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Stunts  
#1 Posted : Saturday, December 27, 2014 4:58:11 PM(UTC)
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Hey guys,

Just finished two Placid HD BP 2.1 boards and both exhibit the same issue: the negative voltage is approx. -2.88V at startup. It seems the exact same issue that miero encountered a couple of months ago.

Upon warming up (<1 minute) the output voltage suddenly rises to the normal value (-14.9V) and remains stable from then on. The problem occurs again after the board has cooled down completely.

The +ve side of both boards, as well as two Placid HD2.1 (non-BP) boards I built, function perfectly well.

All solder joints are double-checked, including the opamps. Of course I can try miero's workaround (replacing R8B by 100R) but that leaves me wondering why both my BP boards (and miero's) are affected on the -ve side while most people don't have this issue. I'd say opamp failure seems rather unlikely...

All parts are stock, except for C1A/B, the resistors, and a larger 225VA 2x15VAC toroid.
TP_VREF- is -1.70V (-2.49V after brief warm-up). TP_VREF+ is 2.49V.

Thanks a lot for your help :)
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avr300  
#2 Posted : Saturday, December 27, 2014 6:49:11 PM(UTC)
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Is the problem present without load, ie. when PSU is unconnected ?
Stunts  
#3 Posted : Saturday, December 27, 2014 6:54:22 PM(UTC)
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Yes, load unconnected.
Martin H  
#4 Posted : Sunday, December 28, 2014 12:10:54 PM(UTC)
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I have exactly the same problem with both my boards. The negative halves only exhibit the problem, as described by Stunts. The voltage output reading looks to be essentially the output of the 2.5V voltage ref (LT1034). After some time, the circuits commence functioning properly, and their outputs rise to the prescribed 15V. But before this happens, the CCS pass element gets warmer than normal, while the shunt element stays cold. Both boards are under load.

Edited by user Sunday, December 28, 2014 12:11:42 PM(UTC)  | Reason: Not specified

Jordo  
#5 Posted : Sunday, December 28, 2014 2:45:43 PM(UTC)
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Hi Guy's,

I have my Placid HD BP V2.1 playing now.
After careful adjustment with the load connected I adjusted the Placid to 55mA Shunt Current and 14.81V
(I can't find the exact recommended voltage. What do you use?)

But when powering the LH channel onto which the new Placid is connected gave a very weak signal.
When checking the voltages I also measure around 2.88V on the - channel.

TP Vref- was approx. 1.65V.

After a little adjusting of the voltage and current I hear a kind of "Click" like a small relay and all voltages were normal again.
TP Vref became 2.50V again and output 14.81V....
Martin H  
#6 Posted : Sunday, December 28, 2014 4:14:54 PM(UTC)
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So, Jordo, it seems you have the same problem. Very strange.

My Placids are set up like yours, almost: shunt current is set to 55mA @ 15.0V.

They are powering Retro phono amp boards. I am worried that the Retro boards will be damaged by the misbehaviour of the Placids.
avr300  
#7 Posted : Sunday, December 28, 2014 4:43:29 PM(UTC)
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Originally Posted by: Jordo Go to Quoted Post

After a little adjusting of the voltage and current I hear a kind of "Click" like a small relay and all voltages were normal again.
TP Vref became 2.50V again and output 14.81V....


If you are using a TPA output stage, the click comes from the relay getting the proper voltage and therefore engages.
avr300  
#8 Posted : Sunday, December 28, 2014 4:46:33 PM(UTC)
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You who has the problem can you try to shunt a bit more current, say 100mA.

I might have seen similar behaviour, don't think it was on 2.1 platform. I can't remember what I did to solve it, perhaps something like more current, more voltage (was running +-12v, now I'm running +-15v)
thanks 1 user thanked avr300 for this useful post.
Jordo on 12/28/2014(UTC)
Jordo  
#9 Posted : Sunday, December 28, 2014 5:23:42 PM(UTC)
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Ok I will increase the Shunt Current. With the extra extra large coolers it must be no problem.

I will also increase the voltage to 15V more precise.

Miero had a bad IC, don't you think there is a bad badge of OPAMP's installed on the Circuit Boards?

My other HDBP V2.1 doesn't have this kind of problem. It was delivered earlier than my recent HDBP.

Best regards,

Jordo
avr300  
#10 Posted : Sunday, December 28, 2014 5:28:08 PM(UTC)
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We need to ask Brian to deal with this hypothesis.
Stunts  
#11 Posted : Sunday, December 28, 2014 5:35:49 PM(UTC)
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OK, more people with the same problem, all with -2,8V on the negative rail. I'd say that sort of eliminates construction errors.

I'm using 402R fixed resistors for an output voltage of +/- 14.8-14.9V.
Increasing the shunt current doesn't solve the problem, since the load is not connected and (almost) all CCS current (approx. 2x300mA) is already shunted.
The problem also occurs when the load (Legato) is connected, as is the case with Jordo and Martin H.

Unfortunately my understanding of the working of the circuit is too limited to really identify the cause of the problem, I only see the symptoms.
(I still feel it is related to the temperature since I can only reproduce it when the boards are cooled down completely.)

Hopefully someone has good ideas :)
Jordo  
#12 Posted : Sunday, December 28, 2014 5:37:06 PM(UTC)
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Yes I hope he reads all this...
Stunts  
#13 Posted : Sunday, December 28, 2014 5:44:57 PM(UTC)
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Originally Posted by: Jordo Go to Quoted Post

Miero had a bad IC, don't you think there is a bad badge of OPAMP's installed on the Circuit Boards?

My other HDBP V2.1 doesn't have this kind of problem. It was delivered earlier than my recent HDBP.


I don't believe it could be a bad batch... nobody seems to have problems on the positive rail.
avr300  
#14 Posted : Sunday, December 28, 2014 5:53:00 PM(UTC)
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Its pretty easy to copy Miero.

Just place a 150R (or something around 150R) in parallel with the 480R and see what happens.
Jordo  
#15 Posted : Sunday, December 28, 2014 5:59:44 PM(UTC)
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@ Stunts: O, he had it on the Positive rail, that's a different OPAMP indeed.

@ AVR300: I might try to do so if it re-occurs frequently.
I just experienced it one time today at start-up.
I hope the higher Voltages and Currents might prevent this of happening.

Thx!

Jordo
Martin H  
#16 Posted : Sunday, December 28, 2014 6:22:49 PM(UTC)
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Originally Posted by: Stunts Go to Quoted Post
Hopefully someone has good ideas :)


It is my sincere hope that "someone" can come up with something better than "cold solder joint." Anxious

Stunts  
#17 Posted : Sunday, December 28, 2014 6:23:05 PM(UTC)
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Originally Posted by: Jordo Go to Quoted Post
@ Stunts: O, he had it on the Positive rail, that's a different OPAMP indeed.


I understood nobody has problems with the +15V, only the -15V is affected, as was also the case with Miero (correct me if I'm wrong). The opamps are identical on the positive half and the negative half (OPA209, marked IC1 and IC2), only the PNP and NPN transistors are different (marked QPx and QNx).
Martin H  
#18 Posted : Sunday, December 28, 2014 6:32:11 PM(UTC)
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Originally Posted by: Stunts Go to Quoted Post
I understood nobody has problems with the +15V, only the -15V is affected, as was also the case with Miero


Correct.

Jordo  
#19 Posted : Sunday, December 28, 2014 6:33:24 PM(UTC)
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Do they have the same p/n's?
I thought I saw different 'Legs' connected to similar components than on the other side so I figured it must be another model OPAMP.
Stunts  
#20 Posted : Sunday, December 28, 2014 6:35:20 PM(UTC)
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Originally Posted by: avr300 Go to Quoted Post
Its pretty easy to copy Miero.

Just place a 150R (or something around 150R) in parallel with the 480R and see what happens.


Just tried that, placed a 221R in parallel with R8B (closest I had), resulting in 170R. Placid started right at +14.8V/-14.9V. But it's a little soon to draw conclusions. Will try again tomorrow morning when Placid is cold.

But I have no idea what I'm doing really by decreasing R8B. Perhaps this is the best solution but hopefully Russ or Brian will be able to shed their light on the problem indeed :)
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