MV1B1 Stuck in service mode

[Ryoandr]

CRE401 can fail internally, on each line there's a pull up resistor, an inline resistor and a capacitor to ground.
Cap failing short makes input always on.
https://wiki.neogeodev.org/index.php?title=CRE401

 

[XianXi]

Check the resistance on the CRE401 that the test button goes to from the JAMMA edge.

To test it, you need to check both the input and output pins between VCC.

For example, if from the JAMMA edge it goes to pin 2, then output is pin 3. Resistance from Pin 1 to pin 2 should be 1k ohm, from pin 1 to pin 3 it should be 1.3k ohm and from pin 2 to pin 3 it should be 300 ohm. If the values are nowhere near that, pull the CRE401 and retest out of circuit. If it's fine out of circuit then it's the custom that handles the i/o but if the CRE401 is bad, much easier fix.

 

[twistedsymphony]

if for whatever reason you do need a new NEO-BUFF you can get a replacement here: https://www.tindie.com/products/furrtek/neo-buf-replacement/