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n_maher  
#1 Posted : Sunday, October 19, 2008 8:39:45 AM(UTC)
n_maher

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So I'm having a problem with my LCBPS (bi-polar power supply). Basically I get zero adjustment on the negative side of the PS. I did make one error during construction, I had D4 in backwards. I powered up the PS once, saw that something was wrong and powered it down and found the mistake. But even after correcting that error it seems like I'm not getting any regulation on that side of the PS so my guess is that IC2 is toast. Anything else I should check before just replacing IC2?
Brian Donegan  
#2 Posted : Sunday, October 19, 2008 2:15:09 PM(UTC)
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Just confirm IC1 is an LM317 and IC2 is an LM337.
n_maher  
#3 Posted : Sunday, October 19, 2008 4:59:04 PM(UTC)
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Yup, IC1 is a 317 and IC2 is a 337.

FWIW the + side adjusts just fine.
minivan  
#4 Posted : Wednesday, October 22, 2008 3:33:53 AM(UTC)
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i have the same problem ,except mine is on the +side, the -side adjust just fine.
initialy i have the transitor on the wrong side, i.e LM317 ON IC2 AND LM337 on IC1, it give mep a reading of
zero voltage on +side ,swapping LM337 TO IC2 AND LM317 to IC1 still give me the same problem ,i.e zero voltage on the +side, in this case, it seem both lm317 and lm337 work fine, so what's the problem with this zero voltage?
Russ White  
#5 Posted : Wednesday, October 22, 2008 3:52:16 AM(UTC)
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If you installed the ICs incorrectly, it is likely one of them is now destroyed...

You should replace them before testing again.
n_maher  
#6 Posted : Wednesday, October 22, 2008 7:53:02 PM(UTC)
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Russ White wrote:
If you installed the ICs incorrectly, it is likely one of them is now destroyed...

You should replace them before testing again.


I installed the IC's correctly, just the diode was wrong. But I'm going to replace the 337 before testing again since it seems like it's either that or the trimpot (which is also installed correctly).

Also, Brian, I sent you an email back but I keep getting a message which I think implies that your mail server has "gray listed" me and won't deliver the message to you. :(
Russ White  
#7 Posted : Wednesday, October 22, 2008 8:08:34 PM(UTC)
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The trimpot can't really be installed incorrectly, it will work fine in either direction. It also is not likely to be bad.

Cheers!
Russ
minivan  
#8 Posted : Wednesday, October 22, 2008 11:06:11 PM(UTC)
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my assumption is: when i had my lm117 installed wrongly on the -side, i did get voltage on the -side
when i switched the lm337 to the -side, again i get voltage on the -side, so my conclusion is: both transistor work
i will wait for n_maher's testing result, before i replaced my lm117, sourcing this transistor is very hard here.
james.evans@frost.com  
#9 Posted : Thursday, October 23, 2008 5:51:19 AM(UTC)
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I had issues with both the lcbps and lcdps relating to the trim pots - I got no adjustment from any of the 4. Replacing them with the appropriate fixed value resistor cured that and the boards are working fine now.

Most odd. Be careful desoldering as well - the pcb pads lift really easily...
n_maher  
#10 Posted : Thursday, October 23, 2008 6:12:04 AM(UTC)
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Russ White wrote:
The trimpot can't really be installed incorrectly, it will work fine in either direction. It also is not likely to be bad.

Cheers!
Russ
Russ,

It just seemed like the only other component that could have any affect on the output adjustment. FWIW I get ~-22V on the output which sure looks to me like the unregulated output of the 15V trafo.

Is there any way to test the reg in circuit? Would two of the pins be shorted or something like that if it's cooked?
n_maher  
#11 Posted : Friday, October 24, 2008 4:40:26 PM(UTC)
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Replaced IC2 with a new 337, no joy, no change, still no adjustment on the output. I'll post up some high res shots tonight of the top and bottom of the pcb. I can't find anything else wrong, I even tried reflowing the joints on the trim pot. I try to make sure it's working and post whatever measurements I take. I can't for the life of me think what else could be wrong...
n_maher  
#12 Posted : Friday, October 24, 2008 4:56:20 PM(UTC)
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n_maher  
#13 Posted : Friday, October 24, 2008 5:02:29 PM(UTC)
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Also, I checked the voltages at the various electrolytic caps: I get ~24VDC at the first two and then ~23.5VDC at the final two. I get the same -23VDC on the output, no adjustment, the trimpot appears to have 0 effect. The trim pot itself appears to measure weird. Set to the middle of its range it should measure around 2.5K, what I get is a continuously escalating resistance, like when measure across a capacitor. Weird. The positive side trim pot is set to ~1.85k to get 12V on the output and while this value drifts up just a bit while I take the measurement it's not at all like the trimmer on the negative side.

Help.Brick wall
Russ White  
#14 Posted : Friday, October 24, 2008 5:13:25 PM(UTC)
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So this is on the negative side?

So -23V?
n_maher  
#15 Posted : Friday, October 24, 2008 5:16:51 PM(UTC)
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That is correct, it's on the negative side only. The + works fine, or at least I was able to dial it down to 12V (unloaded) without issue. I get 18VAC out of the trafo, a rectified - 24VDC at C4 and C6 and -23.5VDC at C8 and C10. I get the same -23.5VDC on the output and the voltage does not fluxuate at all running the trimpot through its entire range.
Russ White  
#16 Posted : Friday, October 24, 2008 5:18:39 PM(UTC)
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forgive me for asking but how do you have the trafo connected to the PS.
n_maher  
#17 Posted : Friday, October 24, 2008 5:24:17 PM(UTC)
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No harm is asking Russ, I'm clearly capable of making mistakes.d'oh! I have one of your (TP's) Avel trafos and have one secondary (red and black) connected to S1 and the other (orange and yellow) connected to S2. On the primary side I have it configured for 115V with purple and blue tied together and brown and gray tied together. The trafo secondaries each put 18VAC into their respective terminals.
n_maher  
#18 Posted : Saturday, October 25, 2008 5:39:06 AM(UTC)
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So I guess I'll try removing the trimpot today and soldering in a fixed resistor in the 2k range. I'll report the results if I have time to experiment.
n_maher  
#19 Posted : Saturday, October 25, 2008 7:00:07 AM(UTC)
n_maher

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Rather than deal with the hassle of removing the trimpot I decided to try and check again if it was working correctly. It's definitely not. The outer two pins, which should simply measure the resistance the pot is set to, show infinite resistance across them. The two pins that are supposed to connected together are, so the pcb seems to not be the fault, it's the pot itself. So I soldered in a 1/2W 2k resistor connecting the outer two pins and viola, -11.8VDC on the output. So, now the question is whether or not to try and replace the fault trimpot or just figure out what value I'd need to get the necessary ouput voltage. Which raises the question, what are you guys currently recommending for the IVY? The IVY docs say 12 to 15V.
james.evans@frost.com  
#20 Posted : Sunday, October 26, 2008 3:01:39 AM(UTC)
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I used 2.7kR instead of the trimpots - all 4 were dead for me. This gives me +-15.4V. I used 1.025kR on the dps to give both at 6.6V.
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