Cymbal decays

Discuss Nava construction and related issues
  • To add to this, VR17 (Hi Tom Decay) doesn't seem to work at all hmm..
  • VR17 fixed by soldering from top of pcb. Repeated for all other pots for good measure.
  • Listening through headphones seems to be a lot more pronounced decays.
  • I've mostly ironed out these issues but one still lingering is the Ride cymbal. I have no decay on this voice for some reason and I can't pinpoint why.

    Things I've tried:

    - All resistor values good, no shorts or breaks
    - All capacitor values good
    - Tried swapping EEPROM with Crash and works normally
    - Tried swapping each IC (4011, 4013, 4040, 4520, M5218's and even the 74HC174's) with Crash and same result
    - 2603 and 798 seem to measure fine in circuit

    I'm stuck as to what else I can try. The issue is real odd, if I trigger the Crash then trigger the Ride directly after, I can hear the decay taper off as the Crash decay tapers off. If I trigger it alone, I get no decay whatsoever. Could this be an issue elsewhere on the board?

    Any suggestions appreciated!
  • Nobody?
  • Try this:

    Swap the ROMs that contain the Ride and Crash samples. This is an easy way to check if your EPROM is faulty. If the Ride works properly in the Crash circuit and the Crash sounds wrong in the Ride circuit, you need to inspect the Ride circuit closer. I would check/ swap IC58. ;) If you don't have another CD4520 left, "borrow" the one that's in the HH circuit.

    Be very careful when pulling the ICs. Use a proper tool (IC extractor) or carefully lift each side with a flat jewelers screwdriver. Otherwise you risk to bend/ break some legs!
  • Thanks for the reply Klang. As above, I’ve already swapped the EEPROMs and the ride works as it should on the Crash circuit. Also as above, I had already tried another 4520.

    I guess I wanted to try and understand which part of the circuit affects decay. Is it the transistors? Perhaps I need to investigate and trace again but it does seem that all ICs are fine when swapped. All that is left is retracing or transistors but was trying to avoid blindly swapping transistors.
  • Image

    I had issues with my crash/ride because of bad 2SA798 transistors. Worth checking.

    Search on this forum, you can hack a replacement from cheaper single transistors. Originals are ridiculously expensive.
  • As mentioned in my post above, all transistors measure fine circuit, including the 798. I have queried a user from another forum who is quite well versed in the 909 circuit and he has provided me with a detailed reply of what to look at for my outstanding issues:
    ..Regarding the Ride Cymbal decay I assume that there is a problem with the envelope generation. You might try to swap the Ride and Crash ROMs since these are completely identical circuits. Envelope generation for Crash and Ride works a bit different compared to the HiHat. In case of the HiHat the envelope is analog and feeds the output VCA. In case of the Crash/Ride the envelope is generated digitally by just DA-converting the actual address of the main ROM being played. The result is a decay-curved shape feeding the output VCA. This is done because Crash/Ride have tune controls. Since the sound get shorter or longer and different tuninge, the envelope will automatically stretch by this way of generation ensuring that there is always the whole sample being played regardless which tuning setting is adjusted. If this envelope generation fails for some reason, the Ride will only be a short pulse or off at all because the VCA does not open at all.
    In order to check if the envelope generation out of the ROM-address works properly, could you check the output of opamp IC52a (referring to the original 909 schematic). There should be some ramp style waveform.

    The fact that you hear the Ride decay when triggering the Crash shortly before might be due to the fact that Crash and Ride use the same Accent (=volume, see above) signal. So you might check the voltage at the Accent signal for the Ride as well (same procedure as in the Toms case). This is the lower side of R435 (Ride side) or R426 (Crash Side). Which voltage do you measure when playing Ride only and then together with the Crash.
    I'll be checking this shortly and will post an update afterward..