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 Forum index » DIY Hardware and Software
Magic Smoke 8k LFO pcb layout - debug
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defog



Joined: Aug 24, 2011
Posts: 113
Location: Philadelphia

PostPosted: Tue Jan 22, 2013 6:32 pm    Post subject: Magic Smoke 8k LFO pcb layout - debug Reply with quote  Mark this post and the followings unread

Let's play a game! The game is called "I suck at making layouts". Obviously I've got something screwed up here. I etched and wired up two of these suckers identically, and both are exhibiting the same behavior; one position turns a test LED on solid, the other turns it off, and the scope shows no LFO, so the comparator is obviously not working. Voltages on the wave outputs are either around solid -10v or +10v, so definitely not the 5v output I should be seeing. I matched the values on the schematic by Tim Servo. I left some space on the bottom left to wire up an LED indicator later on using a 2n3904. I also read stuff about these influencing power rails and having to add a small value resistor and a 10uf cap to aleviate that, haven't gotten there yet though. Red pad +12, Blue -12, Green ground.


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

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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Jan 22, 2013 8:02 pm    Post subject: Reply with quote  Mark this post and the followings unread

The resistors having different numbering makes it hard to map between your layout and the schematic.

I'm not seeing any discrepancy between the two (with the understanding that you subbed your R2 of 500k in place of the fine tune pot).

Have you adjusted the pot? You only mention switching from one position to the other which I assume means flipping S1.
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defog



Joined: Aug 24, 2011
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Location: Philadelphia

PostPosted: Tue Jan 22, 2013 8:32 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
The resistors having different numbering makes it hard to map between your layout and the schematic.

I'm not seeing any discrepancy between the two (with the understanding that you subbed your R2 of 500k in place of the fine tune pot).

Have you adjusted the pot? You only mention switching from one position to the other which I assume means flipping S1.


Sorry bout that, I realized that too after making it. Yes, the 500k is in place to sub for the fine tune pot. Adjusting the pot changes the voltage of whatever waveform is selected slightly, like +/- .1v depending on which waveform is selected. I just rechecked my resistor values and did continuity on the board, and it all SEEMS correct. All my power rails from different components I've wired up on my DIY synth are all in parallel as well, and the -v and +v rails and the ground are good on the IC and going to the power supply. Voltages read as follows on the TL072:

1 -10.2v
2 0v
3 -8.5
4 -12.00v
5 0v
6 between 0v to -0.5v with pot sweep
7 starts at +10.85v & slowly starts dropping on one of the boards and goes fully negative eventually with pot fully counter-clockwise. The other stays at -10.55v regardless
8- +12v

Looks like the negative voltages are winning in the comparator battle. I probably have the wrong value for that 5m resistor, but just can't tell from my crappy multi-meter that only goes up to 2m. I'm color blind so reading the bands won't help in this situation. I'm going to check that and also put some clips to bypass the 500k resistor that is a substitution for that fine tune pot and see what that does.

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elmegil



Joined: Mar 20, 2012
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PostPosted: Tue Jan 22, 2013 9:17 pm    Post subject: Reply with quote  Mark this post and the followings unread

Take a picture of the resistor and post it, and we can read it for you Smile

I would start with checking component values, particularly the cap as well as the 5M resistor.

Another question: are you using a socket or not? If so, what kind of socket? I've had the cheaper leaf-style IC sockets misbehave on me.
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marvkaye



Joined: Mar 14, 2011
Posts: 225
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PostPosted: Tue Jan 22, 2013 10:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

I agree with elmegil... the layout appears to match the schematic once you get the resistor numbers sorted out, and other than your calling out S1 in the BOM an SPST when it's really a SPDT, everything looks spot on. Nice compact routing, BTW, not so bad for someone who claims to suck at it.

I breadboarded it and found that it seems to work just fine using the component values shown on your PCB layout. However, I notice that the square wave doesn't have a 50% duty cycle, it looks more like 60-40 or worse. I didn't get the scope out, just setup a couple LEDs but they're enough to check basic function. I also noticed that if I slowed it down too much it would lock up... about the best I saw before it stopped cycling was each bulb staying on about 3 seconds or thereabouts... a 6 second cycle is a far cry from the expected 60 seconds at the slow end. I wonder what that's all about? Anyway, I'll get the scope out tomorrow and do some more fiddling with it... let you know what I find out.
'
<marv>
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elmegil



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PostPosted: Tue Jan 22, 2013 10:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

Is it possible that when you power on, you're already in a position where you're locked up?
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marvkaye



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PostPosted: Tue Jan 22, 2013 11:16 pm    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Is it possible that when you power on, you're already in a position where you're locked up?


Good thought. I know when I run the pot fully CCW the circuit stops cycling. If you set that pot about midrange it oscillates just fine... slowly decreasing the rate lets you see when it finally stops running. My PS is +/-15v, BTW.
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elmegil



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PostPosted: Wed Jan 23, 2013 6:12 am    Post subject: Reply with quote  Mark this post and the followings unread

Once it stops, will it start again if you swing it back into the mid range? If so then my idea is bunk Smile
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marvkaye



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PostPosted: Wed Jan 23, 2013 7:57 am    Post subject: Reply with quote  Mark this post and the followings unread

elmegil wrote:
Once it stops, will it start again if you swing it back into the mid range? If so then my idea is bunk Smile


Yeah, I can go from locked up to back running again by simply turning the rate pot clockwise. It probably takes 10-15 degrees of rotation before the oscillation starts back up, though, so there is some threshold that needs to be reached to restart the cycling. It could be I have a wonky pot... I'll check that tonight to see if there's a dead (open) spot at full CCW rotation... that could explain a lot.

On another note...I decided to pull out my old Hitachi scope late last night, took awhile to get it lit up but the sweep had a definite hook in it which lasted for a couple minutes and then it finally died, couldn't get a sweep or even a dot on it. Poop! Sucker's got to be 25-30 years old, guess I'll need to start looking at PS caps and such. Fortunately I've got a replacement waiting in the wings, guess I finally get to unbox it and press it into service. Never a dull moment.

<marv>
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defog



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PostPosted: Wed Jan 23, 2013 8:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for your help guys. I appreciate it. I'm going to work on it some more tonight and keep in mind what you've suggested.
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defog



Joined: Aug 24, 2011
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Location: Philadelphia

PostPosted: Thu Jan 31, 2013 9:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

I made a very silly rookie mistake and had two values swapped, and since I was making two of them, I did the same thing on both boards and it was overlooked. Now it's working properly. Thanks for all of your help once again. This layout is now confirmed working for anyone that's interested in building one of these, and with board mounted pots and switches it fits with a smaller sized knob okay as well.
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Tim Servo



Joined: Jul 16, 2006
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PostPosted: Tue Feb 05, 2013 11:51 am    Post subject: Magic Smoke 8k LFO pcb layout - debug Reply with quote  Mark this post and the followings unread

Hi Defog,

Can you tell us which values you had swapped? I've never had a problem with my 8K stalling, so I thought that was interesting. Please note that the part values I have are for +-15V supplies, so +-12V uses may need to change a few things. I might try a slightly lower value for R2 (adjusts the waveform offset to keep it centered around 0V) and R4. Glad you got it sorted though.

Tim (stalling) Servo
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defog



Joined: Aug 24, 2011
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PostPosted: Tue Feb 05, 2013 8:26 pm    Post subject: Re: Magic Smoke 8k LFO pcb layout - debug Reply with quote  Mark this post and the followings unread

Tim Servo wrote:
Hi Defog,

Can you tell us which values you had swapped? I've never had a problem with my 8K stalling, so I thought that was interesting. Please note that the part values I have are for +-15V supplies, so +-12V uses may need to change a few things. I might try a slightly lower value for R2 (adjusts the waveform offset to keep it centered around 0V) and R4. Glad you got it sorted though.

Tim (stalling) Servo


Hey Tim, I had the 5M resistor in the wrong spot and the 100k. It made perfect sense after I switched them that the comparator had nothing to compare with because the negative rail was never sitting where it was supposed to be (stuck negative). I didn't get to testing it into any of the CV sources yet (just watched it in the scope) because I didn't have the depth pot wired up yet, but it looked right there and the voltages were correct, so I think it should be okay with +-12v rails. I can always adjust a few values if the waveform sounds off. Thanks for your help.

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