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Old 12th June 2011, 06:05 PM   #1
tubeing is offline tubeing  Italy
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Default PSU Problem with Placid BP and BII

Hi to everyone,
I've got my boards from TP some months ago, but I had some time to work on them only last week. Today I set up Placid and Placid BP as explained in the manual and all was ok. I connected +5V from Placid to the BII board and everything was ok. Then I connected +/-12 from Placid BP to Legato board and...something went wrong. -12 remains at -12V while +12 went down to about 3V. I switched off immediatly the power but it was too late. Now my positive rail (without load) can reach only 11V max, and I can't see any voltage drop across R2. I measured VR2 and found about 4Mohm , so probably VR2 has gone...Maybe also Q1...I've checked the legato board and there are no shorts between +V and GND...Any suggestion to make things alive again will be appreciated.
Thanks
Stefano
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Old 13th June 2011, 12:12 PM   #2
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Placid and Placid BP are shunt regulated supplies, so a dead short at output should not be fatal because the current is limited.

Do all of the LEDs light correctly on the Placid BP?

My suggestion would to isolate and test the placid BP on its own. Try simply replacing VR2 first. VR2 might have been damaged by adjusting too far past the stops.
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Old 13th June 2011, 04:06 PM   #3
tubeing is offline tubeing  Italy
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Thanks Russ.
The leds are working, I have a new trimmer to replace the old one.
Now, if the placid will work well with the new component, I need to find out the problem that caused this situation...legato?
Thanks again.
Stefano
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Old 14th June 2011, 01:48 PM   #4
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Most likely just a damaged pot.
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Old 18th June 2011, 01:53 PM   #5
tubeing is offline tubeing  Italy
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Hi Russ,
I've changed the trimmer but with the new one I can only reach about 150mA through R2. Even with VR2 set to the min...I think that also QP1 has been damaged...and I don't find anything else in the board or in the legato board that can give me some clue on what it's going on.
Thanks.
Stefano
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Old 18th June 2011, 02:32 PM   #6
tubeing is offline tubeing  Italy
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Some news (bad news): after about 20 min after power on (Irs = 150mA), the Vout went to about 20V (the rectified voltage after the bridge)...with Placid BP disconnected from everything. QP1 is cold, QN3 is very hot, D5 is "off" while D1 e D2 "on". I think I have a complete set of problems...not only one Then, I have to set VR2 near the max to allow Vout to go down to 12V. I this case, both VR2 and VR4 set the Vout. When I set Ir2 to less than about 20mA D5 starts to emit light again but Vout is about 18.4V. Any suggestion?
Thanks
Stefano

Last edited by tubeing; 18th June 2011 at 02:45 PM.
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