Frequency Central XVCO TWO no squarewave [SOLVED]

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artilect99
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Frequency Central XVCO TWO no squarewave [SOLVED]

Post by artilect99 » Sat Sep 12, 2020 3:36 pm

Just built an FC XVCO TWO, my second one. Works fine except there is no squarewave output! I have tried swapping the pots and jacks board, and also the ICs, from the working module, so the problem must be a passive component or trace on the main board. The VCO is a sawcore and the saw is fine so the problem must be located somewhere between there and square output.

I was hoping somebody who knows more about this circuit will be able to help. I have hit up FC for a schematic but haven't heard anything back... it's hard to debug without. I have the original Roland 100M schematic but it hasn't been very enlightening so far.

I generally don't see much on FC modules here but if anybody is experienced with this module or with the roland 100m VCO in general I would sure appreciate any insight. Thanks!

Edit: user Guest was able to figure out I had a wrong resistor in there, I swear that never happens! :oops: got sent a bag of 18k's labelled as 180k... :bang:
Last edited by artilect99 on Sun Sep 13, 2020 8:07 pm, edited 2 times in total.

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Re: Frequency Central XVCO TWO no squarewave

Post by artilect99 » Sat Sep 12, 2020 9:41 pm

Here is the schematic of the original, just for reference. My problem appears to be somewhere in the gray area lower right, since the saw and triangle outputs work fine.

Image

Probing for differences against the XVCO that works, I found the cathode of the circled 1n4148 reads a solid 7.5v (as does the squarewave output jack) whereas on the working unit the squarewave signal is present there, as well as on the 4.7k next to it and at pin 1 of the 072 below... Should I try replacing this diode? I am willing to try anything but I don't want to start desoldering all willy-nilly if it doesn't make sense to do so.
bad VCO.png
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Re: Frequency Central XVCO TWO no squarewave

Post by guest » Sat Sep 12, 2020 9:52 pm

what are the voltages on all pins of IC5A?
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 2:45 pm

Hi Guest, thanks for the reply. Assuming IC5a is the top right TL072 in the pic of the circuit board I posted--
1 = 11.08v
2= -3.21v
3= 0.01v
4= -12v
5= 0.63v
6= 0.63v
7= 1.58v
8= 12v

On the working unit these are:
1= -5 - 5v (squarewave sig I guess)
2= -.3-3v
3= 0.01v
4= -12v
5= 0.63v
6= 0.63v
7= 1.89v
8 = 12v

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 4:51 pm

check all the resistors in the blue area on your highlighted schematic. im betting one of them is the wrong value (22k instead of 220k, etc) or has a cold solder joint. first place i might look is R49. you should see the full scale sawtooth on one side, and an attenuated sawtooth on the other.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 5:32 pm

yeah, R49 (R43 below) is correct value (47k) and the saw is present on one side, attenuated on the other. All those resistor values are correct. I went around and re-flowed that area, looked for bridges, etc.-- no dice. :(

The 180k that goes to -12v reads -12v on one side and ~ -3.21v on the other.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 6:30 pm

the problem is the voltage on pin2 of IC5A. set to a really slow frequency, and see what the min and max voltages are. it should be going above and below 0V.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 6:43 pm

At the slowest frequency pin 2 fluctuates between approx. -3.08v and -3.27v. Pin 1 stays flat at 11.08v.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 6:47 pm

what is the voltage range on the raw saw wave before R43?
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:07 pm

if the saw wave input is 0-6V, then i think either R47 is too low (check it with a multimeter), or there is a short in the board someplace pulling that node down.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:25 pm

These are the voltage readings I get with my less-than-stellar DMM. Sorry I don't have a scope I know that would be much more helpful. Basically on both the good and bad units, all the marked points fluctuate are the same until you get to the top of the 47k (next to IC5a) and then it's fluctuating between -3.03v and -3.27 on the bad unit, and between -0.5v and +0.5v on the good unit.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:28 pm

R47 is the 180k to -12v... It is the correct value, it reads -12v on one side and -3.21v on the other.

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:30 pm

did you measure the resistance with your meter?
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:30 pm

wait, resistance, no, I went by color code. I measured the voltages on it with my meter.

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:33 pm

As far as I can tell the board is identical in all respects (component values, polarity, etc.) to the board in the working unit... it's just not behaving the same. It's really got me bummed
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:34 pm

meaure the resistance of the 180k with your meter. you may have to lift one leg to get an accurate reading.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:39 pm

I measured in situ with the good unit for comparison. Good unit measures 18.5k in situ, bad unit measure 8.8k. (I can still pull one leg up if you want.)

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:42 pm

yeah, we are getting closer to the problem. lift a leg and measure it, might just be a bad resistor.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:46 pm

it measures 18k pulled completely out of the board... I'm going to put in a 180k and see if that does it... if it does, I'm so sorry you had to troubleshoot my stupidity Guest... :eek: remind to get you a nice fruit basket or something...
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:48 pm

must be a misprint (it has a yellow band in third spot?). toss a 180k in there and it should work fine for you.
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:50 pm

yeah it's definitely brown grey black orange brown... wtf

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 7:52 pm

I just measured the rest in that bag and they all measure 18k!! argh I guess a tayda packing error... I kind of got lazy about verifying resistor values, I guess that will teach me. Here's hoping this fixes it!

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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by guest » Sun Sep 13, 2020 7:54 pm

mistakes like that suck, because you dont expect them. i sometimes have trouble telling the difference between the orange and red, is there another red band resistor you can put it next to just to see if it might be an odd tone of red?
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Re: Frequency Central XVCO TWO (Roland 100m) no squarewave

Post by artilect99 » Sun Sep 13, 2020 8:04 pm

Yeah, seeing the 18k and the 180k together it is a little more obvious, though I guess my eyesight is going because even under my shoplight its hard to tell... but yeah I should have caught that. Anyway Guest you sussed it out the hard way, can't tell you how helpful that was. Thanks again! :hail:

Put next to the red bands on a 100k it is more faded, I can see how it happened... Whoever chose red orange and brown to differentiate critical values needs to be dragged out into the street and horsewhipped
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Re: Frequency Central XVCO TWO no squarewave [SOLVED]

Post by guest » Sun Sep 13, 2020 8:08 pm

awesome to hear its working.
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