

It´s always a good start to make sure that all the supply voltages are there and within range. The Midiverbs rectify internally so I started at the power adaptor plug and made my way through the PSU unit. Power on all paths was present but at very low ratings. I unplugged it again and did some continuity measurements. I found a short circuit between the output and ground of the 7805 regulator. I took it out and found that it measured okay out of circuit so it itself couldn´t be the problem. I still had to replace it because the input leg simply broke with no real force applied,

The repair

So there was a short circuit on the 5V supply line. I measured continuity across all capacitors and marked al those that seemed short which I then soldered out. Basically I removed all ripple filter caps of the digital ICs. Subsequently I no longer measured a short circuit across the 7805. It turned out that two of the filter caps had shortened. I replaced those with 470pF that I had left over from the Opra 6 repair and put all the other working ones back in.
That was all it needed. Plugged in the adaptor and there it was up and running again.


I noticed that the unit emitted a slight humn and a significant amount of noise when cranked up fully. I then remebered that the device has always been a little noisy. I then found that it might not be a bad idea to check whether the analog circuit could be improved upon. All opamps where LF347 and a little research revealed that the TL074 is compatibel and seemed to have better ratings. So I decided to replace all LF347 with TL074. I used sockets just in case that I where proven wrong and had to reverse it, which didn´t happen. I am quite stisfied with the improvement and really happy to have the unit back in working condition.

Midiverb 2 TL074
Midiverb II w. TL074