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 Forum index » DIY Hardware and Software » Thomas Henry designs
SN Voice - 3rd time is a char...uhh, problem
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Appliancide*



Joined: Jul 04, 2007
Posts: 119
Location: Paul lives in Thief River Falls

PostPosted: Tue Apr 23, 2013 5:22 pm    Post subject: SN Voice - 3rd time is a char...uhh, problem
Subject description: problem solved!!!
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I have run into a bit of a puzzler on the third SN Voice I've built. Actually two problems, but I'm wondering if they are related.

First, the triangle out is weak. The LFO output and pulse/noise output are both putting out the expected levels. I have been using my scope to eyeball levels and compare them to one of my working units, so please forgive the lack of specific numbers. The output amplifier/buffer seems to be working fine. The triangle just seems to be weaker coming out of the 76477. I checked R51 to make sure it was 220k.

Second, the PWM is not working at all. There is no variation between the switch settings. I triple-checked my switch wiring, doing continuity tests on the PCB itself to rule out the MTA connectors.

I know this circuit has been around for quite a while, so I'm curious if anyone has had similar problems, or maybe ideas about mine.

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Last edited by Appliancide* on Thu May 16, 2013 4:48 pm; edited 1 time in total
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sneakthief



Joined: Jul 24, 2006
Posts: 529
Location: Berlin

PostPosted: Wed Apr 24, 2013 3:10 am    Post subject: Reply with quote  Mark this post and the followings unread

Sorry to be Mr. Obvious, but did you try swapping out the SN for another known working one?
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Appliancide*



Joined: Jul 04, 2007
Posts: 119
Location: Paul lives in Thief River Falls

PostPosted: Wed Apr 24, 2013 8:26 am    Post subject: Reply with quote  Mark this post and the followings unread

I will have to check to see if I have a "known good" wide version. My two working versions have the narrow chips in them.

Has anyone ever come across a partially working SN76477? I ask because most of the functions tied to that chip seem to be working (noise, lfo, envelope, mixer....).

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Appliancide*



Joined: Jul 04, 2007
Posts: 119
Location: Paul lives in Thief River Falls

PostPosted: Wed Apr 24, 2013 6:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

I did have another wide one to check and the behavior was the same, so I must have made a mistake somewhere. I feel like the answer is staring me in the face, especially since there is very little circuitry associated with the functions I am having problems with.
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Appliancide*



Joined: Jul 04, 2007
Posts: 119
Location: Paul lives in Thief River Falls

PostPosted: Fri Apr 26, 2013 12:41 pm    Post subject: Reply with quote  Mark this post and the followings unread

I broke down and pulled one of my working modules out of the case to switch pcbs with the problem module.
The working pcb has no problems when connected to the newest module's faceplate, so I can now say that my wiring is not at fault.
I really wish that I enjoyed troubleshooting more.

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Uncle Krunkus
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Joined: Jul 11, 2005
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PostPosted: Fri Apr 26, 2013 2:36 pm    Post subject: Reply with quote  Mark this post and the followings unread

I hardly ever enjoy troubleshooting while it's happening. But the payoff can be very satisfying. Good luck with it. Smile
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Appliancide*



Joined: Jul 04, 2007
Posts: 119
Location: Paul lives in Thief River Falls

PostPosted: Tue Apr 30, 2013 1:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

I had two pcbs that I stuffed 4-5 years ago that I was going to build in a format that I've since abandoned. I dug one out, cleaned it up a little bit and it works. I used the same sn76477 from the problem module, so that wasn't the problem.

I though that J3-2 and J3-3 might have been interacting in some way on the problem PCB, so I decided to wick some solder off of those pads in case there was a small short under the J3 header. When I did this, it exposed some copper on the little triangular section between the two pads (this is one of the newest PCBs). It isn't shorted to either jumper, but i can't help thinking it could have been causing problems. I wonder if I could have damaged the PCB when I was cleaning the flux off. I used a pretty strong flux remover, so maybe some seeped into the PCB? I can only guess at this point, since I was never able to definitively nail down the problem.

Now that I have two working modules again (third one was for a forum friend, so I just gave him one of mine while I was troubleshooting this one), I will probably de-solder the expensive parts and chuck it in my failure box to mess with another day.

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Appliancide*



Joined: Jul 04, 2007
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Location: Paul lives in Thief River Falls

PostPosted: Thu May 16, 2013 4:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Holy cow, am I a moron! After thinking the old board I swapped in fixed the problem, I noticed the same problems again!

It wasn't the SN chips I tried and it wasn't the pcbs themselves. I also had the same problem with two different panels, so what is the difference between the working modules and the "problem" modules?

Freakin' trimpots! On my first two modules, I used single-turn pots for the offset and modulation adjustments. The modules with the "problem" both have multi-turn pots, that I failed to take through their full ranges before moving to troubleshooting.

I will spare you the details of all of the different things I tried before realizing this. I just wanted to wrap this thread up with a solution instead of the the defeat I expressed in the last post.

It's embarrassing to post this, but maybe it will be a reminder to someone else down the road that sometimes it's best to walk away and come back with a clear head. Obvious solutions are very good at hiding from frustrated people!

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Danno Gee Ray



Joined: Sep 25, 2005
Posts: 1342
Location: Telford, PA USA

PostPosted: Thu May 16, 2013 4:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

We appreciate your candor, and allowing us all tou learn with you from your journey. Do not feel alone in having traveled there.
Congratulations on finding the error yourself.
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elmegil



Joined: Mar 20, 2012
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PostPosted: Thu May 16, 2013 6:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hooray for working SN Voice Smile
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Wobuse



Joined: Dec 07, 2011
Posts: 26
Location: Lyons,il

PostPosted: Sun May 19, 2013 12:32 am    Post subject: Reply with quote  Mark this post and the followings unread

Danno Gee Ray wrote:
We appreciate your candor, and allowing us all tou learn with you from your journey. Do not feel alone in having traveled there.
Congratulations on finding the error yourself.

I second that!
Everyone makes so called silly mistakes, but we only learn from our mistakes...
even silly ones. Wink

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