Rank: Member
Groups: Member
Joined: 4/21/2007(UTC)
Posts: 59
Location: Minnesota
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I hate to bother the forum, but I haven't seen this problem reported. I can't quite figure what is wrong with a new Placid HD BP version 2.1. I just built 2 Placid HD BP supplies - one works great. On the one in question, the plus half also works 100% as expected but the minus side doesn't adjust. I am always very careful with part placement and good solder flow. 15v 25VA transformer in R8 = 220 ohm Example: Trim pots measured in place with caps drained Vr1A 40 ohms Vr1B 40 ohms Vr2A 1330 ohms Vr2B 1330 ohms Then powered up and measured with no load: Vref+ 2.47v Vref- 2.50v CCS+ 0.022v CCS- 1.000v Shunt+ 17ma Shunt- 988ma Vout+ 4.14v Vout- 12.18v No adjustment to either trim pot on the minus side causes any change in the CCS, Shunt current, or voltage out. It is always 12.2 volts with almost 1 amp shunted. The trim pots themselves appear to be functioning normally. ESD not usually an issue this time of year, and Vref- always measures 2.50v. Both minus side heat sinks are warm - not alarmingly hot. With brief load, the CCS and Shunt 1 ohm resistors complained, but survived. All ideas or suggestions much appreciated. Many thanks in advance! Edited by user Saturday, May 30, 2015 10:54:42 PM(UTC)
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Rank: Member
Groups: Member
Joined: 4/21/2007(UTC)
Posts: 59
Location: Minnesota
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Problem identified: Both sets of Q3-Q5 transistors were the same polarity - BC560 in this case. The negative side should have BC550B in positions QN3-QN5. Perhaps the above information can serve as a data point for diagnosis should anybody else have a similar behavior problem. The take-home message is: confirm part identity before soldering! :-) Edited by user Sunday, May 31, 2015 6:07:36 AM(UTC)
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