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Old 2nd January 2007, 04:32 PM   #1
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Default Familiar Sonic Impact Failure Mode?

Hi All,

I've got a modified SI board that was working before I reconfigured it, and now it shows considerable offset on the speaker terminals. I get 9.15v and 8.96v on them, so something is obviously not right. I also read about 1.5v between neg speaker terminals and board ground, and about 10.6v between pos terminals and ground. DCR between the neg terminal and ground is about 140 ohms with the voltmeter neg probe on the terminal and the pos on the ground plane. Open circuit the other polarity.

Mods include the usual stealth, C10, additional low ESR power cap, new larger Dale bobbin inductors and screw terminals for speakers and power in/out. I've done it all before with fine results, although these are inductors I've not used before (Dale IHD-3). SMPS puts out just over 12v.

I've been over both sides of the board with a magnifying glass, and can't find a break or a bridge or any other obvious flaw (doesn't mean there isn't one). When power is connected, chip generates a small amount of heat, as when idling. FWIW, the LED is not connnected right now, but I can't imagine that would cause this problem.

Any suggestions as to what to check next would be most welcome.

--Buckapound
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Old 2nd January 2007, 08:00 PM   #2
Pano is offline Pano  United States
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That sort of DC offset sounds like you have a path to ground on the inputs. That would be AFTER the input cap. Any path to ground, even 1M ohm will pull down the 2.5V bias and cause a DC offset at the other end.

That's the 1st place to look.
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Old 2nd January 2007, 08:30 PM   #3
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Thanks, Panomaniac, very helpful.

Both inputs (post-cap) read just shy of 3 ohms to ground plane with the volume all the way to the left, so there's a problem for sure. I've got one of those counterfeit detented Alps pots on there, don't know if it's maybe shorting to ground. I'll try a different pot and see if there's a problem.

--Buckapound
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Old 2nd January 2007, 10:21 PM   #4
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Check your wiring. Even if your faux Alps is shorted, the short should be on the upstream side of the input caps. Know what I mean?

Your worry is on the chip side of the input caps.
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Old 2nd January 2007, 10:57 PM   #5
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Yes, I think you're right. I'll tear everything up tonight and redo. Thanks for your help.

--Buckapound
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Old 3rd January 2007, 08:07 PM   #6
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Well, of course I had everything screwed up on the input side. There's still something wrong with one channel (Plays very feebly) but one works and the chip seems alive at least.

Thought you might enjoy a look at the unique Jetson's-style case:
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Old 3rd January 2007, 08:11 PM   #7
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And an inside view:
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Old 3rd January 2007, 08:40 PM   #8
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Very Cool!

Nice work, I'd like to add it to my T-Amp Gallery.

BTW, where did you get the feet?
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Old 3rd January 2007, 08:58 PM   #9
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Thanks.

Everything but the nuts and bolts was fabricated in my local adult ed high school welding class. The legs, switch parts on top and knob were all turned on a lathe. Front back and side members are milled aluminum; top and bottom are stainless sheet.

Chassis inside is modelmaker's styrene (or maybe ABS). Easy to cut and work, takes paint well, glues up solidly with superglue. Comes in thicknesses up to about 0.1." You can fabricate quite complex parts with it.

Hard to tell from the crummy photo, but there is a lot of cleaning up to do on the metalwork. Once I get it looking better, and i've taken some proper photos, I'll send you a few. Let me know if you want the temporary ones in the mean time.

The one bad channel turned out to be a funky jack on an iPod dock. Playin' real nice now. Thanks again for your help.

--Buckapound
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