Ibanez RC-99 tip and question.

[digi2t]

Hey all,

Picked up an RC-99 Rotary Chorus recently. Thought it would make a nice project for Phil and I, seeing as how we don't have a chorus over at DEFX. It's a bit on the rarer side of the chorus world too, so we thought it a perfect project. Anywho...

I immediately noticed two problems with original. Don't get me wrong, it's a beautiful sounding machine, but the first problem was ticking from the LFO into the audio. At first I suspected that perhaps the TL022 was going south, so I tried a new one. Nope... ticking still there. Caps perhaps? Replaced a 10uF cap next to it. Nope... that's not it. Dang. Well, I thought maybe one of the trimmers is to blame, so I pulled the main board out, and jigged it up on the bench with all the measuring tools at hand, plugged it in, and even before I touch anything....

The ticking.... has disappeared!

Well, a bit dumbfounded, I reassemble it, go back to the studio, plug it in, and... ticking is back.

Needless to say, I'm bewildered. I decide to pry it apart again, right on the spot. Holding the main board in my hand, I plug it back in. No ticking. Huh?! Put it back into the box. Ticking. Woah!

OK... suddenly the light bulb moment happened. Looking at the internal arrangement, the LFO is located on the control board, while the rest of the circuit, including the board mounted jacks, reside on the main board. Well, guess where the LFO chip sits? Directly above the INPUT JACK! Removing the works out of the box, and plugging it in, I could position the two board a bit further apart, and presto, no ticking.

IBANEZ.... WHY HAVE YOU FORSAKEN ME!!!!

Anyway. I kind of let it go for the evening, and this morning, came up with a plan. I had some strips of copper shielding tape from a past guitar project, so I though to myself, "what if I use this to shield the jacks from the control board?"













I added the strip of foam tape as a guarantee that the copper tape would never come in contact with the solder side of the control board. Did it work? Perfectly. No ticking. Dead quiet. Problem 1 solved.

Now, the other problem I have is that it distorts, but only under certain conditions. Namely, if I use any guitar with hot pickups (I have a couple with Bill Lawrence 500XL's, and one with Motherbuckers). If I hit the low E string, only the low E, open, or up to a few frets up, with any enthusiasm, it distorts with the initial attack, but then the distortion disappears as the note rings on. I tried some other guitars with more reasonable pups, and no matter how hard I strike that low E, no distortion. So my question is this; what can/should I mod within this circuit to try and attenuate the input, without jeopardizing any of the lushness of the sound? It does it in bypass as well, so I'm thinking maybe the input buffer needs to be tweaked? I don't know, I need expert advice here. Here's the schematic;



Thoughts?

[Scruffie]

It shouldn't be too hard to cut the input signal to the BBD, SR1 is series resistance with the BBD so that can be tweaked to reduce the signal level reaching it a bit and if that isn't enough, R3 could be removed but you'll have to tweak R4 & 5 to correct U2's output bias after that.

To get the mix back up to 50/50 after that, the easiest way might be to put some gain in U1B.

[digi2t]

OK. I'll try that. I must admit, after more noodling this morning, I do find that it does seem to boost the signal a tad when engaged, just slightly above unity, so what you're proposing might be a good thing. Of course, it might be just that higher frequencies are being boosted too, giving me the impression of slightly higher output as well. Hard to tell, ears are getting old.

[Mark Hammer]

I'm not seeing anything in either the controls or schematic that would justify use of the descriptor "rotary".  Not unless it simply aims for that zone between flanging and chorus where slow Leslie sounds seem to live.  I once modded a folded-steel-case Small Clone to do that, via a simple cap change, and was quite impressed.

[digi2t]

I'm not seeing anything in either the controls or schematic that would justify use of the descriptor "rotary".  Not unless it simply aims for that zone between flanging and chorus where slow Leslie sounds seem to live.  I once modded a folded-steel-case Small Clone to do that, via a simple cap change, and was quite impressed.

Correto-mundo! It's really not it's forte. Passable I suppose, but the chorus is where it's at.

[digi2t]

It shouldn't be too hard to cut the input signal to the BBD, SR1 is series resistance with the BBD so that can be tweaked to reduce the signal level reaching it a bit and if that isn't enough, R3 could be removed but you'll have to tweak R4 & 5 to correct U2's output bias after that.

To get the mix back up to 50/50 after that, the easiest way might be to put some gain in U1B.

OK, tried tweaking the trimmer, but the problem is that the distortion is coming from upstream. Increasing resistance of the trimmer simply attenuates the effected signal, which is not what I'm looking for. Besides, as I mentioned before, it does it in bypass as well.

Maybe a slight increase in the C1/R1/R2 area?

EDIT: And maybe I should be probing the input and output buffers separately as well? Maybe the distortion is coming from the ouput side? Hmmmmm....

[digi2t]

Upon further investigation, it really was the BBD being overloaded. Trimming back to 50K was a bit better, but hot pups will still overload it in a low E heartbeat. Pulled the 50K and put in a 100K. 88K is the magic number here. No more clipping, and the chorus is still very present. Even with lower output pups. Bizarre that they would only use a 50K here, but then again... LFO next to the audio? Need I say more? The signal is actually at unity now between bypass/effect as well.

Thanks for the insights scruff. Much appreciated.