Need Help Fixing this Board.. (weirdest thing I've seen)

[JJFORTY]

Hi All,

I have this faulty board which does not want to operate unless I touch one of the nodes with finger or DMM probe tip. I have many of this boards 100% operating so it is not down to design error. I have measured all component values and all Ok. I have also replaced all Ic's and C392 which looks to be right in the path of the problem but to no bail.

I have taking all measurements and compared with working board..

Red: Working units
Blue: Faulty units
Black: When node is touched

When node is touched pin 7 Comparator (U315-B) input increases about 10mV and this makes o/p to rise to 3.3V so board works.. I have done this test to a working board and input pin 7 also increases about 15mv ... so what is making the faulty board operate when node is touched???

I appreciate any input..

Thanks

 

[banjo]

Well, it you have checked all the resistors and swapped all the ICs, then there are only three possibilities. First, and most likely, you have a cold solder joint or a broken trace on the PCB. The second choice is flux contamination on the solder side adding extra leakage paths. The third choice is a leaky capacitor.

Scrub the solder side of the PCB with alcohol to remove all flux and contamination and inspect it under a microscope or strong magnifying glass.

Why does touching the node make it work? The U313-D op-amp is being biased off. Touching that node managed to bias it back to the active region.

 

[JJFORTY]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

Hi, banjo Thanks for your input.

I have checked for any cold solder joint o broken tracks under a microscope but this a multilayer board making very difficult fo find if any.

I will try for any leaky capacitor.

thanks again..

 

[banjo]

Did you wash down the board for contamination? You have a number of large value resistors in this circuit. It does not take much to create a leakage path that upsets your bias. Take a hair dryer and warm up the board. Does to start working? If so, it most likely a contamination problem. (The heat from the dryer forces the water out of the contamination temporarily)

 

[JJFORTY]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

As you said banjo applying local heat makes board to work. I have washed tracks with alcohol (IPA) but I can't find any sings of contamination I have also re-soldered all joins but to no bail.

 

[Siniša]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

Check with ommeter all connections on node in question. PCB trace is likely broken between R and amp input.

 

[nael_ee]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

Hi,
Truly amazing problem, am interested what frequency that you are working at ?

Did you considered that your design is marginally stable, did you do any simulation for it? normally in the feedback circuits of OP-AMP there is possible of unstable circuit at a high frequency?

To check if you have unstable board:
1- check the DC input current for a GOOD unit and compare it with a faulty one? if there is a large current difference, then you have a problem (oscillation)?

2- add a high pole capacitor ? small value I think that 0.001 uF is good,the CAP is OC compared with your circuit component. place the capacitor either between the pin 13 & 14 (Lead Compensation method) or pin 13 and the ground (attenuator)?

3- check using OSC, a luxury rarely available?

I know the above explanation looks a little bit strange due to the fact that you have a working boards. remember that you are working in the analog domain so you may be standing at the edge and a little bit variation with-in tolerance may send you from a working board to a non-working.

----------------------
In your experiment did you replaced every component in a working board to a nonworking board. and it is still not working then you have a board problem. not a design.but it is not so obvious in your statement.
and is there is a lot of variation in the boards layout if so maybe you have and undesired coupling and point 3 above should fix it.
------------------------

please report what happened if it worked or not.

thanks a lot.

 

[banjo]

Are the large capacitors ceramic, tan, or electrolytic? If not ceramic, then leakage could be an issue. The voltmeter has an input impedance of 10 meg ohms. I was assuming that this resistance connected between the "node" and ground was enough to bias it back into the active region.
What happens if you connect the voltmeter between VCC and the "node" does it work? If it works, disconnect the negative lead of the voltmeter and touch the "node". Does it still work with only one lead connected?
If it only works when the negative lead is touching ground, then I suspect a pure dc bias problem. If it works with the negative lead disconnected, then I suspect that it is working only because the lead is injecting some power line hum.
Is the board surface mount or leaded components? Surface mount can be very difficult to get the solder flux out from under. Surface mount resistors and capacitors also can develop cracks that are hard to discover. When removing the part and checking with a meter, you apply an inward force to make contact. This has fooled me before by closing the crack and making the part test good. I would start replacing resistors a couple at a time.
After you heated the board, how long did it work? Cracked traces or cracked components will cool pretty quickly and return to a non-working condition. If it works until you power cycle the board, then perhaps you some sort of latch-up condition at startup.
Does it immediately stop working when you stop touching the "node"? If it again keeps working until you power cycle, then a latch-up is suspected.
My guess is that we are only looking at a small sub-circuit of the entire design. If this is true, can you break the circuit before this stage and inject the signal from another board? You can then take the input signal from this defective board and inject it into the known good board. If the problem switches boards you know that the problem is up-stream of the point at which you tapped into the circuit.

 

[JJFORTY]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

Freq: 125Hz

* SMD Ceramic Caps

* Using DMM between VCC and Node: No O/P

* Using DMM between Ground and Node: O/P comes on and stays on as long as contact is maintained.

* Using DMM positive lead only: O/P comes on for abut 1 sec then falls to zero.

* Touching Node with finger: O/P comes on and stays as long as contact is maintained.

* Applying local heat makes board to work but if heat is removed O/P falls almost
immediately.

* When board is powered up O/P appears for about 1 second then falls to zero.

I am in the process of using signals from a working board. I shall keep you posted...

I appreciate your help.

 

[banjo]

Hi,
From your last experiments, it is clear that the problem is too much positive bias. By providing a high resistance path to ground, the bias voltage is lowered and the circuit starts to work.
Did you swap ICs between a working and a non-working board?
Or, did you just replace the ICs with new ones from stock? Are the same IC vendor and lot codes used in both the working and non-working boards?
Please be aware that all vendors ICs that are supposed to be crosses are not internally identical. Also, manufacturer's often do die shrinks to increase the number of chips per wafer. This can also drastically affect the operation. A couple of years ago, we had a problem with Linear Tech Op-Amps not working in a circuit where Analog Device parts had worked for years. The Linear Tech parts, which were supposed to be an exact cross, required a much larger input bias current.
Heating the circuit may make it work because the internal leakage currents increase as the temperature increases. This could overcome the high voltage bias.
Can you apply cold spray to a working unit and it still works? If, not then the op-amps may be close to the inoperable range.

 

[JJFORTY]

Re: Need Help Fixing this Board.. (weirdest thing I've seen

Hi

New IC's are from stock. (same manufacturer)

I applied cold spray to a working unit, it works down to -40C then falls gradually, specs for 0p-amps are -50C max. so I suppose this is OK.

Repeated experiment to my faulty board but unfortunately power on the board went down I think one of the regulators has blown.

Bear with me while I fix this..

Thanks for your posts

 

[banjo]

Hi,

If is works down that far, I'm not concerned. If it stopped with the first blast of cold spray and stayed non-working for several seconds until it heated back up, then I would be concerned.
Are the working and non-working boards from the same vendor and same lot? I just spent most of the day tracking down a problem that ultimately turned out to be a PCB problem. I replaced all the components that could have caused the problem without fixing it. By adding a jumper wire, I proved that it was too small a trace or via on the PCB. (In my case, the current is over 500mA so it is an extreme example.) None of the previous 80 boards had this problem. After finding one detective PCB, I started doing additional tests and found several others that are on the very edge of failure.
My point is not to overlook the only component that cannot be easily changed, the PCB.