Problem

No sound in-game. 10 tones followed by a ‘Sound Section Bad’ error during self-test.

(The owner had reported sporadic in-game pauses but I was unable to reproduce them. These occur when the main CPU is stuck waiting for the sound DSP to respond.)

Diagnosis

10 tones indicates that the sound RAM test failed. This error code is not listed and I only discovered it from disassembling the sound DSP boot ROM. The sound hardware test does not indicate which of the three SRAM ICs are faulty either.

While repeatedly running the sound hardware test, I checked the pins of the three sound SRAMs (U73, U80 and U86) for 5V, 0V and address/data bus activity. I found that U80 had a very low voltage at its VCC pin (28):

One might assume that VCC is connected directly to the 5V power plane, so the likely issue would be either a broken pin or a poor solder connection. This is actually not the case here. The VCC pin of each SRAM is connected to 5V via a ferrite bead:

The ferrite beads filter out high-frequency noise and have a very low DC resistance (<1Ω) . L7 however was filtering out everything – a continuity test revealed that it had gone open circuit…

As there are no Midway Wolf Unit schematics available and no indication as to what type of ferrite bead was used, I picked out what I believed to be a suitable replacement from the dozens of SMD reels at my local electronic surplus store. I chose a Murata BLM21PG600SN1D (0805 package).

Fix

Replace SMD ferrite bead at L7.

Philip Bennett