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Old 18th April 2005, 12:49 AM   #1
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Unhappy Transformer connections

I new to building chip amps and electronic in general, so be gentle...

I completed my BrianGT LM3875 kit today, my first one. I followed the instructions to the letter, double-checking everything along the way.

I mounted eveything onto a slab of masonite, on standoffs, of course. As I didn't know how I wanted to configure the final power supply I was going to use, a bought a transformer for $10 from Radio Shack to make sure everything worked.

It's marked 12.6-0-12.6V, 2A on the secondaries and 120V 60HZ on the primaries, which seems ok based on what I read. There are three wires coming out of the secondary side - yellow-black-yellow. I hooked one yellow to each the AC1 holes on the board, the black wire to the PG+.

I should mention, I 'm using the "legacy" configuration, with the 8 diodes and two small capacitors.

When I hooked the power supply board to the transformer and switched it on, a little smoke came from D2 and D6, and by the time I switched it off, they'd getten so hot they'd melted their solder and were loose in the socket (everything tightened up as it cooled).

I'm not inclined to plug the thing back into the wall until I have some idea what I did wrong. Any ideas?
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Old 18th April 2005, 12:57 AM   #2
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Default Re: Transformer connections

Quote:
Originally posted by gcnewbie
It's marked 12.6-0-12.6V, 2A on the secondaries and 120V 60HZ on the primaries, which seems ok based on what I read. There are three wires coming out of the secondary side - yellow-black-yellow. I hooked one yellow to each the AC1 holes on the board, the black wire to the PG+.

I should mention, I 'm using the "legacy" configuration, with the 8 diodes and two small capacitors.
In the original BrianGT clone thread (and i expect the manual), it shows how to hook up the boards with a centre tapped trafo... with this kind of device you only use 4 diodes i believe.

If this trafo is anything like the 12.6V centreTapped ones you can go in and separate the windings -- we do this to get 2 6.3 V windings for filaments. If this is possible then you would have a dual secondary trafo and could hook it up with 8 diodes (after you replace the frided ones)

dave
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Old 18th April 2005, 01:01 AM   #3
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Default Re: Transformer connections

Quote:
Originally posted by gcnewbie
When I hooked the power supply board to the transformer and switched it on, a little smoke came from D2 and D6, and by the time I switched it off, they'd getten so hot they'd melted their solder and were loose in the socket (everything tightened up as it cooled).
Sorry for your loss.

Refer to Brian's LM3875 manual, page 15. Only stuff rectifier pcb with D1, D3, D6 and D8. Install jumpers where indicated in the diagram.
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Old 18th April 2005, 01:37 AM   #4
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Quote:
Originally posted by gcnewbie
Are those diodes toast now?
Maybe not all of them...when the smoke comes out you can be pretty sur eit is time for fresh ones... a smell quite to my dislike when that happens.

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Old 18th April 2005, 01:38 AM   #5
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Are those diodes toast now?
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Old 18th April 2005, 06:46 AM   #6
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Default Help me understand

OK - I should have used the jumper wires as indicated in Brian's manual, but what I don't understand is why that would have fried the diodes, especially since they would otherwise have been replaced with wire.

Also, is the transformer of the proper size and type?
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Old 18th April 2005, 06:54 AM   #7
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Default Re: Help me understand

Quote:
Originally posted by gcnewbie
Also, is the transformer of the proper size and type?
For testing... it will limit the amount of power & the quality of the amp since it is small with low voltage.

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