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Scott Frankland  
#1 Posted : Saturday, March 21, 2020 8:41:22 PM(UTC)
Scott Frankland

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A friend of mine asked me to troubleshoot his Placid HD BP 2.1. The negative side was latched up at about 5V and could not be adjusted up or down. The positive side could be adjusted and was set to 15V output. I verified that there was 20VDC at the input of both sides, and that all four references were operating at 2.5V.

I was able to adjust the current in both sides, which I set to 280mA at R1A & R1B. I also tried connecting dummy load resistors, but this did not change things.

Next I checked all the component values, and diode tested the transistors. I found one part in each side that was the wrong value. R8A & R8B were both 220 ohms. According to the schematic these should be 470 ohms.

The builder assured me that he took the resistors out of the correct bag, marked 470 ohms. In any case, I removed both 220 ohm resistors and replaced them with 470 ohms. Now both sides are latched up at about 5V output! I checked for oscillations but there were none. I adjusted the current up and down but this did not change things. Both outputs are now stuck at 5V.

I next checked the +/- input voltage at IC1 & IC2. The 2.5V reference voltage is present on each IC+ terminal. I'm able to adjust VR2A & VR2B to get 2.5V on each IC- terminal, but the output voltage remains latched up at 5V. Even if I vary the IC- voltage from 2V to 3V the IC remains latched up. This occurs whether I bring up the supply on a variac, or whether I hard start it from 120VAC.

Has anyone seen this behavior before in a Placid 2.1?
Brian Donegan  
#2 Posted : Tuesday, March 24, 2020 12:49:44 PM(UTC)
Brian Donegan

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I need to dig out my notes on the older Placid. I'll try to get back to you later today.
Scott Frankland  
#3 Posted : Tuesday, March 24, 2020 8:29:56 PM(UTC)
Scott Frankland

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Thanks, Brian.

I was able to get the positive side to come out of latch-up by replacing R8A with a trimpot and dialing it down from 470 to 275 ohms. After that I was able to complete all the settings: Vo = 15V, IC1 = 2.5V, total current = 285mA. The set-up is for a Mercury board.

The negative side is still not cooperating. In order to break out of latch-up I had to set R8B to 332 ohms, and R7B to 19.7 ohms. Unfortunately I'm still not able to adjust Vo with VR2B. I can adjust the feedback voltage and dial it in to -2.5V, but there is no change in Vo. Vo can only be adjusted by changing R7B, which is far from ideal. I'm going to recheck the component values on this board.
Scott Frankland  
#4 Posted : Wednesday, March 25, 2020 12:07:33 AM(UTC)
Scott Frankland

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Some more findings.

I was able to get the negative side to come out of latch up by carefully reiterating VR2B & R8B. I left R7B alone this time.

I found I could set Vo by maintaining the feedback voltage at or near -2.5V, and then adjusting R8B to get Vo = 15V. The problem is that adjusting R8B causes a change in the feedback voltage. So reiteration is necessary.

The negative regulator is still not as rock solid as the positive regulator. After a dropout (or after a cooling period) Vo comes right back to 15V in the positive reg. The negative reg overshoots Vo and then gradually drifts down to nominal. But it's never exactly right. It's a bit off one way or the other.
Scott Frankland  
#5 Posted : Friday, March 27, 2020 10:24:11 PM(UTC)
Scott Frankland

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It looks to be a problem with the OPA209 control IC. When I balance everything out, the Placid regulates AC line voltage, but any change in load current throws off Vo.

I'm going to replace the IC and try again. Not sure how this happened. The supply was brought up slowly with the trimpots pre-set according to the guidelines.
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