Fahz0r

[fahz0r]

LM13700 VCF

This design is originally by René Schmitz and is (half) an MS-20 filter but with an LM13700 chip instead.  I have found a pretty solid layout on a forum called "electro-music.com" by user Clack, but since that topic is from 2010 I decided to try and fix it a bit:

A couple of changes on mine: 1) R13.1 and R13.2 was originally just one 10K resister but I've found that a bit loud and got a more stable but devestating result when putting a 6.8K and 2.7K resistor in parallel with eachother. You could manage using a 10K trimpot here as well of course!

2) The transistors T1 and T2 are reversed now and are 2N3906's. There wasn't a cut between the bases before, there is now. 

3) Cut was removed for IC2's pin 11 connection to -12v rail.

4) I've actually put diodes in this layout, but on my PCB it has LEDs (any diode can be used actually, and they light up a bit when you have the resonance up... Bit useless when it's hidden inside your modular of course haha..).

5) I've used an LF347 instead of TL074 because I had them laying around. 

If you follow this layout your filter should be working with both +15/-15v and +12/-12v.

Will edit this post with audio clips when I can.

[fahz0r]

Double KPR-77 Repair (PART 2)

Board #2 looks a lot cleaner now! Haven't got any time currently to fix it once and for all but this is a start. Oh, and the op-amps arrived.

[fahz0r]

Double KPR-77 Repair (PART 1)

I have recently acquired not one but -two- Korg KPR-77 drum machines for quite a cheap price, but both of them slightly wonky. One of them had quite some wonky buttons and had some separate instrument outputs modded onto the board but no Kickdrum from the main, or any, audio output. The second one was in bad shape; sequencer didn't work, wonky buttons again, but overall all the sounds were working if not some being lower in volume than others. Right upon receiving them I've decided to immediately get rid of the modifications in KPR #1 by looking inside KPR #2 to see what resistors were taken out to do the modification. That's where I found this horror...

It seems we've had an op amp explosion in here. There's some corrosion and there's an obvious void where the 9pin SIP op amp used to be. Decided to quickly just put this aside and focus on which resistor was where so I could replace them in KPR #1.

All good and done (I stupidly forgot to take pictures of the modifications that were made) I've made my way onto replacing the tactile switches. Now I could have searched long and wide for the exact same ones but decided to just go with proper ones that had a slightly larger head for the button cap to slide onto. With a little bit of rounding of the edges they were easier to slide on but impossible to take off (as I've learned with button #12 where I had put in the tactile 90 degrees to the side.. It took a bit of hammering and plastic bending to get it out).

Probably not the best of choices but it worked. Let's hope I won't have to replace them again before the KPR just dies of old age instead. Buttons are working extremely well now though. I've also decided to rig the display light to be on permanently. Because it looks awesome. And that's one working KPR-77 and it sounds great! Apart from a tiny little bit of audio output wheezing somewhere in the background it works perfectly. Now I'll have to wait for the op amps to drop in my mailbox and get work done on the other machine. Until part 2.