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Mongo1
Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC
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Posted: Wed Sep 07, 2011 7:14 am Post subject:
Smoke testing the CGS VCO Subject description: A narrow escape.... |
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Well, a few weeks ago I did something monumentally stupid.
I was doing a little bench work, and wanted to have my CGS VCO laying flat on the bench to do a little probing around. My power cable wouldn't reach, so I elected to use some jumper clips.
I looked at the board's power connector, and saw the little silk screen by the power connector that says +15V, and assumed that that was the 15V pin. Well, its not. It's the -15V pin. Oops!
There was no smoke or anything, but the power supply was clearly not happy (I have little leds for each power rail, and one of them went out). So I swapped leads, and found the VCO dead.
The chips on that board weren't socketed (stupid Mongo!) and after looking at the vast tangle of wires leading to the board, I decided that there were several things I needed to do that didn't involve that VCO.
And so it's been sitting on a shelf for weeks.
Last night I decided to gather my courage and fix it. I plugged it back in, and found that it actually did kinda sorta work a little bit. There was one narrow range on the coarse tuning knob that resulted in oscillation. It wasn't the kind of oscillation you'd normally like to hear - sort of the sound a baby calf makes when its looking for it's momma. But at least it was something.
After a little poking around with the scope, I determined that a good bit of the board was actually working pretty well, and nothing really obvious jumped out. So I did the hard part - I started clipping chips out and replacing them. The cheapest were the TL072s, so I did those first - no cigar.
The only chips left that could be responsible were the CA3140, the LM311 and of course the supermatched transistors. Of those, the only chip I had in stock was the 3140 (actually mouser sold me an NTE replacement for those) so I tried it. And you'd never guess - it worked like a charm.
So the little sick calf is now a mighty bull once again.
I guess the points of this post are
1) Never be lazy or impatient - wait for the sockets
2) Don't assume anything.
3) Ken Stone designs some pretty solid stuff - thanks Ken!
Gary Last edited by Mongo1 on Wed Sep 07, 2011 9:51 am; edited 1 time in total |
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andrewF
Joined: Dec 29, 2006 Posts: 1176 Location: australia
Audio files: 4
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Posted: Wed Sep 07, 2011 9:36 am Post subject:
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Well, i'm pretty curious, where does the 3080 go?
My CGS VCOs don't have any 3080s....do you mean the 3140? |
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Mongo1
Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC
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Posted: Wed Sep 07, 2011 9:45 am Post subject:
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Quote: | My CGS VCOs don't have any 3080s....do you mean the 3140? |
D'oh!
Yep - I was just reading another thread about 3080s, and had a senior moment.
What the heck, I was only 60 off....
Thanks for catching that....
Gary |
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djg
Joined: Nov 10, 2010 Posts: 12 Location: New York
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Posted: Thu Sep 08, 2011 8:58 am Post subject:
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Yeah, the only time I've pulled a reversal was on my first CGS VCO a few weeks after I built it doing some calibration with the bench supply. Smoke and all. I remember a few of the outputs still working. I had spares of all ICs except the LM311 and LM3900, which I was able to get at my local electronics store for cheap. I replaced all the electrolytics just to be safe too. Gave me hope for any future disasters. |
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djg
Joined: Nov 10, 2010 Posts: 12 Location: New York
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Posted: Sat Sep 10, 2011 10:24 pm Post subject:
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Well this is ironic. Did the same goddamn thing on a different CGS vco just now after a long day of general maintenance. Long story short-- too many beers, too late at night, and that same +15v pad that must've thrown me below the normal header's -V. Oh well, live and learn (and forget and relearn). |
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Mongo1
Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC
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Posted: Sun Sep 11, 2011 6:26 am Post subject:
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Quote: | Well this is ironic. Did the same goddamn thing on a different CGS vco just now after a long day of general maintenance. Long story short-- too many beers, too late at night, and that same +15v pad that must've thrown me below the normal header's -V. Oh well, live and learn (and forget and relearn). |
Well, thanks for making me feel better! Maybe we oughta ask Ken to change that damned silkscreen!
Gary |
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macumbista
Joined: Sep 12, 2007 Posts: 398 Location: berlin
Audio files: 3
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Posted: Tue Jul 17, 2012 10:58 am Post subject:
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I *almost* did this yesterday. I didn't have any ferrite beads when I was building, so I used 10 Ohm resistors. The benefit was, when I plugged the power backwards, the resistors burned and not the ICs!!! I've never noticed any noise difference with or without ferrite beads, but I did just discover a good reason to use a resistor instead of a link. A small tip, I suppose... _________________ Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
macumbista.net |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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macumbista
Joined: Sep 12, 2007 Posts: 398 Location: berlin
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Posted: Tue Jul 17, 2012 11:47 am Post subject:
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Well, this happened in my professionally designed Doepfer without me having to do anything except plug a minijack cable in halfway:
Give me bananas every time! _________________ Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
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Mongo1
Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC
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Posted: Tue Jul 17, 2012 9:01 pm Post subject:
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Holy merd!
That's pretty amazing. Did you let doepfer know about that? I can't imagine any engineer worth a damn not wanting to fix that problem.
What kind of chip was that when it was still, you know, NOT glass?
Gary |
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elmegil
Joined: Mar 20, 2012 Posts: 2177 Location: Chicago
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Posted: Tue Jul 17, 2012 9:31 pm Post subject:
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Looks like an LM324.... |
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JingleJoe
Joined: Nov 10, 2011 Posts: 878 Location: Lancashire, England
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Posted: Wed Jul 18, 2012 1:59 pm Post subject:
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I burnt out an LM324 recently aswell, but nothing that spectacular happened, it just got very very hot and gave me another burn to go along with all the other soldering iron burns
I keep a stock of 16pin DIL sockets which I cut down to size when needed for 8 or 14 pin applications. A bit ramshackle but it saved me a lot buying in bulk _________________ As a mad scientist I am ruled by the dictum of science: "I could be wrong about this but lets find out"
Green Dungeon Alchemist Laboratories |
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macumbista
Joined: Sep 12, 2007 Posts: 398 Location: berlin
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Posted: Wed Jul 18, 2012 2:06 pm Post subject:
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macumbista wrote: | I *almost* did this yesterday. I didn't have any ferrite beads when I was building, so I used 10 Ohm resistors. The benefit was, when I plugged the power backwards, the resistors burned and not the ICs!!! I've never noticed any noise difference with or without ferrite beads, but I did just discover a good reason to use a resistor instead of a link. A small tip, I suppose... |
Veering back on topic... I just discovered the downside to using 10 Ohm resistors instead of links or ferrites. CV applied to the VC Shape affects the Square and Saw outputs as well. Ken helped me work that one out, going back to ferrites now. _________________ Esoteric drones and nonlinear distortion
Custom/handmade experimental instruments
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