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Old 3rd March 2013, 03:20 AM   #1
tadakan is offline tadakan  United States
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Default trouble with BrianGT chipamp lm3886

I had my dual mono pcbs fulled loaded with components and tested with light bulb attenuator a month or so ago and then I got swamped with work.

I've since scavenged some heatsinks and made a breadboard with a back to temporarily house everything.

I got the four pcbs screwed down and wired. Plugged it in and blew the fuse. Probably should have used the light bulb but they worked when I put it away.

Tonight I've taken it all apart, and without the amp boards connected the psus check out. 20v secondaries, 26v rectified. Blue LEDs light up. With the amp boards connected, the light bulb attenuator glows bright with 60w lamp in it.

I can't figure out where the amp boards are shorted. I've even removed one of them from the heatsink (although the heatsink is not grounded since it's a wooden case.)

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I'm posting pictures of one of the amp boards. I can post pictures of the psus if I need to, but they seem to be checking out okay.

Any thoughts appreciated.
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Old 3rd March 2013, 03:56 AM   #2
DUG is offline DUG  Canada
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Red for -Vcc

Black for +Vcc

?
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Old 3rd March 2013, 04:00 AM   #3
tadakan is offline tadakan  United States
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Yeah, that probably looks backwards. The connections are correct though. Just less than awesome color choices.
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Old 3rd March 2013, 08:23 AM   #4
tadakan is offline tadakan  United States
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Also, when the amplifier board is connected to psu the voltage measure from v+ to pg+ is only about 1.5v if that helps narrow it down. I can't remember what it was like when the amp was working so I don't know if that's normal, but I'm pretty sure not. The power indicator led on the PSU board does turn on briefly and then drops down to a very low glow with the amp board connected.
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Old 3rd March 2013, 12:38 PM   #5
wwenze is offline wwenze  Singapore
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All boards are shorted?
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Old 3rd March 2013, 12:52 PM   #6
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Probably you didn't insulate the chips from the heatsinks, or poorly.
Are heatsinks grounded or touching the casing?

Last edited by bobodioulasso; 3rd March 2013 at 12:54 PM.
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Old 3rd March 2013, 04:31 PM   #7
bcmbob is offline bcmbob  United States
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"Dual Mono" - does that mean you have two PS boards or are you powering two amp modules from one power supply?
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Old 3rd March 2013, 08:53 PM   #8
tadakan is offline tadakan  United States
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I'm assuming that something is shorted because after I blew a fuse plugging it in, I started using a "light bulb attenuator" to power it up. The light bulb glows brightly. Both amplifier pcbs have the same condition so far as I can tell. Both PSU pcbs seem to be fine (I can power them up, the light bulb attenuator does not stay lit, the power on led in the circuit glows brightly and the rectified DC voltage measures 26v which sounds right for 20v from the xformer secondaries.)

As I said, the chasis is a temporary setup made of plywood, so the heatsinks can't be grounding significantly to the chasis. Also, I removed one of the heatsinks to test that condition and still see a short as indicated by the light bulb attenuator.

Yes, two amp PCBs, to PSU PCBs and two transformers.
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Old 3rd March 2013, 09:21 PM   #9
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Should it be possible that + and - have been inverted?
I ask because i see a red wire for minus, which is an uncommon choice.
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Old 3rd March 2013, 09:44 PM   #10
DUG is offline DUG  Canada
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bobodioulasso .

see post #2, #3
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