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Roland 100m sawcore VCO - squarewave not working [SOLVED]
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artilect99



Joined: Oct 01, 2018
Posts: 45
Location: USA

PostPosted: Sat Sep 12, 2020 1:44 pm    Post subject: Roland 100m sawcore VCO - squarewave not working [SOLVED] Reply with quote  Mark this post and the followings unread

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: It was a bad resistor. Confused

Last edited by artilect99 on Sun Sep 13, 2020 6:18 pm; edited 2 times in total
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hv0190



Joined: Apr 18, 2011
Posts: 25
Location: Munich

PostPosted: Sun Sep 13, 2020 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

I'm far from being an expert, but if you've got an ok sawtooth but no square, I'd check the region around the comparator ... as FC provides no schematics I can only guess where that is on the PCB, but I'd check the TL072 next to the two trannies in the upper quarter of the main PCB and also the single diode in the first top row next to the 4K7 resistor ... especially check if you've got the orientation of the diode right ...
Directly below the said TL072 there's a 100K and a 22K resistor ... these might be parts of the comparator circuit, you might check for continuity and if you've got the resistor values right
But without the schematics this is only guesswork, I'm afraid ... Sad
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artilect99



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PostPosted: Sun Sep 13, 2020 12:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah I was thinking the same... probing for differences between the working and non-working units, that diode reads just a solid 7.5v on the cathode whereas on the working unit the squarewave is present there. If I remember right the anode on both units read the same, so I had zeroed in on that diode being a problem. The orientation is correct, but I could try replacing it just to be sure... I'll also go thru and verify continuity in that section.

Thanks for the reply!
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artilect99



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PostPosted: Sun Sep 13, 2020 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's the relevant bit of the schematic if it helps (found this online, from an older version but appears to be unchanged here)


bad VCO.png
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bad VCO.png



comparatoretc.png
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squarewave comparator
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comparatoretc.png


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Blue Hell
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Joined: Apr 03, 2004
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Location: The Netherlands, Enschede
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PostPosted: Sun Sep 13, 2020 1:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

7.5V is an odd value .. are you measuring that against ground or against the -12V?

When the diode would be a short, you should measure 0V against ground ... when it would be an open circuit you would still see a square wave, but going from around -12V to around +12V, with the diode functioning properly the negative half would be cut off and the square would move between 0V and +12V.

Anyway, is the ouput voltage fluctuating with the PM controls being moved (the PWM 50% trim for instance)?

Can you measure on pin2 of ic4.1 to see of the saw is present there? (best to measure on the IC pin, not on the socket, just to rule out a bad socket as well).

And if so .. check the values of R49 and R50 and see if they are properly soldered in.

And the output (pin1) of ic4.1, what is that one doing?

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Jan
also .. could someone please turn down the thermostat a bit.
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artilect99



Joined: Oct 01, 2018
Posts: 45
Location: USA

PostPosted: Sun Sep 13, 2020 4:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

Here's the measurements on that IC:

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

Unfortunately I have checked and re-checked the resistors and reflowed them, so I'm 99% sure they are correct, barring truly unforgiveable stupidity on my part.

Pin 1 stays at 11.08v and pin 2 at -3.18v no matter how I move the PWM manual or PWM->LFO or duty cycle trimmer.

7.5v was measured against ground... what's weird is there is a separate board for the octave switching that uses 7.5v (set by a trimmer with a test point) ... don't know if it's a coincidence or what. I know the the panel boards work with the other unit's main board plugged in though...


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octave pcb.png


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Blue Hell
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PostPosted: Mon Sep 14, 2020 12:32 am    Post subject: Reply with quote  Mark this post and the followings unread

I see you found a bad resistor, good Cool
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Jan
also .. could someone please turn down the thermostat a bit.
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artilect99



Joined: Oct 01, 2018
Posts: 45
Location: USA

PostPosted: Mon Sep 14, 2020 3:53 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah I had a bag of 180k that actually turned out to be 18k. Nearly drove me nuts! Shocked user Guest on MW figured it out.
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