Sega Model 3 (Blast, NVS4000, etc) no JAMMA sound / buzzing fix

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    • Sega Model 3 (Blast, NVS4000, etc) no JAMMA sound / buzzing fix

      tl;dr: Replace R1. It is a 10 ohm resistor.

      One of my Blasts had two simultaneous sound problems. One of them was that JAMMA audio was more or less inaudible, with a buzzing sound replacing it. One of my Astro power supplies, the NVS-4000, was doing the same thing. Frustratingly, both used to have JAMMA audio working.

      The Blast and NVS-4000 have Sega Model 3 sound amplifiers. This amp accepts stereo line-level audio, and also accepts mono balanced audio (JAMMA). With how the Blast is wired, only line level audio or speaker audio are passed in, and the choice is made by moving the 6-position AMP-UP connector on the JAMMA harness between one of the two sources.

      On my Blast, the previous owner had done some hack work to the harness, and in my fixing it, I simply allowed the JAMMA audio to be disconnected or reconnected using a 4P AMP-UP connector (all I had at the time). Unfortunately that means I was able to accidentally give it line audio at the same time as JAMMA audio.

      On the NVS-4000 PSU in the Astro, there isn't really an obvious way to prevent the user from feeding it both types of audio, so the same situation happened.

      In short, line audio + JAMMA audio at the same time == blown JAMMA audio in the amp.

      If you open up the PSUs you'll find the Model 3 sound amp. Near the sound inputs, there is a vertical resistor, R1, which is supposed to be 10 ohms. On one, it measured 2 megaohms, and on the other, 11. Way out of spec. Replacing R1 with a new 10-ohm resistor fixed it. I used a 1W resistor as some hapless future-proofing, but I don't expect that to prevent it from being damaged.

      Someone had mentioned this resistor long ago, and I couldn't find the post. I just want to keep it in an obvious location for anyone with this problem in the future.

      EDIT: Thinking about this further, this is a fusible film resistor. A next step would be to identify the maximum current the original was rated for, and replace it with a similar fusible resistor.

      The post was edited 2 times, last by Hatsune Mike ().

    • Sorry to bump an old thread, but I am in the process of wiring an NVS-4000 in my New Astro City and I would like to know the correct wiring for the mono JAMMA speaker level to go into the "Audio In" 4 plug socket in the amp, so that I don't fry mine? Do you need to unplug the line level JVS out from the Audio In socket when you switch from speaker level JAMMA and vice versa (which always leaving audio out connected through to the speakers)? If so, which of the 4 pins do you feed the 2 pin mono signal through? Curious to know how the amp can work out if the signal is already amplified or not and act accordingly.
    • I suspect these were supposed to come with a harness for switching between line stereo input and mixed JAMMA input, and mine does not have it. I thought to introduce a 2x3 AMP-UP connector to match how the Blast does it, to allow for selection. Used as-is, the JAMMA audio enters through the large AMP-UP connector on the face, not the 2x2 pin one.
    • Yeah, the Blast has it where either the speaker audio goes to the dedicated input, or the two line inputs go to that dedicated input. Using 6 pins, it makes sure neither set is active at the same time as the other, but there's no rerouting going on.

      I once had a Blast without this harness, the simpler JAMMA connector, and added my own line in connectors. I found the hard way that the resistor will pop if you don't make sure the utilization is exclusive between them.
    • that's exactly what I did years ago, removed the pins of the sound coming from Jamma and put an amp connector to be able to plug the speakers from the cab, or plug them on the sound out connector of the NVS-4000. This allows also to connect amplified stereo sound of some boards straight to the speakers (like MVS slot or Konami boards) by making a cable with a 4 pins JST connector.

      The post was edited 1 time, last by futasan ().

    • That's a great idea regarding the audio wiring. I have wired up harnesses needed to get my NVS-4000 up and running. However from the word go it has the dreaded issue with JAMMA humming, so will wire things to go straight to the speakers rather than through the amp. I'll check my R1 resister as it's likely been blown some time before I got my hands on it (I haven't yet fed any JVS stereo to the audio in at all).

      However, I also note something else about my NVS-4000. Not sure if this is a symptom of what also happens when R1 goes bad: 3.3V and 5V are all good, however my 12V output is only providing 10.5V. When I have a board attached (one which uses 12V) it drops to 9V or so.
    • I replaced my R1 resistor and sure enough sound is great. Thanks for making this thread @Hatsune Mike!


      And cheers @nem, I will get a cap kit for this power supply and hopefully will be pumping out 12V before no time. Anyone know is there is a schematic available for this PSU?

      BTW, thought I would follow up on this thread. I made an audio hardness which connects to the normal Astro JVS harness, but what I did was the following:

      - Removed the 6 pin audio plug from a Blast City Jamma loom, and de-pinned the 4-pin connector from the Astro JVS harness (which normally connects to speaker).
      - Then I re-connected the pins into the plug from the blast city loom labelled (line) and took the little label across.
      - Then I got an 18 pin connector a used that to control how the speaker level jamma routes.
      - At the NVS-4000 it either gets the 4-lines for stereo line level to audio-in, or the 2-lines for speaker level mono but not either, compliments of the 6-pin plug i took from the blast city loom.

      It looks like the attached image.
      Images
      • nvs4000-audio.jpg

        201.15 kB, 960×1,280, viewed 24 times

      The post was edited 3 times, last by zetsurin ().

    • I've been thinking about rewiring my hacked up NVS since the previous owner bypassed the audio amp altogether to wire it for MVS. Is the proper pin to input audio signal from the jamma edge on this PSU/amp through the same connector that the service and test buttons are on, through the 'AF+' and 'AF-' pins?

      Or should I jump 'L+' to 'R-' on the 4pin audio input connector and wire the mono audio output 'speaker+' to 'R+' and 'speaker-' to 'L-'?
    • snakeGrave wrote:

      I've been thinking about rewiring my hacked up NVS since the previous owner bypassed the audio amp altogether to wire it for MVS. Is the proper pin to input audio signal from the jamma edge on this PSU/amp through the same connector that the service and test buttons are on, through the 'AF+' and 'AF-' pins?

      Or should I jump 'L+' to 'R-' on the 4pin audio input connector and wire the mono audio output 'speaker+' to 'R+' and 'speaker-' to 'L-'?
      I wired mine to AF+ and AF-. Doing it this way your have the flexibility of using audio in for stereo line level JVS inputs, but please do note the limitations described by the OP where you need to be careful to avoid feeding both line level JVS and speaker level JAMMA in at the same time as it will blow resistor R1.

      if you never need JVS, you can certainly just plug the speakers into the harness which holds up to the jamma edge (assuming that hasnt been hacked put). I can send through some closeup pics of the harnesses i wired if you want to make up something similar though (overseas at the moment but can do next week)
    • Oh yeah I wanted to go back through the NVS and try to undo this hackjob. I then noticed that I was getting a buzzing from my speakers. Was kind of peculiar because the MVS single slot board induced no buzz/hum but every other jamma board I've tried in my astro has this buzzing if audio is pulled from the jamma edge. Audio is all good when stereo is input though.

      I thought maybe the speakers were hacked and wired in parallel at first but have verified they are wired in an 8ohm config