CP-251 schematics...?

[noddyspuncture]

Hi guys,

Would anyone here have the (later) CP-251 service manual or even just the schematics please?

I would like to recreate one of the 'Attenuator' section circuits in there.

Cheers,
Tom

[thealien666]

An attenuator is a very simple passive circuit, even for Moog.



The value of the potentiometer is 50K.

[EMwhite]

But doesn't the 'Attenuator' on the newer CP-251 offer 'reversible' properties, aka amplify if clockwise. I think that's what he's after (at least that's the way the thread was going on the response over at Muff's). The original w/the red jacks and just what you said, same with VX-351 but the newer (white ring) goes +5 (on the pot label anyway) as far as I know.

I don't have one so can't tell for sure.

EDIT: Ah, I see, so it's flat at full clockwise (around 5pm) (voltage in = unity), at 12 o'clock it's 0 volts then at full CCW it's negative to the extent that it was positive? I assumed it was similar to the Moon 525 but maybe it is??

[latigid on]

This is probably the easiest way:

http://www.thonk.co.uk/shop/fonitronik- ... -chicklet/

Doepfer refers to it as a polariser, but the more trendy name is attenuverter. Often they are set for 2.0x/-2.0x gain at either end of the pot.

[unfiltered37]

But doesn't the 'Attenuator' on the newer CP-251 offer 'reversible' properties, aka amplify if clockwise. I think that's what he's after (at least that's the way the thread was going on the response over at Muff's). The original w/the red jacks and just what you said, same with VX-351 but the newer (white ring) goes +5 (on the pot label anyway) as far as I know.

I don't have one so can't tell for sure.

I am pretty sure it doesn't amplify at all, just switches polarity, right? If you were going to make one, I'd suggest finding a schematic with a deadband or detent, that is the one shortcoming of the cp-251.

[noddyspuncture]

Many thanks for all the replies guys.

Yes, indeed, I do need the one that goes both plus & minus..! Certainly an 'active' circuit.

You gave me few leads to go on and a quick search of the net came up with this link:

http://www.hotrodmotm.com/images/Triple_Atten_Mix.gif

I only need to build one of those op/amp sections... so here goes....

Cheers,
Tom

[thealien666]

Only one thing worries me a little in Davis Bradley's schematics: with +5V connected to the #1 input jack as described, there will be a momentary connection of that +5V to the input source whilst inserting a jack in that source input. Sending a voltage back directly to the output of an LFO or any other source of modulation, however shortly it lasts whilst inserting a jack, can't be good for those circuits.

[CZ Rider]

with +5V connected to the #1 input jack as described, there will be a momentary connection of that +5V to the input source whilst inserting a jack in that source input.

You noticed that too! I have blown up MOTM modules that same way. The MOTM 100/101 has a sample hold that the clock input jack is normalized to an onboard 555 clock. Same thing, when making a connection to break that normalized circuit, you can send voltage straight into the output of the 555. I was using ARP compatible gates (+12 volts) pluged into the external clock input. For the brief moment that positive gate hit the 555 input, it let out the magic smoke. Lucky for me I used my own +/- 15 volt supply that had fuses on both rails, and minimumized the damage from the 555 going south. Poor design to wire jacks that way. Switchcraft did make a nylon isolated break jack. I got them from EML back in the 70's. But I don't see that same 112 nylon isolated switch jack offered anymore from Switchcraft. It had four tabs with the two tabs normally closed and opend via a nylon insulated blade inside.

That top circuit is the same as the reversable attenuator found on the bottom of the Moog model 10/12/15 or the reversable external input to the 904 filter bank. Where the 12 O'clock position is zero volts and variable to unity gain CW and inverted unity gain CCW.
If one needs a two jack and one dial solution, you would still need to add/mix in the original CV to get more than unity gain in either direction.
Something like this:


If this is for precision pitch CV adjustments, this circuit above would still give a coarse range on the dial of going from .5 volt per octave all the way to 2 volt per octave, with 1 volt per octave at center. By dialing in or selecting/changing that 100K resistor at the top, one could get a finer range from the dial. Using a resistor substitution box could find the optimal range for the dial. Those TL1013's are much better at handling precision voltages than one would get using a CP-351 or Moog modular CP-3 or reversable attenuator.

[noddyspuncture]

Hi Terry,

Thanks for your comments...

I did build just the top section of the original schematic I linked to: IN1 > OUT1 ... and it didn't work for my purpose! (The SCALE tweaking trick)

When you say it needs the "original CV mixing through to the output" do you mean that would be instead of using the CV1 switch on the CP3 panel...? If so it wouldn't work anyway - as I already tried it using CV1... although, admittedly without the last two op/amp stages.

Could adding those two other op/amp stages be the answer to my unsuccessful build...?

Cheers,
Tom

[CZ Rider]

Could adding those two other op amp stages be the answer to my unsuccessful build...?

I think so? If you follow the circuit from the input jack the signal splits and goes two ways. The top circuit first goes to a pan pot. Each side goes to a unity gain op-amp with an inverted ouput. The inverted output of the bottom feeds to the top op-amp where it is mixed with the top signal. These two signals (+/-) that are selected via the panpot to make up the reversable attenuator. The output is either unity gain to zero to inverted unity gain, depending on the panpot setting. There should be zero voltage at the 12 O'clock position.
That reversable attenuator output is then mixed in with the original straight split signal via a second set of unity gain op-amps. This mixed output is first inverted and gets inverted again through the next op-amp and sent through the final 1K resistor to output.
Without the second op-amp stages this is only a reversable attenuator. With the additional op-amp stages the original signal is mixed or added to the reversable attenuator. So now the response is unity gain at the 12 O'clock position with little or no gain CCW and 2 times unity gain fully CW. This range can be reduced by subbing in a different value for the marked "selected" resistor. So the pot could be a more tuneable to say .9 unity gain CCW and 1.1 unity gain CW with unity gain at 12 O'clock.
Although I am just guessing here, and could be totally wrong? I haven't actually tried this particular application of that circuit. Time to experiment.

[noddyspuncture]

I understand all that Terry... but in your diagram, with the original CV fed through to the output... I wouldn't be getting ZERO volts with the pot a 12 O'clock would I...??.... as I do with the attenuators in the CP-251 (I'd be getting whatever is on the input?)... .... it is those CP-251 attenuators that I'd like to recreate.

At 12 O'clock the CP-251 attenuators output ZERO - whatever the input voltage is.

Cheers,
Tom

[CZ Rider]

At 12 O'clock the CP-251 attenuators output ZERO - whatever the input voltage is.

Then just the top portion with the two op-amps is all you need. Might want to try again. This is the same function as the Moog modular reversable attenuator. Just need that 1K, 1% resistor at the output and eliminate that second op-amp pair for mixing in the original signal.
On the CP-251, I have an early one, and those circuits are just simple attenuators. Good to know the latter ones have reversable attenuators similar to the one on the model 10.
Something like this:

[noddyspuncture]

Yep... that's exactly what I put together... it didn't work...!
But I did use a TL072 instead of the LT1013. Reckon that might be the problem..?

Cheers,
Tom

[CZ Rider]

I'm curious too, Tom. Might have to try this myself. Perhaps those Dave Bradley schematics are wrong? The TL1013 is a dual op-amp and more of a precision design than the TL072 , not sure about other differences.
To be honest, I do not have a single op-amp in my Moog and refuse to use any circuits that are not descrete. So my working knowledge of IC's are quite limited. I just built the Moog reversable attenuator with the 2N3392's and 2N4058. Not something I would use for precision pitch CV's, but fine for LFO's or EG's.
Kevin Lightner had a schematic using those Minimoog buffer boards on an Ebay auction. A little different design, but tested. Might want to try that one. Or even purchase one?
Ebay auction:
Synthfool auction for reversable attenuator

[noddyspuncture]

Hi Terry, and thanks for the research sir!

Well waddaya know...?! That works. Kevin Lightner is a star - !

All I need to do now is reign in it's severity... for my purposes I don't much variation across the range... maybe a tone and a half up and down max...?

Cheers again.
Tom

I'm curious too, Tom. Might have to try this myself. Perhaps those Dave Bradley schematics are wrong? The TL1013 is a dual op-amp and more of a precision design than the TL072 , not sure about other differences.
To be honest, I do not have a single op-amp in my Moog and refuse to use any circuits that are not descrete. So my working knowledge of IC's are quite limited. I just built the Moog reversable attenuator with the 2N3392's and 2N4058. Not something I would use for precision pitch CV's, but fine for LFO's or EG's.
Kevin Lightner had a schematic using those Minimoog buffer boards on an Ebay auction. A little different design, but tested. Might want to try that one. Or even purchase one?
Ebay auction:
Synthfool auction for reversable attenuator