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For one half of the Placid, say the +ve half :
Would it be possible to isolate QN2/QP2 from the circuit by removing R23, R19 and R3? Thanks.
SMathews
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Update.
Working with one half of the supply, +ve half. I disconnected R1 (1 ohm resistor).
Symptoms
1 - The light bulb in the hot line stopped glowing. With R1 connected, there was a faint glow, which I think is a small resistance to ground.
2 - The output of the secondary reads about 18V AC, when before the R1 disconnect the output did not go above 12V AC.
My Conclusion
1 - This points to a problem in the CCS or/and Shunt transistors i.e. QN1, QN2, QP3 and QP4 (blacked out board).
I would like to reconnect R1 and disconnect R3. R3 connects the shunt section. This will isolate either a CCS transistors (QN1 & QP3) or Shunt transistors (QN2 & QP3).
Would disconnecting R3 put the burden of sinking all the current onto the CCS transistors?
Simmonds
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Removing R3 would not allow any current to be shunted. I am not sure I would do that. the transistors are relatively cheap. Why not just replace each of them?
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Thanks Russ. I was hoping to save effort. I will replace all. What kind of nominal impedance does the Legato 3.1 present to the Placid HD BP?
SMathews
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Got all the Transistors from Mouser. I thought I'd replace one transistor at a time to figure out the offending device.
Starting with just the negative half of the supply and concentrating on the transistors for CCS and Shunt i.e. QN3, QP1, QN4 & QP4 (for blacked out board).
I first changed QN3.
- The LED's (all 3) come on.
- Adjusting VR2 to go up from 0 ma, I get briefly (> 1sec) 120 ma, but settle at 90 ma and I cannot adjust for higher current.
- The shunt current tracks the CCS current and settles at 90 ma.
- What I see, is when increasing the current, the short circuit protection light bulb I have, on the hot power line to the power transformer is not lit. It gets faintly lit when the CCS current reaches the 120 - 90 amp point.
I am planning on changing QP1 next.
SMathews
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Changed out all the CCS and Shunt transistors. Same result.
Plan to measure voltages.
SMathews
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Try it without the lightbulb
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Brian, thank you, appreciate it. Removed the bulb, everything measured correctly. While pondering on what happened, I connected the Legato 3, and trimmed all the Legato 3 voltages to 0V offset. Looks good. I had checked out the Legato on a Lab supply, while debugging the Placed. :d/
Now off to attack the Buffalo.
SMathews
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