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 Forum index » DIY Hardware and Software » fonik's place
VCO 50% working :-\
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Wed Mar 07, 2012 8:45 pm    Post subject: VCO 50% working :-\ Reply with quote  Mark this post and the followings unread

My first 4046 VCO worked perfectly. Second one has a problem.

Saw, Tri and Rampoid outputs are fine.
Freq, Fine, Lin & Exp FM controls and Rampoid pot all work fine.
Sin output = nothing
Pulse output = nothing, but when I sweep the PW knob past 50% there's a click

The problem must be in the WS board (?). The chip is getting +/- 15v as required. The only parts change from my first VCO and this one is on the first one I used box caps for C500 & C600. On this second one I used ceramic discs.

Any suggestions? I have VCOs #3 & #4 waiting to be wired Smile

Drew
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fonik



Joined: Jun 07, 2006
Posts: 3759
Location: Germany
Audio files: 23

PostPosted: Thu Mar 08, 2012 3:25 am    Post subject: Reply with quote  Mark this post and the followings unread

so you are halfway there? good!

sine is created from the triangle, the pulse is created from the sine.

thus we have to solve the issue with the sine output first:
do you have a DC coupled scope? how does your triangle look like?

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cheers,
matthias
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Thu Mar 08, 2012 4:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

That's what I figured... that maybe my Triangle is wacky.

I am going to get out my old Roland S-550 sampler and use the wavescope on it to see what the wave looks like. From "ear only" experimentation, it appears that the Tri Offset and the two Connect trimmers are what I need to play with (?)

Drew
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Thu Mar 08, 2012 8:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK, I hooked up the S-550 (after spending a half hour looking for the boot floppy Rolling Eyes ) for my pseudo scope.

My #1 VCO (that works fine) had a triangle wave that looked terrible! Far far worse than the #2 VCO (half working). Some trimmer adjustment got both of them looking better, but #2 was pretty good to start with.

With the Rampoid pot at zero, the Rampoid wave looks just like the Triangle wave. And it sweeps into a Saw as expected.

Still no Sin or Pulse output, as before.

I check all the resistor values on the WS board as well as the caps. I swapped the IC from #1 VCO and #2 VCO to no effect. I checked the continuity of the wiring that goes to the Sin Out jack. I looked for shorts between wires on adjacent pads.

Are ceramic caps for C500 & C600 OK? That's the only difference between #1 and #2 VCO (that I know of!).

thanks again for your help Smile
Drew
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fonik



Joined: Jun 07, 2006
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Location: Germany
Audio files: 23

PostPosted: Fri Mar 09, 2012 12:38 am    Post subject: Reply with quote  Mark this post and the followings unread

oozitron wrote:
With the Rampoid pot at zero, the Rampoid wave looks just like the Triangle wave. And it sweeps into a Saw as expected.

and the offset is almost zero volts?

Quote:
Still no Sin or Pulse output, as before.

I check all the resistor values on the WS board as well as the caps. I swapped the IC from #1 VCO and #2 VCO to no effect. I checked the continuity of the wiring that goes to the Sin Out jack. I looked for shorts between wires on adjacent pads.

Are ceramic caps for C500 & C600 OK? That's the only difference between #1 and #2 VCO (that I know of!).

thanks again for your help Smile
Drew

so if the triangle is fine now, we have to focus on the sin converter.
the good news is: the circuit/PCB itself has no errors, so there must be an error that you could find and solve.

you could check all solder connections/solder joints and vlaues a third time. if you are 100% sure the network is as it should be, then there could be a faulty component - i have heard of some really nasty errors, i.e. a hairline crack going through a resistor, and the resistor was working when you attached the test prods of the multimeter!
however, i would take a look at the transitors first. is the orientation correct? are they working?
the trimmers could be another possible source of error: measure their resistance and check if they work.

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cheers,
matthias
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Sun Mar 11, 2012 7:38 am    Post subject: Reply with quote  Mark this post and the followings unread

Well I got tired (and frustrated) with troubleshooting, so I put #2 VCO aside and wired up #3 VCO. And it works *perfectly* (just like #1).

So I guess I'm down to:
1) wiring soldering error
2) bad transistor, resistor or cap

At least I can compare the resistor values in #2 vs #3 Smile

Troubleshooting is the one part of DIY I fear...

Drew
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fonik



Joined: Jun 07, 2006
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Audio files: 23

PostPosted: Sun Mar 11, 2012 11:20 am    Post subject: Reply with quote  Mark this post and the followings unread

oozitron wrote:
Troubleshooting is the one part of DIY I fear...

alas, it happens from time to time. however, it gives me a great feeling of success if i finally find the error. so don't give up.

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cheers,
matthias
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Tue Mar 27, 2012 2:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

OK... I'm back! I went ahead and finished VCO #3 and #4 (which work perfectly), and now I'm back onto fixing #2.

I went thru all the resistors (on the WS board) and compared them to one of my working VCOs. Everything matched except for R10 (2k2). It read zero ohms. So I cut that out and was going to put another in, but then I checked the resistance between the two pads for R10; zero ohms!!!

I can tell that the R10 pad near the edge of the PCB should be grounded and it is. But the other pad is as well. It leads to the middle leg of Q4 and then on to the two legs of the R41 trimmer.

And the problem was... a bit of solder from one of the two (joined) trimmer legs had dribbled onto the ground plane and apparently burned thru the red coating. I will power it up tonight (have to make dinner now before my wife gets home!) and I have no doubt that it will be singing.

Thanks for the encouragement! I'll be showing this off at AHMW this weekend and will post some videos as well.

Drew
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fonik



Joined: Jun 07, 2006
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Audio files: 23

PostPosted: Tue Mar 27, 2012 9:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

oozitron wrote:
And the problem was... a bit of solder from one of the two (joined) trimmer legs had dribbled onto the ground plane and apparently burned thru the red coating.

Shocked
never heard of anything similiar before! so the solder stop mask did not it's job?

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cheers,
matthias
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oozitron



Joined: Apr 02, 2007
Posts: 35
Location: USA

PostPosted: Wed Mar 28, 2012 7:58 am    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, it was a bit weird, but no more!

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Drew[/img]
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