CP-251 Mixer output problem

[Yekuku]

Hi all,
new member here, long time moog user though.
I had bought a cp-251 (red nuts) a few years ago, used it for simple routings and I thought it was working ok, but it seems it does not.
Lately I was using the cp 251 mixer section for modulating pitch and I noticed that there is some pitch fluctuation that I was not expecting.
So i connected an oscilloscope to the mixer out and I found out that there is a voltage fluctuation without having anything else connected to the cp251. I also noticed that the lfo rate knob affects the period of the fluctuation.
Here is a pic :


Then I injected a +5V pulse signal in the mixers input and by checking both mixer outputs I got this :



Has anybody experienced this too ? What is causing this voltage fluctuation ?
Any ideas ?
Thanks in advance.

P.S. I have already used 3 different power supplies, all of them exhibit the same behavior.

[latigid on]

Hi, is the power supply harness from the VX-351 still in place?

[Yekuku]

Hi, is the power supply harness from the VX-351 still in place?

No, the 351 is not powered at all and there is no internal power connection between the 2 units.

I have noticed that without having anything connected to the cp251, the "noise" that is produced from both mixer outputs, has a frequency of 2-3Khz with the lfo set to slowest setting and it can be raised up to 12 Khz by increasing the lfo rate.
Any ideas?

[cliffman]

Hi, is the power supply harness from the VX-351 still in place?

No, the 351 is not powered at all and there is no internal power connection between the 2 units.

I have noticed that without having anything connected to the cp251, the "noise" that is produced from both mixer outputs, has a frequency of 2-3Khz with the lfo set to slowest setting and it can be raised up to 12 Khz by increasing the lfo rate.
Any ideas?

I just checked my two CPs, it's clean, no signal or extra noise. Either you have nasty bad power, or it's broke.
Even used the same scope as you (my it's cute)

cliffw

[Yekuku]

Hi cliffman and thank you for taking the time to check this.
Did u test an old cp251? (red nuts )
Damn it, there are no schematics for the new moogs, this will be a PITA to repair.
I am bit disappointed about the overall build quality of the old 251. Now I understand why moog was giving them away for free with every new voyager, a few years ago.
Anyways, the dso nano rocks The only things I wish it had are a sine wave generator and adjustable test wave frequency for generating frequencies related to western music tuning e.g 440Hz, 880Hz etc.

[cliffman]

No, mine are both the newer ones - no red nuts.