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I am building Placid HDBP for my Legato 3.1, the problem is DC output always at ~12VDC and could not change output voltage by VR2.
After checked, I found that all LED did not light up and Vref become 20V (I use 17VAC transformer). I swapped direction of all LED, now they are all lighting up, Vref ~ 3.88V. However, I only can adjust VR2 to get output voltage from 5V to 12V, could not get 15V. Please help.
Thank you very much.
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Check the orientation of D1. Also, check for transistors in the right spots.
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Thank you Brian, D1 is orientation is correct. I could adjust voltage to reach 15V by changing CCS current with VR1.
But when I connect it to my Legato 3.1, voltage on both rail dropped to ~4V. Look like I am facing voltage sag issue when I only can increase voltage by increase CCS current to more than 400mA both rail. I wonder whether it will damage Legato/Placid HDBP or not because the manual said that Legato need 350mA only and should start at 50-60mA.
Could you please advise me what to do now?
Thank you.
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Leave the Legato disconnected for now.
This sounds like a misplaced transistor issue (common problem). Also, can you describe how you have your transformer connected?
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Hi Brian, I disconnected Placid from Legato, recheck transistors and found they are correct QNs are BC550C and QPs are BC560B. I use 02 transformer, each has 17VAC secondary go to each co-responding terminal input of Placid HD BP (One go to S1A and S1B, the other go to S2A and S2B) bitinhvi attached the following image(s):  Wiring.JPG (70kb) downloaded 24 time(s). Placid HDBP.JPG (106kb) downloaded 43 time(s).You cannot view/download attachments. Try to login or register.
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Also check the TO-220 transistors. There isn't much else that can be wrong. I assume caps are all in the correct way...
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I checked 04 TO-220 transistors and found that they are correct. Capacitors are fine because there are only 02 big bipolar capacitors on board. Pls note that I put LEDs at the bottom of board and they are lighting well. I resolder some parts to eliminate cool soldering issue and tried connect again to Legato with CCS 350mA and 13V (no load), Voltage drop become like this: - Negative rail: ~11V - Positive rail: ~4.5V Heat sink are a bit warm so I think transistors are working. Is there any thing need to check on Legato 3.1? Edited by user Monday, September 10, 2012 4:38:06 PM(UTC)
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Can you post pictures of the top and bottom of the board?
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Hi Brian, you can see the top of board on above post. I am attaching bottom of its and Legato bitinhvi attached the following image(s): IMG_9668.JPG (214kb) downloaded 42 time(s). IMG_9672.JPG (221kb) downloaded 32 time(s). IMG_9673.JPG (184kb) downloaded 26 time(s).You cannot view/download attachments. Try to login or register.
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Hi, today I tried build Placid HD BP again from new (I ordered 02 set), but I got same problem when connect to Legato. Voltage on both rail dropped significantly from 12V to 3V. Heatsinks on positive rail is a bit hot while negative is not. Any help is highly appreciated. Thank you Edited by user Tuesday, September 11, 2012 4:22:27 PM(UTC)
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Have you adjusted the shunted current ?? Sounds like you're not shunting enough current.
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Hi avr300,
I think so, however I do not know how to adjust shunted current. I started with 288mA when no load (measure between TP_VIN and TP_CCS), tried adjust VR1 but it change very slow and voltage does not increase much.
Could you pls teach me where to measure and should adjust to how many mA which is suitable for Legato with SE mode?
Thank you.
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I'll try. - Power down the rig. - Disconnect the load, here the Legato. - Adjust the voltage with VR3. When you have reached the wanted voltage don't touch VR3 any longer. (- Adjust VR1 measuring between TP_VIN and TP_CSS, the 288mA is ok for the time now.) - Power down the rig. - Connect the load. - Fire it up again, this time measure between TP_SHUNT and TP_OUT. Tell us your reading. Edited by user Tuesday, September 11, 2012 4:57:29 PM(UTC)
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Hi avr300,
1. No load: +-14.5V, 298mV between VIN and CCS
2. With load
+ postive: 17.1mV (TPSHUNT-OUT)/ 4.1V out
+ negative: 4-6mV (fluctuated) (TPSHUNT-OUT)/-10.1V
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Hi avr300, I follow exactly steps you mentioned, this time heatsinks on both rail are very hot.
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ok, I did miss that you're are running at 14.5v. Try adjusting to 350mA, and check again. The heat sinks are designed to get hot. Alternatively, drop the voltage to +-12v and the current requirements and the heat drops too. Edited by user Tuesday, September 11, 2012 5:07:26 PM(UTC)
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Hi avr300, this time, result are: 1. No load: +-14.5V, 350mV between VIN and CCS - I want to run Legato at 15V as recommend from Russ 2. With load + postive: 17.1mV (TPSHUNT-OUT)/ 4.5V out - CCS become 359mV + negative: 0mV (TPSHUNT-OUT)/-11.1V out Edited by user Tuesday, September 11, 2012 5:17:42 PM(UTC)
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Just to check for any problem with the Legato itself, temporary drop the voltage to +-12V and see if the adjusted current is enough @+-12v
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Meanwhile I checked the board yesterday, I got identically result with today's board.
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If you doesn't shunt enough current the output voltage will sag. That's the nature of a shunt supply.
The higher voltage the higher demand for current. That's why I suggest to drop to +-12v, do the test with 350mA, is the rails stable now. If so, and the rig is producing sound, the rig is healthy.
Increasing the voltage is straight forward - as long as you increase the current as well. But we have to make sure that the load is a load and not a short.
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