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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Micro Sample & Hold issue...
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Mon Dec 31, 2012 12:01 am    Post subject: Micro Sample & Hold issue... Reply with quote  Mark this post and the followings unread

Building a SLMS+ and all the other parts work great, but the S&H not so much.

What I'm seeing is that the "sampling" cap (a .005uF poly cap) doesn't appear to hold its voltage much at all. Viewing it on the scope, I see it spike on the leading edge of the sample clock, and immediately start decay back to ground, taking less than half a clock cycle to get back there.

With no voltage in the circuit, I measure about 750K resistance 'across' the cap. Not sure if that's normal for Drain->Gate with no voltage on it or not, but I'd expect that is the only expected source for that resistance, since the other side of the cap connects to the (presumably very high impedance) input of the LF444.

I have re-washed the board, and looked for solder bridges and come up empty there.

I'm curious if anyone has any suggestion as to whether I'm more likely to have a bad 2N5457 or a bad cap (I have never yet had a problem soldering these poly caps, but there's always a first time to have damaged it with heat).
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elmegil



Joined: Mar 20, 2012
Posts: 1509
Location: Chicago
Audio files: 14

PostPosted: Mon Dec 31, 2012 9:43 am    Post subject: Reply with quote  Mark this post and the followings unread

Indeed it was the transistor; swapped it out as the first thing (seemed most likely) and I now have a working S&H Smile
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