JAMMA cab weirdness

[big10p]

I've a JAMMA cab that's giving me some odd behaviour when running original arcade boards, mostly to do with sound.



I fitted a sound amp to the cab years ago, as I usually use it to run a groovyMAME rig. As most PCBs have a built-in amp, I've just fitted a bypass switch to send sound from the JAMMA edge connector straight to the speaker. I did this as I thought that amplifying already amplified sound might be the cause of the sound issues I'm getting with game PCBs. It hasn't fixed things.



Example: playing an original Pac-Man board using a JAMMA adapter, the sound is flaky, comes and goes and even disappeared completely. The heatsink on the PCB amp gets very hot. Playing on a supergun, all is good and the heatsink stays cool.



I guess this is a wiring issue or maybe PSU related, as another game I tried is working fine on supergun, but flaky in the cab. However, a 60in1 board works fine.🤷‍♂️

Any ideas? Ta.

 

[TheDaddy]

If it getting hot then its most prob a voltage / wiring issue. First thing i would check is voltages , bad caps can cause amps getting hot due to voltage fluctuations (obviously not caps thats just an example as the 2 cause same issue). I would cut the + and - and just stick a speaker straight on bypassing anything else. Check -5v and 12v , 60 in 1 only uses 12v i think. I reckon its your psu but just a guess.

Its going to be trial and error type of job.

I love thses ones myself ! Lol , great when you figure it out.

Dave.

P.s use a cheap booty to test , yoh can certainly wreck an amp if its getting hot quick.

 

[philmurr]

Do you have one side of the speaker connected to GND?

Pacman (and some others) have push-pull final amplifier stage and need to have their 2 speaker connections connected to the speaker (and neither to GND)

 

[big10p]

Thanks for the tips, guys. I have checked voltages, but not while the sound is crapping out. I'll do that.

I didn't know about the push-pull amps on some boards. I just checked the cab edge connector and the negative speaker pin is connected to one of the ground pins, but not any other grounds! I'll try and find out why this is and correct it. Thanks!

 

[Bods]

Dave will send ya one of his cheap booty Wonder Boys to test it if the above doesn't help

 

[TheDaddy]

Dave will send ya one of his cheap booty Wonder Boys to test it if the above doesn't help

How dare you ! Lol. I dont have many......

Dave.

 

[big10p]

Only recently started buying PCBs, so don't have any cheap boots to wreck. 😛

I found why the speaker edge connection was connected to ground, but now I can't work out how to wire the amp bypass switch without fitting a second speaker. Maybe I'm too tired or just thick. Lol.

 

[big10p]

Ok, think this is sorted now, kinda. Possibly blown the little amp I fitted as getting no output despite inputs checking out fine. Anyway, Pac-Man amp heatsink now stays cool.