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The symptoms (black label board):
1 - All LEDs light up
2 - VREF = 3.8V
3 - Cannot adjust past 13.8V. but can adjust to -15V
4 - Shunt current = 5 ma (0.005V across shunt TP)for both polarities
5 - CCS current = 0V for both polarities
6 - QP1, QP2, QN1 & QN2 are cold. Looks like they are not on.
Any quick tips before I load pictures?
SMathews
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Double-check the transistor location. Then take a break and check them again :).
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Thanks Brian. I may have to load pictures. I checked all the transistors using a scope and they looked okay. I will look again. Any intermediate node voltages other than Vref that I can check?
SMathews
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Working at the office at the moment, so don't have any good info in front of me...
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Same here, Lunch break. I will load pictures when I get back home.
SMathews
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I believe I have to post before attaching pictures. Here goes. usmat attached the following image(s):  DSC_1430.jpg (74kb) downloaded 347 time(s). DSC_1431.jpg (68kb) downloaded 345 time(s). DSC_1432.jpg (58kb) downloaded 345 time(s). DSC_1433.jpg (58kb) downloaded 343 time(s). DSC_1434.jpg (53kb) downloaded 342 time(s). DSC_1435.jpg (50kb) downloaded 341 time(s). DSC_1436.jpg (52kb) downloaded 348 time(s). DSC_1437.jpg (61kb) downloaded 341 time(s).You cannot view/download attachments. Try to login or register.
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I had an almost identical problem the other day and it turned out to be one of the pins on a variable resistor wasn't soldered. Guess my eyes just skipped over it and it's a very small pin and pad.
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You do have some cold joints there. Try adding some flux and reheating.
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Thanks Tony, Brian, I will resolder.
SMathews
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I re-touched almost every solder point.
No go.
I then adjusted VR1 & VR2 (blacked out board) and then, voila current.
Was able to adjust for +/- 14.5V and 350 amps on both rails. QP2 and QN2 got hot, quite fast and stayed hot to touch. QN1 and QN2 got hot slowly, but not as hot as QP2 and QN2. Is that normal? I have no load.
Thanks again Tony, Brian, appreciate it.
My soldering sucks.
SMathews
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350 amps!!!! ..... 0.350 amps
SMathews
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Yes, the heat is completely normal. You are always dumping current, regardless of the load, thus the heat.
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So glad you got it sorted!
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Looks like I'm back to no current, after connecting the legato 3. I have a light bulb on the hot power line, that lit up after I connected the Legato, so that would mean a short some-where.
Disconnecting the Legato, I can adjust the Placed for voltage and not current. The current stays at 50 ma. Turning the pot does not change the current.
I guess I can used fixed resistors. I am aiming for 15V, 350 ma.
SMathews
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From the manual for 15V I need to set :
Rf = 1K, that is R18 and R17 in the black label boards
Rg = 750 ohms, htat is R15 and R16
VR2 needs to be found out.
Can someone suggest a fixed resistor value for VR2 to get 350 ma and 15V? I'm trying to avoid getting another VR2 pot.
Thanks
SMathews
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Update :
My trimmer pots seem to be okay. Here's whats happened so far
1) Previously working placid was attached to legato 3.1, which I presume had a short.
2) Disconnected Legato and in my haste connected the secondaries of transformers to the Placid for the -ve side in opposite.
3) Re-connected correctly.
4) Symptoms below
Symptoms :
1) All LEDs light up
2) Current stays a 50ma, and cannot be changed by adjusting VR2/VR1 (blacked out board). Shunt at 50 ma too.
3) But the current can be adjusted using VR3/VR4 to max 150 ma.
4) Light bulb connected to the supply hot lead is glowing very slightly and hence the secondary voltage does not go above 12 to 13 Volts.
5) Vref is okay - 3.8V
6) QP2 and QN2 are very slightly warm.
Any ideas?
SMathews
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Quote:connected the secondaries of transformers to the Placid for the -ve side in opposite. Can you explain what you mean by this?
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The 4 secondary transformer wires black, red, orange & yellow. I mistakenly connected yellow to SA2A and orange to SA2B. I had black on SA1A and red on SA1B.
SMathews
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Ok, that's what I thought you meant. It should have not effect whatsoever on the Placid.
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Brian, thanks. If you have any voltages I can check, that would be great.
SMathews
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