Casio PT-87

[untune]

Howdo all!  I've not been on here in yonks... about a year I think!  That PSS-140 I originally intended to bend is still in bits somewhere... just never found the time!

But a few nights ago I remembered I'd bought this little gem off ebay a while back and put it away in a cupboard without ever getting round to opening it because I didn't think it'd have much going on inside it.  Did a quick search for it and there are plenty of YouTube vids of people showing off with their fancy bent PT-87's, but virtually nothing detailing exactly how the bends are done!

I'm having a fun poking around the board and I've found a few points of interest (mainly points that when connected adjust the pitch) but I'm wondering if anyone else has had a pop at one of these?  Somebody on YouTube had gotten a great pitch bend knob installed on his that took the pitch really far down and growly and the same in the opposite direction... I'm after doing this to mine.  Now on the board there's one of those little tune adjusting pots (I can't remember what they're called exactly, the ones you've got to get into with a screwdriver or something )

I'm wondering if I just whack a bigger potentiometer onto the same points as this, will that give me pretty much the same effect?  Sounds simple enough but i don't want to go boshing it before the fun's started

Also thinking about one of the voltage drop mods too, but any other ideas would be cool

Cheers!

[computer at sea]

Here we go!  I just found it this very instant.

The thing with the screw that you're talking about is a trimmer.  Go ahead and cut it off and swap it out for a 1meg pot.  That'll make for a pretty low pitch drop on its own, but you can actually get it to go much deeper. 

As you look at the board with the keys facing you, the trimmer has two points on top and the center point on the bottom.  They look roughly like this:

                                                A          C


                                                       B
Connect the center of your pot to B, and one side to A.  Leave C alone (or hook it up as a body contact).  Connect the remaining point on your pot to the long diagonal jumper wire that is near the top right of the board.  This will make the pitch go so deep that it becomes a series of clicks.

[computer at sea]

Update on the pitch bend:

I ended up connecting the pin on the pitch pot that I suggested wiring to the jumper to a SPDT switch.  One goes to the above mentioned jumper, and the other goes to the original point C where the trimmer was removed. 

When the pot runs to the jumper you can get the deepest tone, but the sound cuts out when you turn the knob more than about half way.  With the pot running to the point where the trimmer was you don't get as low, but you do get a smooth run through the whole pot when you turn the knob.  Also some of the other bends behave differently depending on what the pot is hooked up to.

Has anyone else worked on one of these?  I'm looking for some more drum bends.  I know they're out there cause there are a couple of nice clips on youtube, and so far I haven't had much luck.  I found one that accents the bass note  and snare resonance, but nothing else.  I've run over all the resistors on the board with a substitution wheel and come up dry.

[computer at sea]

OK!

Hours of trial and error later some stuff has come together.

I closed the machine up,  so I can't post any pics, but I'll try to describe the situation to the best of my abilities.


I found a resistor that has some good stuff on it.  If you're looking at the component side of the board with the keys facing towards you it is the furthest right resistor, directly above the green strip of board that faces upside down.  I think it is a 47k or 4.7k. 

The top of that resistor makes a goodly amount of noise on several different spots, and I wound up throwing in some extra components there to tame stuff down a little.  I wired the center leg of an NPN transistor (NTE 123 AP) to the top of the resistor and connected one of the outer legs of the transistor to the negative voltage on the speaker.  On its own the bend doesn't do too much, but combined with some other bends it makes some great beats.  The distortion changes a bunch depending on which of the four settings the keyboard's volume is on.

Coming from the same resistor I also connected a 100n capacitor to another resistor.  The second resistor is the leftmost in the cluster of resistors located diagonally up to the left.  The cap connects to the top of this resistor, and  accents the cymbal resonance. 

I don't readily recall exactly where the bend that I mentioned in the previous post is, but it happens at three or four locations, so it shouldn't be too hard to find.  Poke around on the vertical resistors that are directly to the right of the line of ceramic caps near the center of the board and you'll find it pretty fast.  It does bass and snare resonance, and combines with the two above mentioned bends to make some excellent beats.

Audio samples coming hopefully tomorrow.

[untune]

Hey

Thanks for sharing this stuff, it's really cool.  So far I've pulled the speaker and replaced it with a mono TRS jack, and I've also removed the trimmer and put the 1M pot in it's place, complete with the switch to swap between the C point and the jumper.  One of them works great giving a nice smooth run down the pitch (which will be the one linked to the C point) but the other one running to the jumper seemed to do nothing, though it was very hard to test trying to keep pressure on the volume switch the whole time, and the jumper was a tough and rather dodgy solder job to say the least

I've left it alone and will go back to it soon enough, I plan to play with a couple of other things yet and I need to paint the case but it's coming along nicely

Cheers

[seaweedfactory]

Here's a video of my take on the Casio PT-87. I removed the ROM functionality as I don't feel it adds much to the keyboard. There are instructions here if you would like to try my four modifications.

http://seaweedfactory.blogspot.com/2011/05/buzz-circuit-bent-casio-pt-87.html