Problems with SFT clone

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dune2k
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Post by dune2k »

Hi,
I recently built an SFT clone according to the (stripboard) layout by Mike Livesly. After putting everything in the enclosure I toyed around a bit with it and also used a power supply to power it.
After some time it made *boom* and the 100uF electrolytic blew up (probably a bad electrolytic or some power spike, I don't know...)...I changed it and so far it works BUT I feel it has less gain than before and also the higher gain settings sound different. More synthlike fuzz than the one on the catalinbread videos...

Any ideas what might be damaged?
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roseblood11
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Post by roseblood11 »

any polarized parts could be damaged, esp.the electrolytics...

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nhavens
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Post by nhavens »

I just finished my SFT clone using the madbean board/layout. It seems to work great except one problem. Every time I plug in an 18v adapter, it makes the magic smoke from the 48ohm resistor (R24 in his layout). It works fine with a battery or a 9V power supply. The caps are all Topmay low voltage polyfilm box, the radials are all 25v and I used 1/4 watt carbon film resistors. I'm a knewb so i know it may be a stupid question but does any of the parts stand out as the reason why. Is it that I used carbon film vs metal for the resistors?

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DrNomis
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Post by DrNomis »

nhavens wrote:I just finished my SFT clone using the madbean board/layout. It seems to work great except one problem. Every time I plug in an 18v adapter, it makes the magic smoke from the 48ohm resistor (R24 in his layout). It works fine with a battery or a 9V power supply. The caps are all Topmay low voltage polyfilm box, the radials are all 25v and I used 1/4 watt carbon film resistors. I'm a knewb so i know it may be a stupid question but does any of the parts stand out as the reason why. Is it that I used carbon film vs metal for the resistors?

The type of resistor shouldn't make much difference at all,I'm guessing that the 25V electrolytic capacitors need to be replaced with 35V types,since the supply voltage could be a bit too high for them,causing them to short-out.... :)
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roseblood11
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Post by roseblood11 »

Are you sure that it is a 18V DC power supply?
And does it have the correct polarity (neg is center!)?

As the polarity protection diode is placed after R24, the resistor acts as some kind of fuse if s.th is wrong with the supply voltage. Maybe this was intended, maybe not.
I'm guessing that the 25V electrolytic capacitors need to be replaced with 35V types,since the supply voltage could be a bit too high for them,causing them to short-out....
Why should 18V supply voltage be "too high" for a 25V type cap? :scratch:

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Hides-His-Eyes
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Post by Hides-His-Eyes »

Maybe your 18V power supply has the wrong polarity.
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nhavens
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Post by nhavens »

I wouldn't think so, It's just an 18v wall wart I got from small bear. http://www.smallbearelec.com/Detail.bok?no=1007.

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nhavens
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Post by nhavens »

Ok, found something weired. Since the power supply does not have a polarity labeled, I did the ole LED/resistor trick. The LED is lighting up whether the the negative or positive lead is placed on center.

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nhavens
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Post by nhavens »

Sun of a biscuit. I just realized that it is a 18vAC not DC. Since it was right under the 9vDC and 12vDC power supplies on the small bear site, I didn't even pay attention.

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