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Rocktek CHR-01 Chorus

8/8/2003 8:31 PM
Chris B
Rocktek CHR-01 Chorus
I just bought a Rocktek chorus off e-bay for $12.11 shipped. :) It's missing the battery cover, and the box looks like crap. Lots of nice sticky tape residue to remove. :(  
Anyway, I was wondering if any one might know of a source of a schematic for this thing? I'm pretty sure I'm going to mount this thing in a new box (a nice Hammond "BB" or possibly a custom, one off box). New pots and a new switch are in order.  
Now I've got two 80's El-Cheapo pedals! How cool is that? ;)  
Chris B
8/8/2003 8:43 PM
Mark Hammer

I had one. This is going to be a pretty basic unit that uses a single FET for switching and won't diverge much from the MN3207/3007 datasheet/appnotes. This is also one of the reasons why they are such great value (as are the Rogue units). Much the same way there were a lot of also-ran tube amps that used a 5Y3, 12AX7 and 6V6 tube compliment in a textbook format/design (which is what the low power Fenders, Gibsons, Supros, etc were) but made a crappy choice about speaker or power transformer, there are plenty of low-end chorus units that all follow the same basic design but made poor choices about chassis or footswitch, etc.. Same basic guts as the good ones, though.  
So, expect to see two dual op-amps (one for LFO, another for input/output stages), 4 bipolar transistors (one for each 2-pole anti-aliasing filter, 2 for a discrete flip-flop for switching the FET), a MN3101 or 3102 and an MN3007 or 3207. You can mod it to do some quasi flanger tricks by subbing different value caps for the one just beside the MN3101/3102. There will probably be something in the 220pf range there right now. Drop it to 75 or 68pf and you should be able to get some decent rotating speaker sounds. If I'm not mistaken, there is a "tone" control on this unit, to take the highs out of the delay signal. You may want to free up that spot for another control.
8/9/2003 4:36 AM
Chris B

I like the idea of the cap change. Maybe I'll put a switch in that selects between cap values there. I know that I'm going to take it out of the plastic chassis, because those pots are flimsy and I want some good Alpha 24mm pots in there. I'm hoping that all of this will fit into a Hammond "BB" sized box.  
Would you go higher value than the 220p for the flanging effects? I know Radio Shack sells a nice small rotary switch that would work well for switching caps.  
Thanks Mark! :)  
Chris B
8/11/2003 2:24 PM
Mark Hammer

There are limits to both how fast and how slow you can clock the BBD chip. The rule of thumb is that you use smaller values to make the clock frequency range higher (which means shorter delay time = flanging range) and larger values to make the delay time longer (more in the chorus range).  
Check out the thread at Aron's DIYstompbox forum on the Zombie clock mod. I detail how to go about adding 3 clock ranges with a 3-position on-off-on SPDT minitoggle.  
Please note that since chorus pedals also use a slower clock rate to produce longer delay times, they also use a lower cutoff frequency on the lowpass filters that get rid of clock-related noise. This means you won't get as bright a flanging sound as you might on a dedicated flanger, but it won't be horrible. You'll be able to get a nice Leslie effect. Just don't expect "jet plane" swooshes.
8/13/2003 5:24 AM
Chris B
Those "Jet plane swooshes" are what I'd be after. So, I guess a Flanger is still on my wish list. Can you recommend a good one that doesn't cost an arm and a leg?  
Thanks again,  
Chris B
8/13/2003 3:57 PM
Ed G.
I've got the cheapo danelectro flanger, and for what it is, it sounds pretty damn good. No noticable noise, too. It doesn't sound as good as an old EH Dlx. Electric Mistress, but it does sound good.
8/14/2003 2:58 PM
Mark Hammer

Unfortunately, you won't get much "swoosh" from the stock pedal with just a cap change. The difficulty with the MN3007/MN3207 type chip is that their input capacitance is too high to tolerate very high clock frequencies and higher clock frequencies are what you need for the jet-plane-down-from-the-skies tone.  
This CAN be overcome with suitable buffering though, and you will see a number of designs that stick multiple parallel invertor/buffer stages between the clock generator and the BBD itself. Think of these like "clock signal preamps" that overcome the way in which the clock input lines on the BBD would normally degrade the clock signal to the point of uselessness. Mike Irwin tells me he has successfully gotten MN3007s to clock well over 1mhz (which the MN3007 spec sheets describe as unlikely) with this method.  
However, sticking a 4049 or other CMOS invertor/buffer chip on the board, lifting and rerouting lines, starts to get complicated. I say be happy with turning a $12 pedal into a $50 pedal with a few more tricks and hope that you can stumble onto a $40 pedal someday that lends itself to transformation into a $200 pedal.  
Recommendations for a flanger? Consider the digital pedals....seriously. I don't think many of the lower end ones do a fabulous job at distortion and some other things, but when it comes to delay-based effects, digital is fine and excellent value for the money. There are plenty for under $100. On the analog end, there are tons of BOSS BF-2 units out there in 2nd hand stores for well under $50. Thse do a pretty good job and are also quite modifiable for other tricks.  
True analog "jet plane" stunts are done nicely by the FoxRox Paradox TZF flanger and almost any issue or clone of the A/DA Flanger, both of which produce some stunning effects.....but at a price.
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