Wednesday, September 13, 2017

Novation Bass Station Schematic

It seems that no one has a schematic of the original Bass Station, so I drew one up. This is from the keyboard version with board markings "Novation Bass Station SM, Issue 4, 1995".

DCO

At the beginning of the signal chain is the DCO. There are two identical DCOs, so we'll just look at one I've dubbed "OSCA".

DCO

At the top is an exponential current source that charges capacitor C10/2. It's built around a transistor pair marked "FMA1". Based on that marking I believe this is the correct datasheet.

At the bottom left is the sync pulse generator. When a pulse or square wave is fed into the "OSCA_SYNC" net a short pulse is output at pin 4 on U20B. The width of this pulse is determined by the RC network formed by R29 and C9. Together they cause a small delay between pins 5 and 6 changing logical state. While these inputs are unequal the output will be high.

This sync pulse enables transistor Q2 to sink the charge stored in C10/2 thereby "resetting" it to VSS. The capacitor is then able to charge back up via the exponential current source. This cycle results in a sawtooth wave that is present on pin 5 of U18B.

U18B serves as a buffer for the sawtooth, but can also be switched to generate a squarewave. Nets OSCA_WF_SW and OSCA can be connected or disconnected via multiplexer U12. If connected the opamp is configured as a noninverting amplifier outputting the sawtooth. If unconnected the opamp is configured as a comparator, comparing the pulse width CV, OSCA_PWM_CV, on pin 6 and the sawtooth on pin 5. This results in a PWM squarewave as pictured below.


Multiplexer
PWM CV

Mixer

Next in the chain is the mixer. The mixer is made of two simple VCAs, one for each oscillator. Each VCA works off of a single linear CV. The mixers serves only to change the relative volume of the oscillators, not to apply the volume envelope.
Oscillator Mixer

VCF

After the mixer is the filter. It's closely related to the OSCar filter that is, in turn, based off of the Wasp filter. It should be noted that the inverting stage present in the OSCar has been removed from the Bass Station. This stage allowed the filter to be switched into a high-pass mode. Without it, the Bass Station is stuck in lowpass mode.

Bass Station VCF
OSCar VCF
I find it interesting that the resonance is under CV control, but cannot be modulated. It does respond to MIDI though.

VCA

Last in the chain is the main VCA. It's another simple, one-OTA VCA. Unlike the mixer VCAs it includes an exponential current source and responds to modulation from one of the envelopes. It also has a trimmer to adjust the offset voltage.
VCA

CV Header

The final thing of note is an unlabeled header. It breaks out most of the internal CV and audio outputs. Almost all of them tap directly after a buffer and, with the addition of a 1K resistor, are safe to connect to other devices.

Unfortunately none of them are really suited to take a signal as input. Nearly every signal would have to be disconnected from its destination and fed through a summing-amp instead. With this summing-amp an external CV signal could then be mixed in with the existing CV.
CV Header

Full Schematic

Here is the majority of the analog section of the Bass Station. I've omitted the dull CV multiplexer and buffers.
Bass Station Schematic







28 comments:

  1. Great job !
    Thank you.
    I am looking for insert a new VCF SSM2044 based.

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  3. Hello,
    great job!
    I have a problem with my old bass station keyboard,
    the envelope amp don't work,
    you know the schematic of this part??

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    Replies
    1. Hi,
      Thank you!
      I'm not totally sure what you mean by envelope amp. The envelopes are digital and get generated inside the CPU. I don't think you will find a replacement for that.
      The VCA, at the end of the signal chain, is made from a LM3080 and a TLC274. Either one can be found easily.

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    2. Thank you for you response!
      No work the envelope (ADSR) for the amp, but work perfectly the ADSR of the filter.

      I thought the envelope was analog.

      I replaced u36 (lm3080) and U24 (TLC274) but no work.
      I only get sound, if I bridge the CV Header pins 12 and 14 (VCA CV and B_VOL_CV) but it does not work ADSR of amp, yes adsr of filter.

      If I remove the bridge there is no sound.
      Do you have any clues to follow?
      Thank you very much for your help!

      Regards,
      Jof

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    3. Jof,

      I understand you now.

      The problem may be in the exponential converter/current source made from U24 and Q4.
      VCA_CV is after it, so applying a voltage there bypasses it.

      I would also test VCA_CV_LIN. This is the envelope that comes from the CPU. If you don't see a voltage here, the CPU might be damaged.

      Let me know what you find!

      - Zack

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    4. Hello Zack,
      I made some measurements on pin 20 VCA_CV_LIN

      4.95 v DC (note off)
      4.87 v DC (note on)

      0.055 v AC (note off)
      0.135 v AC (note off)

      Do you think these values are normal or do you think the CPU is damaged?

      Thanks and Regards,
      Jof

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    5. Hi Jof,

      That does not sound normal. I would expect a much larger swing in votage. I'll test my Bass Station once I get a chance.

      The CPU is still suspect.

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    6. Hi Zack,

      Thanks for helping me!

      I have also made measurements in pin 12 (VCA_CV)

      0.490v DC note off
      0.500v DC note on

      0.011v AC note off
      0.008v AC note on

      I need to check transistor Q4
      (to see if I find an old transistor tester, which I have somewhere)

      Regards,
      Jof

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    7. No problem, Jof.

      VCA_CV_LIN should span 0.1-5.1V relative to VSS. The response is inverted, .8v at full volume.
      VCA_CV should span .3-3V relative to VSS. Response is normal.

      I did some more poking around and found that U16(LM324) is a unity buffer for the CPU signal. It goes through some passives and then into another unity buffer, U12(TLC274). The signals at both of those buffers should be very close to VCA_CV_LIN.

      Let me know what you find at those points!

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    8. Hi Zack,
      I have made some measurements

      Transistor Q4 works correctly (checked with transistor tester).

      U16 (st324):
      pin 1 (out 1) 3 VDC note on
      pin 1 (out 1) 2.6 VDC note off

      pin 7 (out 2) 5 VDC note on
      pin 7 (out 2) 4.96 VDC note off

      pin 8 (out 3) 0 VDC? note on
      pin 8 (out 3) 0.03 VDC? note off

      pin 14 (out 4) 3 VDC note on
      pin 14 (out 4) 2.6 VDC note off

      4 VDC pin 5.8 VDC


      U12 (TLC274):
      pin 1 (out 1) 3.5 VDC note on
      pin 1 (out 1) 3.5 VDC note off

      pin 7 (out 2) 4.5 VDC note on
      pin 7 (out 2) 2.8 VDC note off

      pin 8 (out 3) 3.7 VDC note on
      pin 8 (out 3) 3.7 VDC note off

      pin 14 (out 4) 4.7 VDC note on
      pin 14 (out 4) 4.5 VDC note off

      4 VDC pin 5.7 VDC

      Do you seem normal?

      Thanks and Regards,
      Jof

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    9. Jof,

      You should be seeing approx .1 and 5.1V at pins 5,6,7 of U16 and pins 12,13,14 of U12.
      Pin 5 of U16 connects directly to the CPU, so if you aren't seeing the correct value there, the issue is in the CPU.

      Things like the sustain level, and velocity can affect this voltage. Maybe try experimenting with them.
      See if it responds to MIDI velocity differently than the keybed.

      - Zack

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    10. Hi Zack,
      I have some voltage variation on pin 5 of u16,
      but I do not know if it's enough.

      These are the measures:

      U16
      pin 5 - 4.8 vdc (note off) - 2.2 vdc (note on)
      pin 6 - 4.9 vdc (note off) - 4.9 vdc (note on)
      pin 7 - 5 vdc (note off) - 4.9 vdc (note on)

      U12
      pin 12 - 4.92 vdc (note off) - 4.88 vdc (note on)
      pin 13 - 4.75 vdc (note off) - 4.56 vdc (note on)
      pin 14 - 4.75 vdc (note off) - 4.56 vdc (note on)

      I have tried with an external keyboard with different velocitys, there is little difference.

      Regards,
      Jof

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  4. Hi Zack,
    thanks for your help!

    I have tried to make a bridge in pin 20 VCA_CV_LIN to pin 14 B_VOL_CV and I have sound but without ADSR amp.

    I will test the double transistor Q4, since I replaced U24 and it did not make a difference.

    Regards,
    Jof

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  5. Hi Zac.... I have a bassstation keyboard with a non functioning modwheel.. I replaced the pot and resoldered the connections from the pot to the pcb.. I also checked the mod switch pitch/off/filter for continuity and seems to check out ok.. The pitch bend wheel works fine.. Not sure where that leaves me... I can see that the mod wheel seems to feed into a quad opamp, but cant trace the pitch wheel.. I would have assumed they were also feeding into the same quad amp, as the leads all terminate around this IC... But im thinking if the pitch wheel works then so should the mod wheel if it is operating on the same quad opamp.... I dunno ,Ive reached a dead end now!!

    You wouldn't by any chance have a schematic of the pitch and mod section laying around ?

    Cheers

    Paddy

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    Replies
    1. Hi Paddy,

      I did partially draw the CV routing. There's almost nothing to the pitch and mod wheels though. They both go into U5(?) that quad opamp, then straight out into the CPU. I believe it has two ADC channels.

      See if you get the pitch/mod signals boosted on pins 1 & 7. If not, I'd replace the opamp. Otherwise, it's the CPU and you're out of luck.

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  6. Thanks for the quick reply Mate... Much appreciated ;]
    Ok ill have a look again at the quad opamp..and check the outs..
    ill let you know the results !!

    Thanks again

    Paddy

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  7. No joy Im afraid..the Opamp seems fine ;[
    Funny how the pitch bender works ok though !!

    Cheers

    Paddy

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    Replies
    1. That's a disappointment. Any chance the bend range is turned to 0 semitones? I forget how, but there's a way to change it.

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  8. Not totally sure about that one... I have since given the synth back to a friend.. he actually owned it..
    I suppose a factory reset couldn't hurt... Like you were saying, its such a simple circuit that its odd for it to just not work.. I would have thought that if the Cpu was damaged then so would the pitch bender..
    I would imagine the routing for the mod pitch and bender pitch head to the same destination.. Then again because ive not been able to use the mod section.. Is the pitch mod destination possibly just lfo to pitch ammount ? That would make it a slightly different routing I would think..

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    Replies
    1. They go to different pins on the CPU, but they're probably inputs to the same internal ADC. Hard to say what could go wrong inside the CPU.

      I think the mod wheel controls the LFO > Pitch amount, but it's done digitally.

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  9. hi Zack.
    first of all, GREAT JOB!
    I got an answer about a bass station rack I recently bought. the module works well , sometimes some note stay ON... but it has the original Power supply and I read that there are problems about it (though I tested it and I've found a fierce 9.02 V). Anyway, the problem is another one. I noticed there's a little background noise that seems a modulation-lfo , hangin somewhere. I opened the rack and I saw only one trimmer. I read that it adjust VCA offset voltage. near there are some Test points. do you know the refereces values (P-P Voltage)? thx a lot. Marco

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    1. Sorry, I don't have any documentation or calibration info. Would love to find a service manual!

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    2. THX anyway Zack. Another question. I tried another power supply but... nothing changes. after playing some sequence , one random note remains ON and stay on until i change preset. I've already read about this problem. MIDI optoisolator problem? Suggestions?

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    3. Are you playing it over MIDI? An optoisolator issue would explain that.

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    4. hi Zack. sorry... just a suggestion. I'm not an electronic but I've done some easy fix in the past. the question is... In your opinion what is the chip I have to replace? I think that the chip is the 6N138 (and I hope that's the right one, it's the only one easy to remove...). thx again !

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    5. Yes, 6N138 would be an optoisolator. If you have access to an oscilloscope, you can view the output to see if it's working.
      If not, just be careful desoldering the chip. Since it's a very cheap/common chip, you can cut the legs from the body to make it easier.

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  10. Hi Zack and Unknown,

    Zach, thank you very much for reverse engineering!

    Unknown, are you probably talking about a problem in the Super Bass Station?

    Best regards,
    Andrew

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