6803 MPU Help

[EdGs]

Hello to all,

A friend of mine asked me to take a look at his Motordome machine for him. It had been working, then was powering on, but not starting the game.

He told me he tried to check the fuses on the power supply board, and when he tried to test the first fuse, there was a sizzle sound, followed by a burnt component.

I pulled the psu board out and upon inspection found R2 (2.2 ohm 1/4W) cooked and the LM323 reg shorted on the input side. I replaced those components, reinstalled the board, and got good readings on all psu voltages.

The CPU board however, did not have the green power on LED lit. The sound board gives 3 flashes. I had a spare 6803 chip and also replaced the dead battery (which was probably the only thing wrong in the first place).

I then was able to get the power on led to light, but no flashes.

I have not had a chance to go into it deeper yet, and I would like to make up a power supply so I can power it up on the bench, but I have been preoccupied with life's other problems.

I am thinking when the regulator shorted it may have taken out some of the 74 series chips and/or the PROM's or more.
.
Are there any quick checks I can do to narrow it down a bit once I can power up on the bench? Thanks for any help you can give

[gwhorto]

Ed,
I hope someone decides to give you a little guidance with this, You sound like you could fixit with a little nudge!

[EdGs]

I ended up replacing the control board and eproms, and the game is now functioning. I will make up a rig so I can power the original up on the bench. The only thing left is that there is no sound.

The owner of the machine said that he had to turn the volume pot all the way up and that it was still very weak, so I am hoping that it is just a bad/dirty volume pot. I have not had a chance to check it yet.

I could not locate the pot in the schematics or parts lists, so can anyone tell me the value and if it is a linear or audio taper?

Thanks

[spalmer]

I doubt it is the volume POT. More likely the audio amp.

[EdGs]

I finally got time to check the weak sound problem, and when I powered the machine up, it had decent sound. However, the next power up it was very weak again. So, as I had a ball in play, I gently tapped on VR1 and the sound came back for that instant. I powered the machine down and worked VR1 back and forth several times and reset it to where it was originally (approximately mid range). The sound worked flawlessly after that through several power cycles.

The volume is very stable with the coin door pot set at mid range, and gets very loud at max (which is all the way counter-clockwise, the pot wires must be reversed). If it doesn't hold up, I will replace VR1.

So it seems the machine is back in action. Now all it needs is some new rubbers and a good cleaning and polishing.