TC10 Twin Cam Chorus LFO

[Chillums]

Hi there all... Have a question.  So I finally got around to populating a PCB I made from the schematic by Dirk Hendrick and I spotted something I missed while translating it to PCB.  If you take a look at the snapshot I took of the part in question you will see the top Width pot connects to pin 7 of the 353 and the same pot below does not.  Was it designed that way for the dual chorus effect?  Or is this why my LFO is not working on my build.  On my schematic I have neither Width pot connected to the output of the first opamp so I'm assuming if I connect the pot up like in Dirks schematic (the top LFO) that should fix my LFO? 
My symptoms are no chorus.  If I move the 200k trimpot I get chorus.  If I touch the pins of the trimpot I get chorus.  I cant find a variable voltage anywhere on those two op amps so I'm assuming my LFO is not working correctly.
Sorry about the large blank space in the schematic.  I guess I had my paint resolution set way to high...

[anotherjim]

Its a drawing error I'd say. The joining dot on the upper portion shouldn't be there, it would make the pot work funny if it was joined across the ends like that.

[Chillums]

That's the way I have it now on my pcb... See attached.  So I'm thinkin I should hook them up the other way.  Worth a shot.

[Chillums]

Now that I think about it... Your right... There is no way that the pots are hooked up from pin 1 to pin 3.  Hmmm...think I'm going to breadboard a section and see what's up.

[Chillums]

So I have one LFO on my breadboard and there is no variable voltage after the 220k resistor (R20) until I moved the Width pot.  Voltage goes from 4.6 at full CCW to 8.2 at full CW position.  Any thoughts on what could be the problem?  Free PCB to anyone who helps me figure this thing out.  (I'll install whatever the kludge is for you)  Know of any good gutshots of this unit?  Might just have to buy one...

[anotherjim]

I think the bottom of the width pot should have 2.5v from the +5v divided by the 3k3 resistors.
It looks to me like "Width" is more of a depth control and full CCW should be zero modulation from that LFO.
Do you have a full schematic?

[Chillums]

Have not had the time to mess around with this for the last couple days but hopefully i can figure it out tonight.  I got it on the breadboard and yup the Width pot(s) are really Depth pots.  I get a variable voltage on my breadboard without the transistors installed after the two LFOs.  So it looks like the problem is somewhere after the 220k resistors.  Schematic used for the pcb is attached .  For completeness here is a link to Dirks schematic that I used also. 

https://www.dirk-hendrik.com/temp/Ibanez_BCL_TC10_schematic.pdf

[anotherjim]

I'd check every point that should have a 5v supply on it. I don't think the voltage swing you got on a "width" pot should be anything like as high as you got.

[Marcos - Munky]

The VR4 pot, when wired as top part of the schematic on original post, acts like two parallel resistors between wiper and the outer lugs. With the pot in the middle, it acts as two 50K resistors in parallel, or a single 25K resistor. With the pot rotated to let's say 1/4 of the rotation, you have 25K between lugs 2 and 1 and 75K between lugs 2 and 3, that makes a 18.5K. If you rotate it to 3/4, you have 75K between lugs 2 and 1 and 25K between lugs 2 and 3. So it's basically a pot with a "reference" setting at the center, and if you rotate it to the left you got exactly the same variation as if you rotate it to the right. For two pots that are a dual gang one, it may makes sense for some applications. Since both pots are two single ones, it makes no sense at all and it's a drawing error.

If I move the 200k trimpot I get chorus.  If I touch the pins of the trimpot I get chorus.  I cant find a variable voltage anywhere on those two op amps so I'm assuming my LFO is not working correctly.

I'd bet the trimpot is faulty and have some mechanical contact issues. It happened to me a few times, the wiper wasn't really touching the rest of the trimpot, and when I "forced" it by adjusting it or by holding it with my fingers. It was a brown trimpot, not those blue ones. Replace the trimpot, I think this will solve your problem.

I get a variable voltage on my breadboard without the transistors installed after the two LFOs.  So it looks like the problem is somewhere after the 220k resistors.

That practically confirms your trimpot is faulty. Your LFO is working and the transistor is there as a current booster (I think). But the 200K trimpot is in the way between the LFO and the MN3102 acting as a variable resistor. So if the trimpot is faulty, current from the LFO isn't reaching the MN3102, that's why there's no chorus. To be sure, you can check if you're getting a variable voltage at the point D4 is connected to the trimpot. If yes, then blame on the trimpot.

[Chillums]

Hey guys thanks for the replies... So when I read Jims comments on checking all the 5v spots I noticed that I had the op amp for the LFO connected to the 9v rail and not 5v.  Cut the track and installed jumper so now its getting 5v.  Guess what.... Still nothing... I get different voltages now on the width pot lug 2.... 2.5v CCW and 1.3v CW.  I will change the trimpot later tonight and see if thats it cause if not I'm stumped.  Thought I (Jim) found it.... Damn.  I will report back later this evening....its too nice out to stay inside

Edit: I also have no variable voltage at the connection of D4 and the trimpot. Hmmmmm

[anotherjim]

There's still a lot to get through after the LFO's! But they need to be put right first of course.
If you can read the LFO swing on R64 (4k7), then the LFO section should be good.

[Chillums]

No luck changing the trimpot... Time to start looking at the trannies... Might just start a new board and just install LFO and see if that works.  I guess ive been spoiled with to many "worked first time builds".  I will figure this out.  It sounds and works great if I take my breadboarded LFO and connect it to the 220k resistors on the PCB.  Speed and Width work as they should.  It's gotta be my @#$% up somewhere, but I cant find it.  Rrrrrrrr

[Marcos - Munky]

Ok, so check if you're getting a variable voltage on those parts. Don't bother with writting down measurements, just check for variable voltage.
- U6 pin 7
- U7 pin 7
- R20
- R35
- R19
- wiper of trimpot