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 Forum index » DIY Hardware and Software » fonik's place
Problem with x4046 vco
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bubzy



Joined: Oct 27, 2010
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PostPosted: Tue Jul 05, 2011 11:30 pm    Post subject: Problem with x4046 vco Reply with quote  Mark this post and the followings unread

Dont know if anyone else has experienced this problem, but im at a loose end. i cannot get this vco to oscillate at low frequencies, if i turn the coarse pot down, it fades away to no oscillation and then if i turn the pot back up it "creeps" back to a higher oscillation (the response time is slow to return) in addition there is no v/oct tracking on this at all, a 1v input and a 2v input will both be of a very similar voltage out of the transistor array. The odd thing is, is that if i remove Q1 and leave Q2 in (tried with LM394 and single 23904's) it will oscillate all the way through turning the coarse pot. I have tried this module with +-12 and +-15v with no change, i am using a MC14046 (or so it says, hopefully i havent been diddled)

I have built 2 of these boards, using 1% resistors, i have checked and double checked all the resistor values, i am using good quality components so i cannot see where the problem is, if anyone has a clue as to the problem i would be most grateful because i have 3 of these pcbs waiting to go Smile
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fonik



Joined: Jun 07, 2006
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PostPosted: Wed Jul 06, 2011 1:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

no clue for now.
so both builds behave in the same unruly manner?

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matthias
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bubzy



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PostPosted: Wed Jul 06, 2011 2:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

yeah, have tried 5 different motorola chips in each too. most confusing Very Happy i look forward to a solution, but for now im just going to build other stuff
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fonik



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PostPosted: Wed Jul 06, 2011 11:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

it is normal that the VCO is not oscillating across the whole potentiometer range. thomas did design this by purpose. the main coarse potentiometer has a wide range to compensate offsets that may add up via additional CVs.

what puzzles me is that if you turn up the pot again, the VCO will "follow" up slower...!?

anyways, in the 1st rev of the documentation there has been an error that will keep your VCO from tracking properly:
R2 has to be 390R NOT 100k!

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bubzy



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PostPosted: Sat Jul 30, 2011 1:25 am    Post subject: Reply with quote  Mark this post and the followings unread

well, i just must be retarded. tried 3 different 4046 chips (including NXP HEF4046), checked all my soldering a million times and still i get the same problem. very small range, scale and HF pots do nothing. heeeeeeelp Razz
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fonik



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PostPosted: Sat Jul 30, 2011 2:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

i will pull out my VCOs on monday, and do some measurements for cross reference. hopefully this will help to identify the error. hold on.
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fonik



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PostPosted: Tue Aug 02, 2011 3:39 am    Post subject: Reply with quote  Mark this post and the followings unread

1st point to check was the voltage at pin 2 of the matched pairs.
i measured a change of about 18mV per 1V on the CV input.

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fonik



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PostPosted: Fri Aug 05, 2011 2:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

someone on the muffwiggler forum reported about the screw drills:
Quote:
Actually, some of the pads are a little close... for example something like C13 on the 4046 Core PCB or even J1 on that board, which isn't quite as close but barely clears a 4-40 screw head. I use plastic standoffs and washers so it's never a problem, but they definitely do make contact with solder pads in a few places.

this could lead to a short. obviously i use standoffs and screws that are small enough? just a guess.

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bubzy



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PostPosted: Fri Aug 05, 2011 11:13 pm    Post subject: Reply with quote  Mark this post and the followings unread

hi fonik, sorry i havent replied im working my ass off all the time at the moment and havent had a chance to check it yet. unlikely to be anything to do with pads as this pcb is still on the desk Very Happy
hopefully ill get to check it out this weekend
thanks
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bubzy



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PostPosted: Fri Aug 19, 2011 4:49 pm    Post subject:   Reply with quote  Mark this post and the followings unread

what a dumbass. i didnt check the power connections, and the GND doesnt seem to be attached to the front and rear pins of the idc connector. its just at the "back" pins, 1 jumper later and i have a working vco
many thanks!!!Smile
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fonik



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PostPosted: Sat Aug 20, 2011 8:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Very Happy
things happen. i am glad you found the error. and it has been an easy fix!

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