mini 3 problem
Jun 28, 2008 at 10:34 PM Thread Starter Post #1 of 43

lacrossebowe8

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I just finished soldering up my mini 3 and started to go through the initial checks and everything was fine until I got to the output dc offset check. I had below 5 mV for OG and OL but for OR my reading was around 180 mV.

How should I proceed from here? Does this probably mean I ruined an op amp?

Thanks for the help.
 
Jun 28, 2008 at 10:56 PM Post #2 of 43

UglyJoe

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Really carefully check you soldering on U5. A bad joint can easily cause that, and fixing it should be easy.

EDIT: If all joints are good then the fate of U5 is in doubt.
 
Jun 29, 2008 at 3:27 AM Post #3 of 43

lacrossebowe8

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could anything other than u5 be damaged? or does this test only the functionality of the op amps?
 
Jun 30, 2008 at 2:35 AM Post #5 of 43

lacrossebowe8

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i tried reflowing the solder joints and still had a high offset in that channel. What I don't get is my job on u5 looks much better than my job on u4. I'm not sure what to do now because replacing that opamp will be hard and I would be pretty frustrated if that did not fix the problem. Could anything else be wrong with the amp or would it just be the u5 opamp at least from this test?

Also how do you suggest i proceed from here?
 
Jun 30, 2008 at 2:43 AM Post #6 of 43

amb

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Replacing the opamp is difficult, especially for a newbie, but if it's where the fault is, you need to do it. Based on the info you provided it's almost certain that U5 is blown.
 
Jun 30, 2008 at 3:59 AM Post #7 of 43

UglyJoe

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lacrosse, really carefully check those pins with your continuity test on your multimeter before giving up on U5. Getting those joints right is not the easiest thing in the world, especially when working with the rest of the amp populated.

I'd check the top of each pin (right next to the body of U5) and the pad that is the nearest neighbor (look at the schematic of the board layout file on amb's site). DO NOT PUSH DOWN while doing this test. It can give you a false positive, making the joint look good when it's not. You have to simply touch the pins, don't apply pressure.


PS - make sure that you've removed the batter and discharged the caps before doing the above, please.
 
Jun 30, 2008 at 2:04 PM Post #8 of 43

lacrossebowe8

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I'll try this test and report back. I'm hoping for the best because removing and replacing the op-amp will be challenging.
 
Jul 4, 2008 at 3:16 PM Post #9 of 43

lacrossebowe8

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Well I just can't seem to fix this. I tried the continuity test suggested and my meter simply displayed an open circuit. I tried reflowing and it did not help, so I guess my opamp is blown. I'm not sure what to do now because replacing that opamp will be hard and I'm not convinced it will fix everything. Suggestions?
 
Jul 4, 2008 at 3:32 PM Post #10 of 43

holland

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check the other pins on the side. your opamp may be slightly lifted on one side. You may have to heat and push down on each pin. If you have a large tip that can cover one entire side of the opamp (I use this), you can heat the entire side and push down to make sure you get pad contact.

recheck continuity after that.
 
Jul 4, 2008 at 6:45 PM Post #11 of 43

UglyJoe

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If you aren't getting continuity on the above check then your opamp is almost assuredly okay, and those joints have got to be fixed. You have any desoldering braid? Go ahead and lay it up and down the length of both sides of the opamp and desolder away to make sure the opamp has good contact with the pads (not big blobs of solder in between them. Once done with that, clean your iron, apply a SMALL dab of solder to the tip and wipe the iron tip along one of the pins, and repeat ONE AT A TIME for the others. Let's get that opamp in there good before we start worrying about wither or not it is blown.
 
Jul 6, 2008 at 5:02 PM Post #12 of 43

lacrossebowe8

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ugly joe, I just followed your suggestion, but I still got a high right channel offset. My meter also read "1" for the left offset, which I think means there is no circuit or that the offset is higher than the selected range. What further recommendations does everyone have? I really appreciate all the help.
 
Jul 6, 2008 at 5:13 PM Post #13 of 43

holland

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are you getting continuity on the pins now?

check the joints on your resistors. A cold joint on one of the gain resistors can cause offset as well. cold joints on opamps are way more common, but who knows, reflow all the through-hole components on the whole amp. It only takes a minute or two with a good iron.

visually inspect with a loupe. if all the joints are good, the opamp noted is bad.
 
Jul 6, 2008 at 5:27 PM Post #14 of 43

lacrossebowe8

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I didn't check the continuity yet. I will reflow everything later today and report back. My meter isn't so great and has no continuity buzzer so checking it is hard, and the pins are harder to check since there are so many components around them, so I find this test difficult and sometimes inconclusive.
 
Jul 6, 2008 at 5:50 PM Post #15 of 43

holland

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use resistance mode. continuity is 0 ohms or some very tiny fraction very close to 0.

From pin to the next junction point in line, it's essentially a wire. The reason you are measuring at the top of the opamp (pin) is because it needs to go through the leg, through the pad, into the wire, and to the probe point on the next closest pad.
 

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