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 ().