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Old 6th April 2006, 08:43 AM   #1
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Question Need Help With a True center tap Transformer

Hi All,

A newbie here. Could some one please explain to me how to wire up a true center tap transformer with 3 wires V+, 0 & V- on Brianís version 3 power supply board found here http://www.chipamp.com/lm3875rev3.pdf. I purchased two Toroid transformers from Apex Jr., 120v 2x25 here is the site page. http://www.apexjr.com/miscellaneous.html#Toroids For testing purposes, I only wired up 1 transformer. I followed Steveís instructions from Apex Jr. (he has a loyal customer for life, great PR) regarding the center tap that gave me about 26v ac per secondary. I solder V+ red to outside AC1, 0 black center tap to AC1 inside and AC2 inside, V- orange to outside AC2. With AC power applied to power supply board I get normal rectification around +/- 35 per rail. All seems to work as it should , After I soldered the amp boards to the power board, I get beautiful music but the transformer hums slightly as soon as I plug it in, gets hot and begins to smoke.
Any help would be appreciated.

Thanks
Jim
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Old 6th April 2006, 10:34 AM   #2
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hi Jim,

Got a picture? How did you mount the toriod?

regards
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Old 6th April 2006, 11:13 AM   #3
AndrewT is offline AndrewT  Scotland
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Hi,
could you post a PSU schematic.

I think the dual rectifier version needs dual secondaries.

I think the solution you will need is to reconfigure the diodes to allow centre tapped to work correctly.

But I need the schematic.
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Old 6th April 2006, 01:24 PM   #4
eddog is offline eddog  United States
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I made the same mistake you did. Heres what you need to do.
Only four diodes should be mounted to the rev 3 power supply board, these are D8,D4,D5 and D1. Run a jumper from D6(outside hole) to D7(outside hole). Then run a jumperfrom D2(outside hole) to D3(outside hole). Also run a jumper from D2(inside hole) to D3(inside hole). Your centertapped transformer connects as such(on my working amp anyway), Your centertap wire goes to PG-(yes that is correct), your V- goes to AC2(inside) and your V= goes to AC2(outside). You now have a working rev 3 amp. Hope your X-former didnt get ruined. If smoke came out of it I have a bad feeling about that.
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Old 6th April 2006, 06:48 PM   #5
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Default Need Help With a True center tap Transformer

Thanks Everyone
Eddog,

After posting, and before your reply, I tried to follow Brianís version 1 power board as it relates to true center tap but only got +20vcd and -15vcd per rail. I did not connect the center tap wire to PG- because I thought that had to be a mistake. Thank you all very much for the quick responses I will try it tonight and let you know if I damaged 1 of my transformers.

Thanks
Jim
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Old 6th April 2006, 06:48 PM   #6
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Default Need Help With a True center tap Transformer

Thanks Andrew,

Here is Brian's version 3 board(nice little kit) at
http://www.chipamp.com/lm3875rev3.pdf.
Brian also has a users guide for the 3875 but for a previous version.

Thanks
Jim
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Old 6th April 2006, 08:37 PM   #7
eddog is offline eddog  United States
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Good luck Jim, I was lucky enough to have a variable X-former to gradually bring the AC voltage up with. So I noticed the x-former getting way to hot for its own good. Only then did I go back and look at Brians directions and asked around here. I was informed that the way I(and you) had hooked up our x-formers cuases a short across the former each half-cycle, creating tons of heat!!
Funny thing is it's totally clear to me now why it must be hooked up the way Brian has it shown.
Live and learn
If I can be of further help just ask!

Mark
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Old 6th April 2006, 09:02 PM   #8
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Default Need Help With a True center tap Transformer

Thanks again Eddog.

I cant wait to test it out. Might even leave work early to do so.

Thanks Again
Jim
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Old 7th April 2006, 06:44 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi,
and remember to put a light bulb in series with the mains side of that modified circuit.
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Old 8th April 2006, 01:56 AM   #10
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Default Need Help With a True center tap Transformer

Thanks guys, I followed Eddogs instructions and it worked like a charm. My transformers out side cover, thou a little crispy, seem to have been unaffected (at least for now). Rail voltages were slightly off at first but I think I know why now. After approx 15 min of playing time I unplugged the amp and soldered the amps board ground lead to chasse ground (this is where it gets interesting). When I plugged the amp back in that little 10uf cap on the AC2 side of the power supply board began a very impressive stream of fire and smoke that was no match for Brianís thin fiberglass board or the solder which held it in place. Following the caps own internal countdown it blasted off with such force even NASSA would have been impressed. I was laughing so hard it must have taken me at least 15 minutes to pull the cap out of the ceiling tile in my garage. The funny thing was that little amp didnít even skip a beat and just keep playing during that whole ordeal.

Now my Question is, was I supposed to leave the 10uf caps out of Brianís modified version board?


P.S Thinking about taking up a different hobby

Thanks
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