My Upgrade List
Home ROM Version 2.0
Ringmaster hole protector
Modified sound levels (see below)
Blue Backbox Ball
Auto ball stop
Menagerie Insert Flasher - likely sample machines only
Playfield Upgrade Kit A-22270 (Pinball Resource is the last known supplier)
Backbox bell - standard 4" bell in backbox
Purple flippers and rubbers
Proto skinny ringmaster - not recommended unless you have spare gearboxes!
New speakers - haven't figured out how to do the second 5.25" in the head.
Reinforced yellow targets - reduces airballs
Balancing the Sound - Getting Back the Bass
Here's the first problem: the sound is well out of balance. It's way loud even at level 1, but has no bass - even though the cab speaker is working. For whatever reason, the CV backbox design just seems to pump out a lot of sound relative to others. As I plan to upgrade the speakers, I might as well start on this now.
There are two possible solutions. One is to put a series load resistor in the backbox. Easy. but it doesn't really address the problem as we waste energy in the resistor and have less headroom in the amp. The other is to change the gain on the buffer amplifier on the AV board, which is what I chose to do here.
Here's the schematic. We are going to add a 4k7 resistor across R17. This resistor reduces the gain of the output buffer. I did a bit of trial and error. Your tastes may be different; you might want a higher value, but you won't want to go less than this.
(By the way, this schematic is incorrect - pin 6 of U1 should connect to pin 7 of U1, and pin 9 to pin 8. I'll check this with a meter later.)
|Here's the general area that needs the modification. This is the top right board is a WPC-95 backbox.||And here's a closeup of the fix. I have tacked a 4.7kOhm resistor across pin 8 of U1 and the nearby end of R23. I could have achieved the same result by connecting it across R17 - I'll fix that later. I would rather not change the resistor, as the change will not be obvious to a future owner.|
I don't see any easy way to do this mod without a tiny bit of soldering. It could be tidier than my version right none, but this is easily removed and will leave no lasting marks on the machine. There's also something to be said for the mod being visible.
The end result is that my CV now has bass.