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GEAR: Octave Cat SRM II

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  • GEAR: Octave Cat SRM II

    1980 date codes on some chips, been in the studio and in constant use since 11th June 1994. Unit was found in the old Voyetra/Octave warehouse in Yonkers by Doubletake Studio Services in NY for me.. Modded for various patch points by Tom Carpenter in the late 1990s.

    Diagrams and service manual on the internet are for the SRM model only, not the SRM II. The SRM II is much much rarer than the SRM, in fact it appears production was pulled during the only run, and there are no service manuals diagrams or docs in the wild for it, as it was never properly sold and supported. My unit was assembled from parts removed from the production line and stored. it's Ser # is G1164, and it is marked as being 220v, but is in fact 110v. It's likely the case was a return? Why scrape a ser# and voltage into the case if it was never shipped? My unit also does NOT have a temperature control resistor over the filter chip.

    You likely have already downloaded the service manual for the SRM if You are reading this - it's likely it has a mystery page in it referring to a mod for the minimoog? Yup. That's the level of trust you need to have with that manual. It's a start, and better than nothing, but You need to be prepared to take a leap of faith if You want this thing to be a proper studio tool.

    SO, the holes on the front. This is what i think they do.

    Octave Cat SRM Service Manual.jpg
    Attached Files
    Last edited by playthatbeat; Sun 25-09-2022, 1:04 PM.
    jUst plAythAtbEAt

  • #2
    You can get the service manual for the CAT SRM anywhere. It will help you calibrate the machine, use the info above to translate what to do. Follow the calibration as in the service manual as best You can. Most of it is easy and good practice.

    IF You are having issues where the unit plays fine by hand, but over CV/GATE has 13 or 11 notes in an octave, calibrate it using a Midi-CV converter during the procedure NOT the keyboard in the CAT itself. Then You can set it up to whatever range of voltages it thinks it is getting. You will easily calibrate the unit to play by hand, but by CV/GATE it tracks poorly.

    You really need a scope on the audio out which can show frequency to calibrate this thing - THEN you can do each oscillator's range and tuning using the scope frequency reading. For example: Tune for 440hz (an 'A' key on your midi keyboard) then play an octave up and an octave down - should be 880hz and 220hz when You are dialled in. Use the Coarse trimmer (not tuning knobs if you can help it) to re tune to 440hz after each try at adjusting the range pots, the check again for an 880hz 1 octave up, and 220 1 down. You will get the hang of it. Feel free to fiddle with the pots marked "?" during this if it helps. best centre the "?" pots actually, and tweak them if you find yourself near the limits of the others for some extra room. IDK, Whatever.

    My unit had no pot in hole 9, i marked it on the above sheet to annoy anyone who downloads it without reading this text. (T&C apply, i may be too lazy to re-do that .jpg) The REAL poly/mono balance pot is on the back board, and is pointless if you are running the unit in CV/GATE mode anyway! Also, when i say "back board" i mean the PSU, jacks and memory board that is mounted to the inside of the rear of the case, and when i say "back of board" - i mean on the solder side of the circuit board.

    Maybe the service manual will work for you, and everything will be perfect, or maybe You will need to just do what i did to get a studio-ready instrument. I am betting it will not be entirely one or the other.

    Gotcha: If You set it up to track incoming CV/Gate, This will make the Keyboard on the unit suddenly have 11 or 13 notes per octave when played by hand, but hey, who wants to play this by hand? Nobody, that's who. Keyboard is shite.

    Remember, this thing is built in 1980, it drifts all the time. You may get 2 or 3 octaves to track each other perfectly, but they will change over time, it is the nature of the beast.

    Once calibrated, you should expect exactly 1 octave + and - on the pitch bend slider, +2 and -2 on the switch, and you should have tracking that can phaze (beat) the 2 oscillators on all 3 Cs of the keyboard - you should have 3 octaves of really tight range sync.

    You can of coarse use a custom tracking range if You have a fancy MIDI/CV convertor that can do other than 1v per octave to play by CV/GATE, and retain the sound internal keyboard range for showing off, but as i said, horrible keybed, pass.

    Yes, i know YOUR MILEAGE MAY VARY. You may have a perfect unit that tracks perfectly when played by hand AND by CV/GATE in. You would be a likely spoofer, or doing it wrong, but it's possible, i suppose.

    Oh, and while i am here, You do NOT need to recap this thing. Sure, caps dry out, but this is not an explody cap device, and there are no RIFA caps or anything in there. Just calibrate it, and next time it goes mad, blame the caps.

    Last edited by playthatbeat; Sun 25-09-2022, 12:31 PM.
    jUst plAythAtbEAt


    • #3
      I modified mine to allow the S&H to be triggered by the system TRIG (a note starting), instead of the LFO. Not the GATE, which may also work, but the tiny pulse generated every new note - the TRIG.

      This involves lifting the left leg of the cap above the LM324 circled in the picture below, then run a wire from the leg to one side of a 2 way toggle switch, the centre of the toggle switch to the pad you lifted the leg from, and the other side of the toggle switch to the TRIG bus - pick that up from the ribbon cable between the back board and the other (larger) board with the VCOs on it - next to the pins for the connector it says on the board what each pin does This will allow the LFO to continue to the S&H trigger input when the switch is in one position, as usual, but the other position will pass the TRIG instead, causing the S&H to sample. Both the LFO and the TRIG are close voltage-wise, so there is no danger here. Now You can option to have a new S&H sample taken at the start of every new note played, instead of using the LFO in random bollocks mode - fine for drones, but no use for sexy sync'd S&H fun.

      If You look at the circuit diagram, you can see this cap takes a tap from the square wave LFO on the board and sends it to the S&H. We are just making this happen thru' a switch that can route the nice 10-12v pulse from the TRIG bus there instead, no big deal.

      Attached Files
      jUst plAythAtbEAt


      • #4
        The OTHER mod I have still to do is the one everyone has been howling for. The LFO retrigger mod. You need to look for the 555 timer chip, to the right of the LM324 - it's in a socket next to a C3080, and at a distance looks like one big 16 leg DIL, it's not, it's 2 x 8 pin chips. Anyway, pin4 of the 555 is RESET - sending this LOW will reset the 555, resetting the LFO.

        You can use the TRIG bus again - it's a nice 10-12v, enough to flip a relay to send this pin to GND when the TRIG happens, then return it to connected again when the relay is at rest. It is not enough to disconnect this pin, you need to GND it to reset the 555.

        You can use a flip-flop circuit, transistor switch or a relay. I have a bag of relays. I am lazy. I will use a small relay.
        Last edited by playthatbeat; Sun 25-09-2022, 12:32 PM.
        jUst plAythAtbEAt


        • #5
          There is an interesting blog about the SRM, with some adjusted schematics and s
          so on, you need to check it also:

          The blog post is not really about the SRM II, but is interesting in that it shows how there are so many variants out there.

          These synths were labelled as the poor-man's ARP Odyssey at the time, and were often slammed as clones, and of poor quality. They are not clones of the ARP, but they DID have the moog transistor ladder filter in the pre-SRM models, cloned from the ARP Odyssey Mk1 (Black & Gold), so were likely subject to some legal drama behind closed doors. The use of the SSM2040 for filter in MY Cat shows how things eventually went.

          They were NOT poor quality units, nor were they poorly assembled. They were just in constant development, and there was never a mass-issue of a build. This feeds the theory that the cat sucked for quality, it did not, but it's fair to say, the older Your model, the more likely it is to malfunction due to lack of revisions.

          Last edited by playthatbeat; Sun 25-09-2022, 12:58 PM.
          jUst plAythAtbEAt


          • #6
            For those of You lucky enough to have a CAT SRM II, here is a patch you can set up and leave to loop forever. It's one of those legendary "ever evolving" patches. I would love to know how it sounds on a Behringer CAT.. I imagine it would sound like $300, instead of the $1,400,000 You see and hear below.. (valuation c/o WATA musical instruments, 2022, probably..)

            Note: You can run this patch WITHOUT a 16yo Tortoiseshell cat snoring in the background, but it may not sound the same.


            Last edited by playthatbeat; Sun 25-09-2022, 2:09 PM.
            jUst plAythAtbEAt