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| Agilent 34401A Bubble on PCB ... can it be saved ? (SOLVED) |
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| Kleinstein:
The votlages at pins 5 and 6 of U201 are a bit different, but this is likely from the loading from the DMM to measure these voltages. The voltages look reasonable. Being some 4 % off with the 1 M resistor supports that the test current is too low. Maybe try if the reading drifts just after turn on / warm up. If the issue is from leakage in the semiconductor, it is suspected to get quite a bit worse (like 2 x the leakage) with warm up. With leakage from a dirty PCB less temperature effect is expected. |
| The_Spectrum.A_idiot:
--- Quote from: Kleinstein on September 22, 2024, 09:03:34 pm ---The votlages at pins 5 and 6 of U201 are a bit different, but this is likely from the loading from the DMM to measure these voltages. The voltages look reasonable. Being some 4 % off with the 1 M resistor supports that the test current is too low. Maybe try if the reading drifts just after turn on / warm up. If the issue is from leakage in the semiconductor, it is suspected to get quite a bit worse (like 2 x the leakage) with warm up. With leakage from a dirty PCB less temperature effect is expected. --- End quote --- Well I didn't leave the instrument warm up with its covers on for 5h. ( I don't have such time on my hands) The pcb was cleaned with IPA as best as possible. Next step is to use high pressure contact cleaner to get under every IC and diode ( i can't imagine any other way to get under that clean.) I am hoping for Dr. Frank to give some insight to what might be at fault. I am almost certain that some componet I selected is not appropriate. And I should had just changed the OP AMP only. never the less, time will tell. If further warmup is needed I will do such test with different resistors becuase the 1M was the biggest I had in my arsenal at the moment. Thank you in advance |
| Kleinstein:
The current is rather far off, well more than expected for leakage at Q211 or Q202 or the OP-amp bias current. The transistors in the protection would not effect the low current, even if bad or broken. A point could be if the OP-amp is genuine or a Ebay/Ali part. The change in current from doing the extra cleaning step points strongly towards a dirt problem, though it could still also be a thermal effect (e.g. higher temperature in the 2nd test). No need for a full warm up test. The first half hour should show most of the thermal effect. Contact cleaner may or may not be suitable, as some include lubricants and can leave a resudue. They may also not be good solvents, but more a thing to remove oxide layers. A spray can can also contaminate other areas from splatter. HIgh pressure is more a thing for cleaning a whole board, not so much for loacal claning. That is more a thing for brush, seringe and tissue. One can still use IPA or similar around / under the chips. It will flow also under the chip and can be sucked out / wicked out with clean tissue. The dirt is removed by the solvent removed, not by the solvent added. IPA is also only good for some contaminations and there are different purities / grades. More salt like contaminations are better removed with water or a IPA / water mix. More organic / grease like dirt would want high purity / low water content IPA or even a different solvent. It may well need multiple cleaning steps. Usually water or a water+IPA mix first and at the end clean IPA. One could try / test cleaning of flux residue on a different PCB. So do experiments separate first, to see what works with the flux that was used in the repair. For the flux the soldering step can change things - heated flux is different from fresh or only partially heated. Not all residue from the flux is actually bad - some contain some parafin / wax component, that is hard to remove, but isolating. |
| The_Spectrum.A_idiot:
All parts are from mouser. I would never get ebay parts unless are unoptenium. But what ever I changed / replaced are original mouser parts. All replaced with soldering iron and soldering wick. Flux I have a syringe ( i need to find the brand name I can't think of it on top of my head) Cleaning, before soldering anything on I cleanred the area and pads after wicking them with IPA and a q-tip ( the thing used (incorrectly) for ear canal cleaning) and then soldering on the parts 1 by 1 ( 1 leg at a time) Let me know. Thank you in advance |
| The_Spectrum.A_idiot:
Flux Type : SMD291/30CC - Chip Quik |
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